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Explicitly accounting for phenotypic differentiation together with environmental heterogeneity is crucial to understand the evolutionary dynamics in hybrid zones. Species showing intra-specific variation in phenotypic traits that meet across environmentally heterogeneous regions constitute excellent natural settings to study the role of phenotypic differentiation and environmental factors in shaping the spatial extent and patterns of admixture in hybrid zones. We studied three environmentally distinct contact zones where morphologically and reproductively divergent subspecies of Salamandra salamandra co-occur: the pueriparous S. s. bernardezi that is mostly parapatric to its three larviparous subspecies neighbours. We used a landscape genetics framework to: (i) characterise the spatial location and extent of each contact zone; (ii) assess patterns of introgression and hybridization between subspecies pairs; and (iii) examine the role of environmental heterogeneity in the evolutionary dynamics of hybrid zones. We found high levels of introgression between parity modes, and between distinct phenotypes, thus demonstrating the evolution to pueriparity alone or morphological differentiation do not lead to reproductive isolation between these highly divergent S. salamandra morphotypes. However, we detected substantial variation in patterns of hybridization across contact zones, being lower in the contact zone located on a topographically complex area. We highlight the importance of accounting for spatial environmental heterogeneity when studying evolutionary dynamics of hybrid zones., This work was supported by National Funds through FCT – Foundation for Science and Technology (SALOMICS: PTDC/BIA-EVL/28475/2017); the authors also acknowledge funding from other FCT projects and FEDER funds through the Operational Programme for Competitiveness Factors – COMPETE (EVOVIV: PTDC/BIA-EVF/3036/2012; FCOMP-01-0124-FEDER-028325 and UIDB/500027/2020); and by National/FEDER funds through Ministerio de Economía y Competitividad (CGL2012-40246-C02-02). G.V.-A., A.L. and P.T. were supported by FCT (IF/01425/2014 and CEECIND/00937/2018, PD/BD/106060/2015, DL57/2016/CP1440/CT0008, respectively).

Many animals generate propulsive forces by coordinating legs, which contact and push against the surroundings, with bending of the body, which can only indirectly influence these forces. Such body–leg coordination is not commonly employed in quadrupedal robotic systems. To elucidate the role of back bending during quadrupedal locomotion, we study a model system: the salamander, a sprawled-posture quadruped that uses lateral bending of the elongate back in conjunction with stepping of the limbs during locomotion. We develop a geometric approach that yields a low-dimensional representation of the body and limb contributions to the locomotor performance quantified by stride displacement. For systems where the damping forces dominate inertial forces, our approach offers insight into appropriate coordination patterns, and improves the computational efficiency of optimization techniques. In particular, we demonstrate effect of the lateral undulation coordinated with leg movement in the forward, rotational, and lateral directions of the robot motion. We validate the theoretical results using numerical simulations, and then successfully test these approaches using robophysical experiments on granular media, a model deformable, frictional substrate. Although our focus lies primarily on robotics, we also demonstrate that our tools can accurately predict optimal body bending of a living salamander Salamandra salamandra.

Insights into the causal mechanisms that limit species distributions are likely to improve our ability to anticipate species range shifts in response to climate change. For species with complex life histories, a mechanistic understanding of how climate affects different lifecycle stages may be crucial for making accurate forecasts. Here, we use mechanistic niche modeling (NicheMapR) to derive "proximate" (mechanistic) variables for tadpole, juvenile, and adult Rana temporaria. We modeled the hydroperiod, and maximum and minimum temperatures of shallow (30 cm) ponds, as well as activity windows for juveniles and adults. We then used those ("proximate") variables in correlative ecological niche models (Maxent) to assess their role in limiting the species' current distribution, and to investigate the potential effects of climate change on R. temporaria across Europe. We further compared the results with a model based on commonly used macroclimatic ("distal") layers (i.e., bioclimatic layers from WorldClim). The maximum temperature of the warmest month (a macroclimatic variable) and maximum pond temperatures (a mechanistic variable) were the most important range-limiting factors, and maximum temperature thresholds were consistent with the observed upper thermal limit of R. temporaria tadpoles. We found that range shift forecasts in central Europe are far more pessimistic when using distal macroclimatic variables, compared to projections based on proximate mechanistic variables. However, both approaches predicted extensive decreases in climatic suitability in southern Europe, which harbors a significant fraction of the species' genetic diversity. We show how mechanistic modeling provides ways to depict gridded layers that directly reflect the microenvironments experienced by organisms at continental scales, and to reconstruct those predictors without extrapolation under novel future conditions. Furthermore, incorporating those predictors in correlative ecological niche models can help shed light on range-limiting processes, and can have substantial impacts on predictions of climate-induced range shifts.

Complex life-histories may promote the evolution of different strategies to allow optimal matching to the environmental conditions that organisms can encounter in contrasting environments. For ectothermic animals, we need to disentangle the role of stage-specific thermal tolerances and developmental acclimation to predict the effects of climate change on spatial distributions. However, the interplay between these mechanisms has been poorly explored. Here we study whether developmental larval acclimation to rearing temperatures affects the thermal tolerance of subsequent terrestrial stages (metamorphs and juveniles) in common frogs (Rana temporaria). Our results show that larval acclimation to warm temperatures enhances larval heat tolerance, but not thermal tolerance in later metamorphic and juvenile stages, which does not support the developmental acclimation hypothesis. Further, metamorphic and juvenile individuals exhibit a decline in thermal tolerance, which would confer higher sensitivity to extreme temperatures. Because thermal tolerance is not enhanced by larval developmental acclimation, these 'risky' stages may be forced to compensate through behavioural thermoregulation and short-term acclimation to face eventual heat peaks in the coming decades., This research was supported by MINECO (CGL2012-40246) and MEC (CGL2017-86924-P) Grants. U.E.U. was supported by a Ph.D. award (BES-2013-063203) from MEC.

The evolution of complex traits is often shaped by adaptive divergence. However, very little is known about the number, effect size, and location of the genomic regions influencing the variation of these traits in natural populations. Based on a dense linkage map of the common frog, Rana temporaria, we have localized, for the first time in amphibians, three significant and nine suggestive quantitative trait loci (QTLs) for metabolic rate, growth rate, development time, and weight at metamorphosis, explaining 5.6–18.9% of the overall phenotypic variation in each trait. We also found a potential pleiotropic QTL between development time and size at metamorphosis that, if confirmed, might underlie the previously reported genetic correlation between these traits. Furthermore, we demonstrate that the genetic variation linked to fitness-related larval traits segregates within Rana temporaria populations. This study provides the first insight into the genomic regions that affect larval life history traits in anurans, providing a valuable resource to delve further into the genomic basis of evolutionary change in amphibians.

