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Mothers who produce multiple offspring within one reproductive attempt often allocate resources differentially; some maternally derived substances are preferentially allocated to last-produced offspring and others to first-produced offspring. The combined effect of these different allocation regimes on the overall fitness of offspring produced early or late in the sequence is not well understood, partly because production order is often coupled with birth order, making it difficult-to-separate effects of pre-natal maternal allocation from those of post-natal social environments. In addition, very little is known about the influence of laying order on fitness in later life. In this study, we used a semi-natural captive colony of black-headed gulls to test whether an offspring's position in the laying order affected its early-life survival and later-life reproductive success, independent of its hatching order. Later-laid eggs were less likely to hatch, but among those that did, survival to adulthood was greater than that of first-laid eggs. In adulthood, the laying order of females did not affect their likelihood of breeding in the colony, but male offspring hatched from last-laid eggs were significantly less likely to gain a breeding position than earlier-laid males. In contrast, later-laid female parents hatched lower proportions of their clutches than first-laid females, but hatching success was unrelated to the laying order of male parents. Our results indicate that gull mothers induce complex and sex-specific effects on both the early survival of their offspring and on long-term reproductive success through laying order effects among eggs of the same breeding attempt.

Parasites may have strong eco‐evolutionary interactions with their hosts. Consequently, they may contribute to host diversification. The radiation of cichlid fish in Lake Victoria provides a good model to study the role of parasites in the early stages of speciation. We investigated patterns of macroparasite infection in a community of 17 sympatric cichlids from a recent radiation and 2 older species from 2 nonradiating lineages, to explore the opportunity for parasite‐mediated speciation. Host species had different parasite infection profiles, which were only partially explained by ecological factors (diet, water depth). This may indicate that differences in infection are not simply the result of differences in exposure, but that hosts evolved species‐specific resistance, consistent with parasite‐mediated divergent selection. Infection was similar between sampling years, indicating that the direction of parasite‐mediated selection is stable through time. We morphologically identified 6 Cichlidogyrus species, a gill parasite that is considered a good candidate for driving parasite‐mediated speciation, because it is host species‐specific and has radiated elsewhere in Africa. Species composition of Cichlidogyrus infection was similar among the most closely related host species (members of the Lake Victoria radiation), but two more distantly related species (belonging to nonradiating sister lineages) showed distinct infection profiles. This is inconsistent with a role for Cichlidogyrus in the early stages of divergence. To conclude, we find significant interspecific variation in parasite infection profiles, which is temporally consistent. We found no evidence that Cichlidogyrus‐mediated selection contributes to the early stages of speciation. Instead, our findings indicate that species differences in infection accumulate after speciation., Species composition of Cichlidogyrus infection was similar among the closely related cichlid species of the Lake Victoria radiation (in orange), but differed between this lineage and two distantly related species (belonging to nonradiating sister lineages, in blue).

Ecological speciation is facilitated when divergent adaptation has direct effects on selective mating. Divergent sensory adaptation could generate such direct effects, by mediating both ecological performance and mate selection. In aquatic environments, light attenuation creates distinct photic environments, generating divergent selection on visual systems. Consequently, divergent sensory drive has been implicated in the diversification of several fish species. Here, we experimentally test whether divergent visual adaptation explains the divergence of mate preferences in Haplochromine cichlids. Blue and red Pundamilia co‐occur across south‐eastern Lake Victoria. They inhabit different photic conditions and have distinct visual system properties. Previously, we documented that rearing fish under different light conditions influences female preference for blue versus red males. Here, we examine to what extent variation in female mate preference can be explained by variation in visual system properties, testing the causal link between visual perception and preference. We find that our experimental light manipulations influence opsin expression, suggesting a potential role for phenotypic plasticity in optimizing visual performance. However, variation in opsin expression does not explain species differences in female preference. Instead, female preference covaries with allelic variation in the long‐wavelength‐sensitive opsin gene (LWS), when assessed under broad‐spectrum light. Taken together, our study presents evidence for environmental plasticity in opsin expression and confirms the important role of colour perception in shaping female mate preferences in Pundamilia. However, it does not constitute unequivocal evidence for the direct effects of visual adaptation on assortative mating., Pundamilia cichlids exhibit light‐induced opsin expression plasticity, which correlates with female mate preference, but does not explain species differences in preference. Instead, female preference covaries with allelic variation in the LWS opsin in broad‐spectrum light, confirming the important role of colour perception in shaping Pundamilia mate preferences.

