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Environmental variation can shape the gut microbiome, but broad/large‐scale data on among and within‐population heterogeneity in the gut microbiome and the associated environmental factors of wild populations is lacking. Furthermore, previous studies have limited taxonomical coverage, and knowledge about wild avian gut microbiomes is still scarce.We investigated large‐scale environmental variation in the gut microbiome of wild adult great tits across the species' European distribution range. We collected fecal samples to represent the gut microbiome and used the 16S rRNA gene sequencing to characterize the bacterial gut microbiome.Our results show that gut microbiome diversity is higher during winter and that there are compositional differences between winter and summer gut microbiomes. During winter, individuals inhabiting mixed forest habitat show higher gut microbiome diversity, whereas there was no similar association during summer. Also, temperature was found to be a small contributor to compositional differences in the gut microbiome. We did not find significant differences in the gut microbiome among populations, nor any association between latitude, rainfall and the gut microbiome.The results suggest that there is a seasonal change in wild avian gut microbiomes, but that there are still many unknown factors that shape the gut microbiome of wild bird populations. [ABSTRACT FROM AUTHOR]

Anthropogenic climate change has led to globally increasing temperatures at an unprecedented pace and, to persist, wild species have to adapt to their changing world. We, however, often fail to derive reliable predictions of species' adaptive potential. Genomic selection represents a powerful tool to investigate the adaptive potential of a species, but constitutes a 'blind process' with regard to the underlying genomic architecture of the relevant phenotypes. Here, we used great tit (Parus major) females from a genomic selection experiment for avian lay date to zoom into this blind process. We aimed to identify the genetic variants that responded to genomic selection and epigenetic variants that accompanied this response and, this way, might reflect heritable genetic variation at the epigenetic level. We applied whole genome bisulfite sequencing to blood samples of individual great tit females from the third generation of bidirectional genomic selection lines for early and late lay date. Genomic selection resulted in differences at both the genetic and epigenetic level. Genetic variants that showed signatures of selection were located within genes mostly linked to brain development and functioning, including LOC107203824 (SOX3‐like). SOX3 is a transcription factor that is required for normal hypothalamo‐pituitary axis development and functioning, an essential part of the reproductive axis. As for epigenetic differentiation, the early selection line showed hypomethylation relative to the late selection line. Sites with differential DNA methylation were located in genes important for various biological processes, including gonadal functioning (e.g., MSTN and PIK3CB). Overall, genomic selection for avian lay date provided insights into where within the genome the heritable genetic variation for lay date, on which selection can operate, resides and indicates that some of this variation might be reflected by epigenetic variants. [ABSTRACT FROM AUTHOR]

Anthropogenic pollution is known to negatively influence an organism's physiology, behaviour, and fitness. Epigenetic regulation, such as DNA methylation, has been hypothesized as a potential mechanism to mediate such effects, yet studies in wild species are lacking. We first investigated the effects of early-life exposure to the heavy metal lead (Pb) on DNA methylation levels in a wild population of great tits (Parus major), by experimentally exposing nestlings to Pb at environmentally relevant levels. Secondly, we compared nestling DNA methylation from a population exposed to long-term heavy metal pollution (close to a copper smelter), where birds suffer from pollution-related decrease in food quality, and a control population. For both comparisons, the analysis of about one million CpGs covering most of the annotated genes revealed that pollution-related changes in DNA methylation were not genome wide, but enriched for genes underlying developmental processes. However, the results were not consistent when using binomial or beta binomial regression highlighting the difficulty of modelling variance in CpGs. Our study indicates that post-natal anthropogenic heavy metal exposure can affect methylation levels of development related genes in a wild bird population. [ABSTRACT FROM AUTHOR]