The genetic architecture of speciation, i.e., how intrinsic genomic incompatibilities promote reproductive isolation (RI) between diverging lineages, is one of the best-kept secrets of evolution. To directly assess whether incompatibilities arise in a limited set of large-effect speciation genes, or in a multitude of loci, we examined the geographic and genomic landscapes of introgression across the hybrid zones of 41 pairs of frog and toad lineages in the Western Palearctic region. As the divergence between lineages increases, phylogeographic transitions progressively become narrower, and larger parts of the genome resist introgression. This suggests that anuran speciation proceeds through a gradual accumulation of multiple barrier loci scattered across the genome, which ultimately deplete hybrid fitness by intrinsic postzygotic isolation, with behavioral isolation being achieved only at later stages. Moreover, these loci were disproportionately sex linked in one group (Hyla) but not in others (Ranaand Bufotes), implying that large X-effects are not necessarily a rule of speciation with undifferentiated sex chromosomes. The highly polygenic nature of RI and the lack of hemizygous X/Z chromosomes could explain why the speciation clock ticks slower in amphibians compared to other vertebrates. The clock-like dynamics of speciation combined with the analytical focus on hybrid zones offer perspectives for more standardized practices of species delimitation., This study was funded by the Swiss NSF (Grant No. 31003A_166323 to N.P. and Fellowship No. P2LAP3_171818 to C.D.) and the Russian Foundation for Basic Research (Grant No. 20-04-00918 to S.N.L.). C.D. and M.V. were further supported by the Taxon-Omics priority program (SPP1991) of the Deutsche Forschungsgemeinschaft (Grant No. VE247/19-1)

Due to the speed of climate changes, rapid buffering mechanisms such as phenotypic plasticity – which may depend on breeding phenology – could be key to avoid extinction. The links between phenology and plasticity, however, remain understudied. Here we explored the matching between phenology and the thermal sensitivity of standard (SMR) and routine metabolic rates (RMR), metabolic scope (i.e. the difference between RMR and SMR), survival and growth-development trajectories in larvae of a prolonged breeder amphibian Alytes almogavarii acclimated to 10 and 20°C, belonging to three cohorts: autumn pre-overwintering, autumn overwintering and spring tadpoles. At 20°C, survival of autumn pre-overwintering larvae was lower than for the rest. Although all cohorts showed acclimation potential, patterns for SMR and RMR differed, leading to differences in metabolic scope. Regardless of temperature, overwintering tadpoles arrested growth and development, while pre-overwintering and spring tadpoles showed higher growth and development at 20°C. At 10°C pre-overwintering tadpoles allocated more energy to development compared to spring tadpoles to advance development before winter. Overall, we demonstrate that the effects of temperature depend on phenology, consistent with future, expected thermal regimes. This suggests that extreme events can yield different vulnerability to climate change within populations (e.g. associated to discrete within-year cohorts), and not only between species or populations.

The ability to bear live offspring, viviparity, has evolved multiple times across the tree of life and is a remarkable adaptation with profound life-history and ecological implications. Within amphibians the ancestral reproductive mode is oviparity followed by a larval life stage, but viviparity has evolved independently in all three amphibian orders. Two types of viviparous reproduction can be distinguished in amphibians; larviparity and pueriparity. Larviparous amphibians deliver larvae into nearby ponds and streams, while pueriparous amphibians deliver fully developed juveniles and thus do not require waterbodies for reproduction. Among amphibians, the salamander genus Salamandra is remarkable as it exhibits both inter- and intraspecific variation in the occurrence of larviparity and pueriparity. While the evolutionary relationships among Salamandra lineages have been the focus of several recent studies, our understanding of how often and when transitions between modes occurred is still incomplete. Furthermore, in species with intraspecific variation, the reproductive mode of a given population can only be confirmed by direct observation of births and thus the prevalence of pueriparous populations is also incompletely documented. We used sequence capture to obtain 1,326 loci from 94 individuals from across the geographic range of the genus, focusing on potential reproductive mode transition zones. We also report additional direct observations of pueriparous births for 20 new locations and multiple lineages. We identify at least five independent transitions from the ancestral mode of larviparity to pueriparity among and within species, occurring at different evolutionary timescales ranging from the Pliocene to the Holocene. Four of these transitions occurred within species. Based on a distinct set of markers and analyses, we also confirm previous findings of introgression between species and the need for taxonomic revisions in the genus. We discuss the implications of our findings with respect to the evolution of this complex trait, and the potential of using five independent convergent transitions for further studies on the ecological context in which pueriparity evolves and the genetic architecture of this specialized reproductive mode., Financial support came from National Funds through FCT – Foundation for Science and Technology (SALOMICS: PTDC/BIA-EVL/28475/2017); the authors also acknowledge funding from other FCT projects and FEDER funds through the Operational Programme for Competitiveness Factors – COMPETE (EVOVIV: PTDC/BIA- EVF/3036/2012; FCOMP-01-0124-FEDER-028325 and UIDB/500027/2020); and by the office of the Associate Director of Science at the Smithsonian’s National Museum of Natural History. KPM was funded by an FCT predoctoral grant (PD/BD/52604/2014) and GVA by FCT research contracts (IF/01425/2014 and CEECIND/00937/2018), all from the Portuguese “Fundacão para a Ciência e a Tecnologia,” funded by Programa Operacional Potencial Humano (POPH)-Quadro de Referência Estratégica Nacional (QREN) from the European Social Fund.

Effective management of exploited populations is based on an understanding of population dynamics and evolutionary processes. In spatially structured populations, dispersal is a central process that ultimately can affect population growth and viability. It can be influenced by environmental conditions, individual phenotypes, and stochastic factors. However, we have a limited knowledge of the relative contribution of these components and its interactions, and which traits can be used as reliable predictors of the dispersal ability. Here, we conducted a longitudinal field experiment aimed to identify traits which can be used as proxy for dispersal in juvenile brown trout (Salmo trutta L.). We measured body size and standard metabolic rates, and estimated body shapes for 212 hatchery-reared juvenile fish that were marked with individual codes and released in a small coastal stream in northwest Spain. We registered fish positions and distances to the releasing point after 19, 41, 60 and 158 days in the stream. We detected a high autocorrelation of dispersal distances, demonstrating that most individuals settle down relatively soon and then hold stable positions over the study period. Body size and fish shape were reliable predictors of dispersal, with bigger and more robust-set individuals being more likely to settle closer to the release site than smaller and more elongated fish. In addition, the analysis of spacing and spatial patterns indicated that the dispersal of introduced fish could affect the distribution of resident conspecifics. All together, these results suggest that stocking programs aimed to the enhancement of overexploited populations at fine spatial scales can be optimized by adjusting the size and shape of the introduced fish to specific management targets and environmental conditions., This study was supported by grants CN-07-164 (Principality of Asturias), MEC-CGL2004-03239/BOS and MMA/86-2003-1 to A.G. Nicieza, and a FICYT PhD fellowship (BP04-147) to J.R. Sánchez-González.