Local adaptation can be a potent force in speciation, with environmental heterogeneity leading to niche specialization and population divergence. However, local adaption often requires nonrandom mating to generate reproductive isolation. Population divergence in sensory properties can be particularly consequential in speciation, affecting both ecological adaptation and sexual communication. Pundamilia pundamila and Pundamilia nyererei are two closely related African cichlid species that differ in male coloration, blue vs. red. They co-occur at rocky islands in southern Lake Victoria, but inhabit different depth ranges with different light environments. The species differ in colour vision properties, and females exert species-specific preferences for blue vs. red males. Here, we investigated the mechanistic link between colour vision and preference, which could provide a rapid route to reproductive isolation. We tested the behavioural components of this link by experimentally manipulating colour perception – we raised both species and their hybrids under light conditions mimicking shallow and deep habitats – and tested female preference for blue and red males under both conditions. We found that rearing light significantly affected female preference: shallow-reared females responded more strongly to P. pundamilia males and deep-reared females favoured P. nyererei males – implying that visual development causally affects mate choice. These results are consistent with sensory drive predictions, suggesting that the visual environment is key to behavioural isolation of these species. However, the observed plasticity could also make the species barrier vulnerable to environmental change: speciesassortative preferences were weaker in females that were reared in the other species’ light condition.

Sexual selection on male coloration has been implicated in the evolution of colourful species flocks of East African cichlid fish. During adaptive radiations, animals diverge in multiple phenotypic traits, but the role of physiology has received limited attention. Here, we report how divergence in physiology may contribute to the stable coexistence of two hybridizing incipient species of cichlid fish from Lake Victoria. Males of Pundamilia nyererei (males are red) tend to defeat those of Pundamilia pundamilia (males are blue), yet the two sibling species coexist in nature. It has been suggested that red males bear a physiological cost that might offset their dominance advantage. We tested the hypothesis that the two species differ in oxidative stress levels and immune function and that this difference is correlated with differences in circulating steroid levels. We manipulated the social context and found red males experienced significantly higher oxidative stress levels than blue males, but only in a territorial context when colour and aggression are maximally expressed. Red males exhibited greater aggression levels and lower humoral immune response than blue males, but no detectable difference in steroid levels. Red males appear to trade off increased aggressiveness with physiological costs, contributing to the coexistence of the two species. Correlated divergence in colour, behaviour and physiology might be widespread in the dramatically diverse cichlid radiations in East African lakes and may play a crucial role in the remarkably rapid speciation of these fish.

We investigated in the black-headed gull whether female deposition of antioxidants and immunoglobulins (enhancing early immune function), and testosterone (suppressing immune function and increasing early competitive skills) correlate suggesting that evolution has favoured the mutual adjustment of different pathways for maternal effects. We also took egg mass, the position of the egg in the laying sequence and offspring sex into account, as these affect offspring survival. Yolk antioxidant and immunoglobulin concentrations decreased across the laying order, while yolk testosterone concentrations increased. This may substantially handicap the immune defence of last-hatched chicks. The decrease in antioxidant levels was greater when mothers had a low body mass and when the increase in testosterone concentrations was relatively large. This suggests that female black-headed gulls are constrained in the deposition of antioxidants in last-laid eggs and compensate for this by enhanced testosterone deposition. The latter may be adaptive since it re-allocates the chick's investment from costly immune function to growth and competitive skills, necessary to overcome the consequences of hatching late from an egg of reduced quality.

Female birds of several species have control over the production of daughters and sons. However, most studies failed to find a relationship between egg size and sex. This is intriguing as adjustment of egg size would constitute a powerful tool for the female to meet different resource demands of the sexes, particularly in size dimorphic species. Our results show that, within clutches of black-headed gulls (Larus ridibundus) the proportion of males was positively associated with egg mass. This applied for all three laying positions, independently of the absolute egg mass. There was a significant relationship between the distribution of the sexes over the laying sequence and the egg mass change. When egg mass decreased over the sequence, first-laid eggs were male biased and last-laid eggs female biased, and vice versa. The potential adaptive value of this allocation strategy is evaluated with regard to male sensitivity to egg quality and competitive differences between the sexes.

Sympatric speciation driven by sexual selection by female mate choice on a male trait is a much debated topic. The process is problematic because of the lack of negative frequency-dependent selection that can facilitate the invasion of a novel colour phenotype and stabilize trait polymorphism. It has recently been proposed that male-male competition for mating territories can generate frequency-dependent selection on male colouration. Rare male cichlid fish would enjoy a fitness advantage if territorial defenders bias aggression towards male cichlid fish of their own colour. We used blue (ancestral type) and red phenotypes of the Lake Victoria cichlid species complex Pundamilia. We tested the aggression bias of wild-caught territorial blue male cichlid fish from five separate populations for blue vs. red rival male cichlid fish using simulated intruder choice tests. The different populations vary in the frequency of red male cichlid fish, and in the degree of reproductive isolation between red and blue, reflecting different stages of speciation. Blue male cichlid fish from a population that lack red phenotypes biased aggression towards blue stimulus male cichlid fish. The same was found in two populations where blue and red are reproductively isolated sister species. This aggression bias may facilitate the invasion of a novel colour phenotype and species coexistence. Blue male cichlid fish from two populations where red and blue are hybridizing incipient species biased aggression towards red stimulus male cichlid fish. Thus, after a successful invasion of red, aggression bias alone is not likely to generate frequency dependence required to stabilize the coexistence of phenotypes. The findings show that aggression bias varies between stages of speciation, but is not enough to stabilize the process of speciation.