Variation and population structure play key roles in the speciation process, but adaptive intraspecific genetic variation is commonly ignored when forecasting species niches. Amphibians serve as excellent models for testing how climate and local adaptations shape species distributions due to physiological and dispersal constraints and long generational times. In this study, we analysed the climatic factors driving the evolution of the genus Alytes at inter- and intraspecific levels that may limit realized niches. We tested for both differences among the five recognized species and among intraspecific clades for three of the species (Alytes obstetricans, A. cisternasii, and A. dickhilleni). We employed ecological niche models with an ordination approach to perform niche overlap analyses and test hypotheses of niche conservatism or divergence. Our results showed strong differences in the environmental variables affecting species climatic requirements. At the interspecific level, tests of equivalence and similarity revealed that sister species were non-identical in their environmental niches, although they neither were entirely dissimilar. This pattern was also consistent at the intraspecific level, with the exception of A. cisternasii, whose clades appeared to have experienced a lower degree of niche divergence than clades of the other species. In conclusion, our results support that Alytes toads, examined at both the intra- and interspecific levels, tend to occupy similar, if not identical, climatic environments.

Subdivided Pleistocene glacial refugia, best known as “refugia within refugia”, provided opportunities for diverging populations to evolve into incipient species and/or to hybridize and merge following range shifts tracking the climatic fluctuations, potentially promoting extensive cytonuclear discordances and “ghost” mtDNA lineages. Here, we tested which of these opposing evolutionary outcomes prevails in northern Iberian areas hosting multiple historical refugia of common frogs (Rana cf. temporaria), based on a genomic phylogeography approach (mtDNA barcoding and RAD-sequencing). We found evidence for both incipient speciation events and massive cytonuclear discordances. On the one hand, populations from northwestern Spain (Galicia and Asturias, assigned to the regional endemic R. parvipalmata), are deeply-diverged at mitochondrial and nuclear genomes (~4 My of independent evolution), and barely admix with northeastern populations (assigned to R. temporaria sensu stricto) across a narrow hybrid zone (~25 km) located in the Cantabrian Mountains, suggesting that they represent distinct species. On the other hand, the most divergent mtDNA clade, widespread in Cantabria and the Basque country, shares its nuclear genome with other R. temporaria s. s. lineages. Patterns of population expansions and isolation-by-distance among these populations are consistent with past mitochondrial capture and/or drift in generating and maintaining this ghost mitochondrial lineage. This remarkable case study emphasizes the complex evolutionary history that shaped the present genetic diversity of refugial populations, and stresses the need to revisit their phylogeography by genomic approaches, in order to make informed taxonomic inferences., We thank C. Cabido, I. Garin, A. Gosá, F. Martínez, J. Rubines, X. Rubio, and G. Sánchez‐Montes for help in sample collection. This study was funded by the Swiss National Science Foundation (fellowship P2LAP3_171818 to CD, and grant 31003A_166323 to NP). MV was supported by the Deutsche Forschungsgemeinschaft (grant VE247/16‐1–HO 3492/6‐1) in the framework of the ‘TaxonOmics' priority program, SNL by the RFBR (grant 20‐04‐00918), and AGN by the Ministerio de Ciencia, Innovación y Universidades, Gobierno de España (MICINN grants CGL2012‐40246 and CGL2017‐86924‐P).

The reduction in fecundity associated with the evolution of viviparity may have far-reaching implications for the ecology, demography, and evolution of populations. The evolution of a polygamous behaviour (e.g. polyandry) may counteract some of the effects underlying a lower fecundity, such as the reduction in genetic diversity. Comparing patterns of multiple paternity between reproductive modes allows us to understand how viviparity accounts for the trade-off between offspring quality and quantity. We analysed genetic patterns of paternity and offspring genetic diversity across 42 families from two modes of viviparity in a reproductive polymorphic species, Salamandra salamandra. This species shows an ancestral (larviparity: large clutches of free aquatic larvae), and a derived reproductive mode (pueriparity: smaller clutches of larger terrestrial juveniles). Our results confirm the existence of multiple paternity in pueriparous salamanders. Furthermore, we show the evolution of pueriparity maintains, and even increases, the occurrence of multiple paternity and the number of sires compared to larviparity, though we did not find a clear effect on genetic diversity. High incidence of multiple paternity in pueriparous populations might arise as a mechanism to avoid fertilization failures and to ensure reproductive success, and thus has important implications in highly isolated populations with small broods.

Identifying the evolutionary processes that underlie morphological variation at the intraspecific level is cornerstone for understanding the drivers of phenotypic diversity at higher macro-evolutionary scales. The fire salamander, Salamandra salamandra, exhibits exceptional intraspecific variation in multiple phenotypic traits (i.e. body size, head shape, coloration, reproductive modes). Pueriparity (females laying fully metamorphosed juveniles) in S. salamandra entails modifications during embryonic development in comparison to larviparity (females laying aquatic larvae), which is the ancestral reproductive mode. These heterochronic modifications involve a general acceleration of development and mainly focus on cephalic structures to facilitate intrauterine active feeding, which might impact head shape in the adult stage. In the present study, we (i) describe the main features of head shape variation in adults of the two distinct subspecies of Salamandra salamandra that independently evolved to pueriparity, and (ii) explore the morphological consequences of developmental and functional changes related to this major evolutionary shift. Our results show that evolutionary history, and not reproductive mode, is the main driver of head shape variation in S. salamandra. These results suggest different evolutionary processes acting differentially on each subspecies, at least at the adult stage. The present study highlights the importance of comparative studies integrating evolutionary histories and ontogenetic trajectories to explore the different sources of observed morphological diversification., This work is funded by the Ministerio de Economía y Competitividad (Grants Nos. CGL2012-40246-C02-02 and CGL2017-86924-P); by FEDER funds through the Operational Programme for Competitiveness Factors – COMPETE (FCOMP-01-0124-FEDER-028325 and POCI-01- 0145-FEDER-006821); and by National Funds through FCT – Foundation for Science and Technology (EVOVIV: PTDC/BIA-EVF/3036/2012; SALOMICS: PTDC/BIA-EVL/28475/2017). L.A.-R. was supported by a FPU grant (FPU14/03015) from the Ministerio de Educación, Cultura y Deporte (MECD, Spain) and and G.V.-A. and A.K. by IF contracts (IF/01425/2014 and IF/00641/2014, respectively). from Fundação para a Ciência e a Tecnologia (FCT, Portugal).

Adaptation to warming climates could counteract the effects of global warming. Thus, understanding how species cope with contrasting climates may inform us about their potential for thermal adaptation and which processes may hamper that ability (e.g. evolutionary trade-offs, phenology or behavioural thermoregulation). In addition to temperature, time constraints may also exert important selective pressures. Here, we compare the thermal sensitivity of locomotion of metamorphic and adult European common frogs (Rana temporaria) originating from populations along an elevational gradient. We employed the template mode of variation (TMV) analysis to decompose the thermal sensitivity of locomotion and explore the existence of trade-offs ('hotter is better' and 'specialist-generalist') and the degree of local adaptation. To that end, we studied the relationship between TMV parameters and local environmental conditions. Further, we compared preferred temperatures to assess whether behavioural thermoregulation could dampen the effects of thermal variation, reducing the intensity of selection and limiting thermal adaptation (i.e. 'Bogert effect'). We suggest that behavioural thermoregulation has promoted the conservatism of thermal sensitivity in R. temporaria. Yet, we observed a trend towards narrower thermal niches shifted towards warmer temperature in populations with severe temporal constraints, conforming to the 'generalist-specialist' trade-off. Apparently, this enables time-constrained populations - especially in the case of metamorphs - to effectively exploit resources during the warmest periods. The limited potential of R. temporaria for thermal adaptation suggests that forecasts of global warming should incorporate thermoregulation and explore its potential to buffer species from rising temperatures.

The vulnerability of species to climate change is jointly influenced by geographic phenotypic variation, acclimation and behavioural thermoregulation. The importance of interactions between these factors, however, remains poorly understood. We demonstrate how advances in mechanistic niche modelling can be used to integrate and assess the influence of these sources of uncertainty in forecasts of climate change impacts. We explored geographic variation in thermal tolerance (i.e. maximum and minimum thermal limits) and its potential for acclimation in juvenile European common frogs Rana temporaria along elevational gradients. Furthermore, we employed a mechanistic niche model (NicheMapR) to assess the relative contributions of phenotypic variation, acclimation and thermoregulation in determining the impacts of climate change on thermal safety margins and activity windows. Our analyses revealed that high-elevation populations had slightly wider tolerance ranges driven by increases in heat tolerance but lower potential for acclimation. Plausibly, wider thermal fluctuations at high elevations favour more tolerant but less plastic phenotypes, thus reducing the risk of encountering stressful temperatures during unpredictable extreme events. Biophysical models of thermal exposure indicated that observed phenotypic and plastic differences provide limited protection from changing climates. Indeed, the risk of reaching body temperatures beyond the species' thermal tolerance range was similar across elevations. In contrast, the ability to seek cooler retreat sites through behavioural adjustments played an essential role in buffering populations from thermal extremes predicted under climate change. Predicted climate change also altered current activity windows, but high-elevation populations were predicted to remain more temporally constrained than lowland populations. Our results demonstrate that elevational variation in thermal tolerances and acclimation capacity might be insufficient to buffer temperate amphibians from predicted climate change; instead, behavioural thermoregulation may be the only effective mechanism to avoid thermal stress under future climates.

Phenotypic plasticity can be viewed as the first level of defense of organism homeostasis against environmental stress and therefore represents the potential to deal with rapid environmental changes. Transitions between low complexity, artificial environments and complex, natural habitats can promote phenotypic plasticity. Here, we conducted an experimental introduction with juvenile brown trout to evaluate the plasticity of shape in response to a transition between contrasting environments. We released 202 juvenile trout reared under hatchery conditions in a natural stream and analyzed changes in shape and morphological variability after 5 months. A geometric morphometrics approach based on 14 landmarks was used to compare changes in body shape for 37 fish recaptured at the end of the experiment. A similar number of hatchery and wild fish caught at the receptor stream were used as controls for shape in the two environments. After 5‐months, fish showed significant change in shape, shifting from elongated to robust shapes, and affecting to the relative position of the caudal peduncle. These new shapes were closer to wild than to the hatchery shapes, suggesting a process of rapid phenotype change. Moreover, these changes were concomitant with a marked increase in morphological variability. Our results support the hypothesis that phenotypic plasticity is a major potential for adjustment to environmental change but not the idea that shape can be constrained by initial shapes. We confirmed the “increased” variance hypothesis and phenotype convergence with wild morphs. This has important implications because stresses the role of phenotypic plasticity as a buffer that allows organisms to cope with important environmental discontinuities at time scales that preclude the onset of adaptive adjustments. We suggest that environmental conditioning and shape plasticity can overcome both reduced morphological diversity and phenotype uncoupling with habitat characteristics resulting from initial rearing in low complexity artificial environments.

The existence of two or more distinctly coloured phenotypes among individuals of an interbreeding population is known as colour polymorphism. In amphibians, this phenomenon is pervasive among anurans, but rare or absent among salamanders and caecilians, respectively. Here, we examine whether various distinct phenotypes of Salamandra salamandra in North Spain, used as a basis to describe the subspecies S. s. bernardezi and S. s. alfredschmidti, indeed warrant separate taxonomic status or that these co-occur and belong to a single taxon. Based on a sample of 1147 individuals from 27 local populations, six phenotype classes were designated. Although two phenotypes that are attributable to S. s. alfredschmidti show some degree of geographical restriction, these co-occur with those representing typical S. s. bernardezi. A fifth phenotype class could not be unambiguously attributed to either subspecies due to an overlap in previously suggested diagnostic characteristics. Mitochondrial (cytochrome b) and nuclear (β-fibrinogen) DNA analyses revealed S. s. alfredschmidti to be nested within several subclades of S. s. bernardezi, without displaying unique lineages. Furthermore, no significant divergence was recovered by means of niche overlap analyses. As a result, we revoke the subspecies status of S. s. alfredschmidti, which should be regarded as a junior synonym of S. s. bernardezi. The current findings confirm the existence of colour polymorphism in S. salamandra and the family Salamandridae, which provides exciting possibilities for future research.

© 2015, Springer-Verlag Berlin Heidelberg. In organisms such as fish, where body size is considered an important state variable for the study of their population dynamics, size-specific growth and survival rates can be influenced by local variation in both biotic and abiotic factors, but few studies have evaluated the complex relationships between environmental variability and size-dependent processes. We analysed a 6-year capture–recapture dataset of brown trout (Salmo trutta) collected at 3 neighbouring but heterogeneous mountain streams in northern Spain with the aim of investigating the factors shaping the dynamics of local populations. The influence of body size and water temperature on survival and individual growth was assessed under a multi-state modelling framework, an extension of classical capture–recapture models that considers the state (i.e. body size) of the individual in each capture occasion and allows us to obtain state-specific demographic rates and link them to continuous environmental variables. Individual survival and growth patterns varied over space and time, and evidence of size-dependent survival was found in all but the smallest stream. At this stream, the probability of reaching larger sizes was lower compared to the other wider and deeper streams. Water temperature variables performed better in the modelling of the highest-altitude population, explaining over a 99 % of the variability in maturation transitions and survival of large fish. The relationships between body size, temperature and fitness components found in this study highlight the utility of multi-state approaches to investigate small-scale demographic processes in heterogeneous environments, and to provide reliable ecological knowledge for management purposes., Funds were provided by grants FAIR CT95-0009 to F. Braña and FAIR CT97-3498 and MCYT-AGL2000-3181-CE to A. G. N.; A. F.-C. was supported by a FPU grant from the Spanish Ministry of Education (ref. AP2008-04476)

This study probes the underlying locomotion principles of earliest organisms that could both swim and walk. We hypothesize that properly coordinated leg and body movements could have provided a substantial benefit toward locomotion on complex media, such as early crawling on sand. In this extended abstract, we summarize some of our recent advances in integrating biology, physics and robotics to gain insight into tetrapod locomotor coordination and control principles. Here, we observe crawling salamanders as a biological model for studying tetrapod locomotion on sloped granular substrates. Further, geometric mechanics tools are used to provide a theoretical framework predicting efficacious body motions on yielding terrain. Finally, we employ these coordination strategies on a robophysical salamander model traversing a sandy slope. This analysis of salamander-like robotic motion in granular media can be seen as a first application of how tools from geometric mechanics can provide insight into the character and principles of legged locomotion.

In species with simultaneous polyandry, male-biased operational sex ratio is expected to increase the risk of sperm competition and thus sperm traits affecting siring success can differ among populations. Here, we test the hypothesis that high male–female ratios will enhance sperm competitiveness of Rana temporaria males. In this species, local populations can show either prolonged or explosive breeding. In a context of sperm competition and in controlled laboratory conditions, prolonged-breeding males sired a higher proportion of eggs than explosive-breeding males, regardless of female origin. This study demonstrates intrapopulation variation in siring success under a situation of sperm competition, consistent with the prolonged-explosive dichotomy of breeding strategies.

[Aim] The climate variability hypothesis (CVH) states that a positive relationship may exist between the breadth of thermal tolerance range and the level of climatic variability experienced by taxa with increasing latitude, especially in terrestrial ectotherms. Under CVH, we expected to find a correspondence between both thermal tolerance limits (CTmax and CTmin), ambient extreme temperature and the range sizes of species. We examined the validity of these predictions in a lowland tropical and a temperate tadpole assemblage., [Location] Lowland Neotropics (Bahia, Brazil) and Palaearctic (Iberian Peninsula and North Africa)., [Method] We employed phylogenetic eigenvector regression (PVR) and Pagel's lambda to analyse phylogenetic signals in CTmax and CTmin. We used phylogenetic regression analyses (PGLS) to test the relationships between thermal limits, range size and temperature predictors (measured at the macroscale and microhabitat levels) and phy-ANOVA to compare both the physiological traits and thermal regimen in both tropical and temperate assemblages., [Results] We documented moderate-to-strong phylogenetic signal in both heat and cold tolerance. Temperate-zone tadpoles had broader thermal tolerances than tropical ones. Thermal tolerance range was correlated with range sizes and was explained by seasonal thermal range predictors at the global scale. Both macro- and microclimate temperature variables provided the best predictive multivariate models of thermal limits at the global scale. Microclimatic predictors, however, were the main determinants of CTmax and CTmin variation at the local level of tropical and temperate communities respectively., [Main conclusions] Thermal tolerance range increases with latitude in tadpoles due to the higher increase in cold tolerance in temperate tadpoles. At the global scale, both macro- and microenvironment thermal information were reliable predictors of critical thermal limits and thermal tolerance range, as CVH predicts. However, thermal limits were best predicted by temperatures of the micro-habitat at the regional level, thus suggesting that physiological thermal boundaries may be governed by thermal selection., The CMAOT of JJAA granted the permits for conducting the experiments (Ref. 12_44‐Tejedo). This study was supported by the Spanish MICINN grant (CGL2009‐12767‐C02‐02) and MINECO award (CGL2012‐40246‐C02‐01) to M. Tejedo and a grant from the Ministerio de Ciencia e Innovación (BES‐2010‐032912) to L.M. Gutiérrez‐Pesquera.

Seasonal time constraints can pose strong selection on life histories. Time-constrained animals should prioritise fast development over predation risk to avoid unfavourable growing conditions. However, changes in phenology could alter the balance between anti-predator and developmental needs. We studied variation of anti-predator strategies in common frog (Rana temporaria) tadpoles in four populations from the two extremes of a latitudinal gradient across Sweden. We examined, under common conditions in the laboratory, the anti-predator responses and life histories of tadpoles raised with predatory Aeshna dragonfly larvae in two consecutive years with a difference of 20 days in breeding time in the north, but no difference in breeding time in the nouth. In a year with late breeding, northern tadpoles did not modify their behaviour and morphology in the presence of predators, and metamorphosed faster and smaller than tadpoles born in a year with early breeding. In the year with early breeding, northern tadpoles showed a completely different anti-predator strategy by reducing activity and developing morphological defences in the presence of predators. We discuss the possible mechanisms that could activate these responses (likely a form of environmentally-mediated parental effect). To our knowledge, this is the first study to show that a vertebrate modifies the anti-predator strategy of its offspring in response to natural variation in reproductive phenology, which highlights the need to consider phenology in studies of life-history evolution.

1. As size is tightly associated with fitness, compensatory strategies for growth loss can be vital for restoring individual fitness. However, immediate and delayed costs of compensatory responses may prevent their generalization, and the optimal strategy may depend on environmental conditions. Compensatory responses may be particularly important in high-latitude habitats with short growing seasons, and thus, high-latitude organisms might be more efficient at compensating after periods of unfavourable growth conditions than low-latitude organisms. 2. We investigated geographical differences in catch-up growth strategies of populations of the common frog (Rana temporaria) from southern and northern Sweden in two factorial common garden experiments involving predation risk and two different causes of growth arrest (nutritional stress and low temperatures) to evaluate how the compensatory strategies can be affected by context-dependent costs of compensation. Larval and metamorphic traits, and post-metamorphic performance were used as response variables. 3. Only northern tadpoles exposed to low food completely caught up in terms of metamorphic size, mainly by extending the larval period. Low food decreased survival and post-metamorphic jumping performance in southern, but not in northern tadpoles, suggesting that northern tadpoles have a better ability to compensate after periods of restricted food. 4. Both northern and southern tadpoles were able to metamorphose at the same size as control tadpoles after being exposed to low temperatures, indicating that consequences of variation in temperature and food availability differed for tadpoles. However, the combination of low temperatures and predation risk reduced survival in both southern and northern tadpoles. Also, predation risk decreased energy storage in both experiments. 5. Our results highlight the influence of climatic variation and the type of stressor as selective factors shaping compensatory strategies.

Species inhabiting habitats with different predators are expected to show divergent phenotypes for antipredator traits. Here, we used a predator–prey system of dragonfly larvae and tadpoles to determine if vulnerability to a common predator differs in species with contrasting antipredator strategies. We examined the vulnerability of tadpoles of Rana temporaria and Bufo bufo to predation by Aeshna larvae when the two species co-occur in the same arena. Our results demonstrated that tadpoles of Bufo were more vulnerable than tadpoles of Rana despite the observation that dragonfly larvae did not show initial preferences for either prey species. Differences in susceptibility to predation seem to be associated with their low performance in evasive responses. Most important, our data suggest that despite chemical protection that effectively prevented the consumption of B. bufo by Aeshna larvae, injured tadpoles that otherwise had survived are at a high risk of being cannibalized. This loss of survival ...

The timing of many life history events shows phenotypic plasticity in response to the risk of predation. Theory predicts that increased risk of mortality in an early stage should select for switching earlier, while a higher risk after the transition should select for switching later. Here we examined the effects of stage-specific predation risk on the timing of hatching of Ranatemporaria. Embryos were exposed to chemical cues from either an egg predator (Haemopissanguisuda) or a tadpole predator (Aeshna cyanea) to evaluate three specific hypotheses: (1) a fixed intermediate response, (2) a ‘fixed predator’ response (i.e., either anticipation or delay), and (3) a specific predator response (both anticipation and delay). Rana temporaria embryos did not discern between pre- and post-hatching specific predators, and they hatched prematurely regardless predator type. These results suggest that R. temporaria embryos respond to predation risk in a fixed way by hatching early, and that they use cues stemming from injured conspecifics, which provides a simple, conservative mechanism of risk assessment. In conclusion, our data are not anticipated by the theoretical consideration that organisms should spend less time in more dangerous environments, but they confirm an invariable adjustment of hatching time in response to an inscrutable predation risk (response to a fixed-predator) in connection with a consistent mechanism mediating the perception of predation risk.

In species with complex life cycles hatching plasticity can provide an effective escape from egg predators, but theoretical studies predict a predation-risk trade-off across egg and larval stages. In this study, we examine whether the presence of an egg predator can alter the timing of hatching in an anuran, Rana temporaria, and the consequences of hatching plasticity after transition to the terrestrial habitat. Predator cues induced earlier hatching, and hatchlings were smaller, less developed and had relatively shorter and deeper tails than control hatchlings. The predator–induced differences in developmental time were compensated throughout the larval period; there was no predator effect on metamorph age or size. Surprisingly, the effects of egg predators were perceptible after metamorphosis. Juveniles emerging from the predator and the no-predator treatments differed in several size-adjusted morphological dimensions. Seemingly these morphological differences were not large enough to give rise to suboptimal growth or locomotor performance after metamorphosis. Thus, our results suggest only a short-term effect on juvenile phenotype, but not a trade-off between hatching time and juvenile performance.

We investigated the role of constitutive morphology and previous experience in predator avoidance in two anuran species associated with different larval habitats. In Rana temporaria, deeper tails and larger body size conferred selective advantage against drag- onfly predation. Previous experience with predators had a positive influence on the survival of R. temporaria tadpoles equivalent to predator selection. By contrast, survival in Bufo bufo seems unrelated to tail shape or experience. This suggests that B. bufo lacks con- stitutive morphological defenses against insect predators, and that morphological and behavioral defenses could result more effective than chemical deterrents for these insect predators. A key novelty of this study is the observation that Rana tadpoles having prior experience with predators have an enhanced success in further encounters, and this occurs before the morphological induced defense has been established. This induced modification for R. temporaria, and its lack of for B. bufo, may be an important determinant of larval survival.

We examined the influence of habitat size, growth opportunity, and the thermal conditions experienced during early development on the standard metabolic rate (SMR) of juvenile brown trout (Salmo trutta) from six natural populations to contrast the hypothesis of countergradient selection in metabolic rate. The study populations differed significantly in SMR. Population means for SMR changed in response to the temperature experienced during the yolk-absorption stage, when the risk of oxygen deficit increases and the vulnerability to hypoxia is highest. We also found a strong negative correlation between the temperature experienced during the first 2 months after yolk resorption and SMR, which supports the hypothesis of countergradient variation. Moreover, we detected a strong negative correlation between an index of growth opportunity and relative lipid content, suggesting that the risk of energy shortfall could be a major force in the evolution of storage strategies. Our results suggest that temperature can shape the evolution of metabolic rate during the yolk-absorptive stage or the first feeding stage, while energy storage levels may be more sensitive to thermal constraints acting on growth rates.

Summary 1Adaptation to cold environments of ectotherm populations is expected to result in increased standard metabolic rates if resources are not limiting. However, it is still unclear how the maintenance of high standard metabolic rates would affect locomotor performance and its impact at the population level. 2We compared standard metabolic rate and swimming performance of Brown Trout populations inhabiting rivers with contrasting thermal conditions. To avoid environmental confounding effects, we tested offspring raised in controlled conditions. There were significant differences among populations and families for both traits, suggesting that the observed differentiation pattern has a genetic basis. 3We found a significant negative relationship between standard metabolic rate and swimming performance at the population level. On the other hand, standard metabolic rates tended to be negatively related to the temperature in the rivers of origin, whereas swimming performance was positively related to river temperature, but these trends were not significant. 4These results suggest that selection for traits positively linked with standard metabolic rates may result in lower relative prolonged swimming performance. The potential evolutionary consequences of the observed negative relationship are discussed.

In salmonids, there seems to be a positive correlation between standard metabolic rate and growth rate under artificial rearing conditions. Several recent studies have suggested that phenotypic correlations between physiological or behavioural traits and developmental or life history responses might be common when assayed in low-complexity habitats but rare in those with a high degree of spatiotemporal complexity. This study provides the first test of the connection between metabolic and growth rates of juvenile brown trout (Salmo trutta) in natural streams. In two out of four streams, there was no relationship between metabolic rates and subsequent growth, whereas in the two others, growth and metabolic rates were negatively correlated. Furthermore, survival rates were either unaffected or negatively correlated with metabolic rates. These results reveal complex relationships between metabolic rate, growth, and environmental variability and suggest that (i) in the wild, negative selection on high metabolic rates may result from both juvenile mortality and reduced growth rates, (ii) the conclusions derived from laboratory experiments are not directly applicable to natural populations, and (iii) the correlations between metabolic rate and growth can prove useful after selection of the appropriate spatial and temporal scales.

Juvenile brown trout Salmo trutta from natural populations reacted to the presence of piscivorous brown trout by increasing the use of refuges. In contrast, second-generation hatchery fish and the offspring of wild fish raised under hatchery conditions were insensitive to predation risk. The diel pattern of activity also differed between wild and hatchery brown trout. Second-generation hatchery fish were predominantly active during daytime regardless of risk levels. Wild fish, however, showed a shift towards nocturnal activity in the presence of predators. These findings emphasize the potential role of domestication in weakening behavioural defences. They support the idea that the behavioural divergence between wild and domesticated individuals can arise from a process of direct or indirect selection on reduced responsiveness to predation risk, or as a lack of previous experience with predators.

Summary 1 Density can influence growth through exploitation competition and behavioural or chemical interference. Negative effects of population density on the growth rate of Sea Lamprey larvae have been reported in several field and laboratory studies, but the mechanisms involved are poorly understood. 2 In this study the role of population density and waterborne-mediated interference on the growth rate of Sea Lamprey larvae was assessed in two laboratory experiments. The effects of these factors were evaluated by comparing growth of larvae reared at three different densities (27, 75 and 128 individuals m−2) or in preconditioned water (i.e. containing Sea Lamprey larvae at the same three densities). 3 Larval density had a negative effect on growth rates. Water conditioning had a negative but weaker effect on growth. Larvae reared in water preconditioned at the two higher densities showed a lower mass increase than those growing in water preconditioned at the lowest density. 4 The results are consistent with the existence of some chemical or biological agent released into the surrounding water and influencing growth in larval Sea Lampreys. However, chemical interference is not the only factor influencing density-dependent growth.

Ali, M., Nicieza, A., Wootton, R. J. (2003). Compensatory growth in fishes: a response to growth depression. Fish and Fisheries, 4, (2), 147-190. Sponsorship: Royal Society of London – Chinese Academy of Sciences Exchange award

Summary 1. Anurans exhibit high levels of growth-mediated phenotypic plasticity in age and size at metamorphosis. Although temperature and food quality exert a strong influence on larval growth, little is known about the interacting effects of these factors on age and size at metamorphosis. 2. Plasticity in growth rates, maximum larval mass, mass loss, larval period and size at metamorphosis was examined in Iberian Painted Frogs ( Discoglossus galganoi Capula, Nascetti, Lanza, Bullini & Crespo 1985) under different combinations of temperature and diet quality. 3. Temperature and diet had strong effects on the maximum size reached by tadpoles throughout the premetamorphic stages. Larval body mass varied inversely with temperature. The effect of diet depended on temperature; larvae fed on a ‘carnivorous’ diet (rich in protein and lipids) achieved a larger size than larvae offered an ‘herbivorous’ diet (rich in carbohydrates) at 17 ° C but not at 12 or 22 ° C. 4. Larval period was insensitive to diet composition, and varied only with temperature. Primarily the interacting effects of food quality and temperature affected size at metamorphosis. Size at metamorphosis varied inversely with temperature under the plant- and the animal-based diets. However, the carnivorous diet resulted in bigger metamorphs at 17 and 22 ° C, but did not influence final mass at 12 ° C. Maximum size over the larval period explained most of the variation in mass loss after the premetamorphic growing phase.

This study assesses the influence of thermal regime on the development, survival rates and early growth of embryos of sea lamprey Petromyzon marinus incubated at five constant temperatures (7, 11, 15, 19 and 23° C). The time from fertilization to 50% hatching and from hatching to 50% burrowing were inversely related to incubation temperature. All the embryos incubated at 7° C died at very early stages, while those maintained at 11° C did not attain the burrowing stage. Survival from fertilization to hatching was 61, 89, 91 and 89% at 11, 15, 19 and 23° C, decreasing to 58, 70 and 70% from hatching to burrowing at 15, 19 and 23° C, respectively. Larvae reared during the first 3 months of exogenous feeding in a common environment at constant 21° C, revealed maximum survival for an incubation temperature of 15° C (43% of burrowed larvae) decreasing strongly at 19° C (16%) and 23° C (one suvivor among 240 larvae). Body length at the burrowing stage was maximum for embryos incubated at 19° C, but body mass increased in the interval 15–23° C. Mean incubation temperatures experienced by 117 broods during the embryonic development in the source river were estimated in 15·3±2·30° C and 16·7±1·76° C (mean±1 s.d.) for the periods fertilization-to-hatching and hatching-to burrowing, respectively.

Age and size at metamorphosis are two important fitness components in species with complex life cycles. In anurans, metamorphic traits show remarkable phenotypic plasticity, especially in response to changes in growth conditions. It is also possible that the perception of risk directly determines changes in larval period and the size of metamorphs. This study examines how the perception of predation risk affects the timing of and size at metamorphosis in common frogs (Rana temporaria). I raised tadpoles at two risk levels (fish-conditioned water or unconditioned water) crossed with the availability or lack of food at night (all tadpoles had food available in the day). Tadpoles reacted to chemical cues from predatory fish by decreasing activity. A novel behavioural result was a predation×food interaction effect on refuge use, which also accounted for most of the predator main effect: predation risk only caused increased refuge use in the night-starved treatment. Despite these behavioural modifications, the perception of predation risk did not affect growth rate and mass at metamorphosis in a simple way: the effects of food regime on growth and size at metamorphosis were dependent on the level of predation risk as revealed by significant predation×food interaction effects. Tadpoles who had food withheld at night metamorphosed at the smallest size, suggesting a negative relationship between size at metamorphosis and refuge use. Tadpoles raised in fish-conditioned water had longer larval periods than those in unconditioned water, but these differences were significant only if food was available at night. These results conflict with the hypotheses that tadpoles should reduce their larval period or growth rates (and hence metamorphose at a smaller size) as the risk of predation increases. In contrast to predation risk, food availability strongly affected the length of the larval period: night-starved tadpoles metamorphosed relatively early with or without fish stimulus. Thus, early metamorphosis resulted from periods of low food availability, but not from a heightened "perceived risk" of predation. This example counters the hypothesis of acceleration of the developmental rate (which shortens the time to metamorphosis) as a mechanism to escape a risky environment.

1. This study examines the aggregation behaviour and activity of larvae of the Common Frog Rana temporaria in relation to the early social environment, ontogeny and the presence of chemical cues from a predatory fish to address three main questions: (i) Does previous social interaction influence aggregation behaviour in later developmental stages? (ii) To what extent does aggregation behaviour depend upon the risk level perceived by the individual? (iii) Does aggregation behaviour change through ontogeny? 2. Tadpoles were reared from eggs either in small groups or in complete isolation. In relatively early stages of development, tadpoles showed no preference for the side of the test container containing siblings over the side containing no larvae regardless of the social context experienced (isolation or contact with siblings). 3. The presence of chemical cues from a potential predator did not trigger the aggregation behaviour of tadpoles, but it had a remarkable effect on their activity and spatial distribution. Tadpoles exposed to water preconditioned by a predator spent significantly less time swimming and avoided the central area of the test container more frequently than did controls exposed to unconditioned water. 4. Tadpoles were more active, avoided the central section and associated preferentially with conspecifics (siblings) at later stages of development regardless of the social conditioning they had previously experienced. 5. Individuals reared in groups were twice as active as individuals reared in isolation. This suggests that the early social environment experienced by larvae can influence future behaviour and thereby growth and development rates. 6. The expression of conspecific attraction is probably linked to an ontogenetic shift in larval behaviour. However, reduced activity, rather than aggregation, appears to be the basic antipredator mechanism in larval Common Frog.

1. Agonistic behaviour can be an important trait in determining individual success under interference competition, but it also generates energetic and physiological costs. The ensuing trade-off is expected to be dependent on the individual state. This study tests whether aggression patterns of juvenile Atlantic Salmon Salmo salar are linked to the recent growth history and the developmental pathway adopted by fish (early vs late migrants). 2. By manipulating growth rates over 1 month in the autumn, three groups of fish were produced differing in body size and thereby in the length increment necessary to achieve the threshold size for migration in the following spring. Fish that had experienced reduced rations or abnormally low temperatures during the manipulation period experienced ‘catch-up’ growth after the resumption of standard growth conditions. 3. Aggression was strongly connected to growth bimodality and the underlying developmental polyphenism. Fast-growing (upper modal group, UMG) fish (which would migrate in the spring) were more aggressive than slow-growing (lower modal group, LMG) fish (which would delay migration for another year), but they were also more vulnerable to being attacked by conspecifics. 4. Aggressive acts initiated by UMG fish were strongly biased towards fish of the same category (57–77% of aggressive acts being against other UMG fish). Under illumination conditions simulating overcast nights, UMG individuals sharply reduced their aggressiveness and were not selective in their attacks. The aggression by LMG fish was more evenly distributed between modal groups (44–74% of attacks were against UMG) regardless of light levels. 5. Recent growth history had no consistent effect on aggression rates, although under overcast night conditions the rate of attacks received by non-manipulated UMG fish was twice as high as that for fish undergoing compensatory growth. 6. These results demonstrate a link between developmental pathways and aggression patterns. They also show that individuals maintaining a fast growth strategy over the winter experience a more aggressive social environment than individuals adjusting feeding rates to ensure safe maintenance levels. The reduced involvement of LMG fish in agonistic encounters conforms to a strategy of avoidance of those energetic and physiological costs that ultimately could compromise overwintering survival.

Both in foraging groups and in a sequential prey encounter context, learning had a visible eVect on the pattern of selection for three live prey types (Ecdyonurus larvae, Hydropsyche larvae, and Gammarus) by juvenile Atlantic salmon Salmo salar. Compared to wild-caught fish, naive, hatchery-reared fish that had not been exposed to natural prey ate Hydropsyche larvae in a remarkably low proportion, and consumed a higher proportion of Gammarus. Ecdyonurus experienced a high and rather steady predation rate across the experience gradient, but after a short period of experience with live prey the consumption rate for Hydropsyche increased drastically, and that of Gammarus decreased, matching the selection pattern exhibited by wild fish. Individual fish oVered prey in a sequential encounter context increased consumption rates of all the prey types as they gained experience, but the improvement was higher for the prey that were less consumed initially. Fish became more selective as they approached satiation, conforming to the prediction of optimal foraging theory that higher predator’s energy requirements, as well as low food availability, result in reduced selectivity. The results also suggest that fish from distinct populations can diVer in the degree of diet selectivity according to their energetic requirements for growth. The fast learning response of Atlantic salmon parr towards novel prey probably allows fish to maintain a high foraging eYciency when faced with frequent changes in the availability of diVerent prey types. ? 1998 The Fisheries Society of the British Isles

This study examines behavioral and physiological responses of juvenile At- lantic salmon (Salmo salar) adopting alternative life history patterns following a period of reduced growth. We manipulated the growth rates of premigratory and nonmigratory salmon by either reducing food availability or maintaining water at low temperature (4-60C). A third group of fish was kept at ambient temperatures (12-14'C) and fed ad libitum to provide a control. Fish in both experimental groups exhibited compensatory growth after the manipulation period, even though the manipulations had slowed growth rather than caused mass loss. The timing and duration of compensatory growth were affected by the nature of the constraint and the developmental pathway adopted. Compensatory responses were more persistent and stronger among premigratory fish than among nonmigratory. Fish kept at low temperature did not accelerate growth immediately after transfer to ambient temperatures, but they subsequently grew faster than controls for up to 215 d after the end of the manipulation period. This mitigated the effects of the period of low temperatures, although by the end of the experiment they were still smaller than the controls. Fish on reduced rations showed no such time lag, and they grew significantly faster than controls immediately upon regaining access to full rations. These fish attained the same body size as controls by the end of the experiment (day 215). The manipulations caused fish to reduce their growth in mass more than their rate of skeletal growth, but all fish achieved "normal" mass for their length (as compared to controls) within a week of transfer to full feeding or ambient temperature. The main mechanism underlying compensatory growth rates was apparently the increase of intake rates, although this was insufficient to explain the strong compensation shown by temperature-manipulated fish in the presence of larger (and thus competitively superior) individuals. Instead these fish enhanced their growth rate by ap- parently increasing the duration of the daily feeding period, and avoiding aggressive in- teractions. We interpret the observed compensation for periods of slowed growth as indi- cating that growth rate is normally submaximal and can be increased if the animal has fallen below its expected trajectory; thus premigratory fish may have shown a greater compensation because survival rates during migration are strongly size-dependent.

The effects of light level, developmental pathway, and previous growth history on the foraging attack distances of juvenile Atlantic salmon Salmo salar were examined in circular rearing tanks. Former manipulation of growth rates had no significant influence on distances moved to intercept food items despite the fact that it caused substantial differences in post-treatment growth. Attack distances of fish that were entering a state of overwinter dormancy (lower modal group; LMG) were shorter than those of actively feeding (and growing) fish (upper modal group; UMG). These differences were explained generally by differences in body size between the two groups, suggesting that actual effort per attack was unassociated with growth requirements. Significant differences between growing and non-growing fish in attack distances could contribute to the variation in growth rates through their effect on feeding rates, but were unlikely to have affected energetic costs. This may be due to the fact that attack distances were consistently short throughout the study period as is also evident from the pattern of change between night and daytime. Whereas in the first experiment (daylight v. twilight) fish moved further to reach food during the day, in the second (daylight v. overcast night) nocturnal attack distances matched (LMG fish) or exceeded (UMG fish) diurnal attack distances. Thus diurnal attack distances were probably minimized in the second experiment. These results are interpreted within a framework of overwintering strategies.