Species with a circannual life cycle need to match the timing of their life history events to the environment to maximize fitness. However, our understanding of how circannual traits such as timing of reproduction are regulated on a molecular level remains limited. Recent studies have implicated that epigenetic mechanisms can be an important part in the processes that regulate circannual traits. Here, we explore the role of DNA methylation in mediating reproductive timing in a seasonally breeding bird species, the great tit (Parus major), using genome‐wide DNA methylation data from individual females that were blood sampled repeatedly throughout the breeding season. We demonstrate rapid and directional changes in DNA methylation within the promoter region of several genes, including a key transcription factor (NR5A1) known from earlier studies to be involved in the initiation of timing of reproduction. Interestingly, the observed changes in DNA methylation at NR5A1 identified here are in line with earlier gene expression studies of reproduction in chicken, indicating that the observed shifts in DNA methylation at this gene can have a regulatory role. Our findings provide an important step towards elucidating the genomic mechanism that mediates seasonal timing of a key life history traits and provide support for the idea that epigenetic mechanisms may play an important role in circannual traits. see also the Perspective by Melanie J. Heckwolf and Britta S. Meyer [ABSTRACT FROM AUTHOR]

The timing of breeding is under selection in wild populations as a result of climate change, and understanding the underlying physiological processes mediating this timing provides insight into the potential rate of adaptation. Current knowledge on this variation in physiology is, however, mostly limited to males. We assessed whether individual differences in the timing of breeding in females are reflected in differences in candidate gene expression and, if so, whether these differences occur in the upstream (hypothalamus) or downstream (ovary and liver) parts of the neuroendocrine system. We used 72 female great tits from two generations of lines artificially selected for early and late egg laying, which were housed in climatecontrolled aviaries and went through two breeding cycles within 1 year. In the first breeding season we obtained individual egg-laying dates, while in the second breeding season, using the same individuals, we sampled several tissues at three time points based on the timing of the first breeding attempt. For each tissue, mRNA expression levels were measured using qPCR for a set of candidate genes associated with the timing of reproduction and subsequently analysed for differences between generations, time points and individual timing of breeding. We found differences in gene expression between generations in all tissues, with the most pronounced differences in the hypothalamus. Differences between time points, and early- and late-laying females, were found exclusively in the ovary and liver. Altogether, we show that finetuning of the seasonal timing of breeding, and thereby the opportunity for adaptation in the neuroendocrine system, is regulated mostly downstream in the neuro-endocrine system. [ABSTRACT FROM AUTHOR]

Chromosome inversions have clear effects on genome evolution and have been associated with speciation, adaptation, and the evolution of the sex chromosomes. In birds, these inversions may play an important role in hybridization of species and disassortative mating. We identified a large (⁠ ≈ 64 Mb) inversion polymorphism in the great tit (Parus major) that encompasses almost 1,000 genes and more than 90% of Chromosome 1A. The inversion occurs at a low frequency in a set of over 2,300 genotyped great tits in the Netherlands with only 5% of the birds being heterozygous for the inversion. In an additional analysis of 29 resequenced birds from across Europe, we found two heterozygotes. The likely inversion breakpoints show considerable genomic complexity, including multiple copy number variable segments. We identified different haplotypes for the inversion, which differ in the degree of recombination in the center of the chromosome. Overall, this remarkable genetic variant is widespread among distinct great tit populations and future studies of the inversion haplotype, including how it affects the fitness of carriers, may help to understand the mechanisms that maintain it. [ABSTRACT FROM AUTHOR]

Gathering information about the environment, such as the location and quality of food, is crucial for an animal's survival, particularly in a changing environment. An animal can collect 'personal information' by interacting with the environment itself, or it can collect 'social information' by observing the behaviour of others. The use of these two types of information varies across different situations and between individuals. Personality is a concept that captures consistent interindividual differences in behaviour and could be one of the factors driving interindividual variation in information use. We tested this by conducting behavioural experiments based on a colour association task in captive great tits, Parus major , originating from lines bidirectionally selected for high and low exploratory behaviour. We quantified personal information use by measuring to what extent a bird relied on previously rewarded options instead of novel options. Social information use was measured by recording how birds chose according to social information provided by video playbacks of a conspecific. Here, we demonstrate that variation in the use of both personal and social information is indeed personality related. In their decision making, slow explorers relied more on prior knowledge, from both personal and social origins, whereas fast explorers tended to ignore the available information and chose more randomly. The differences between the personality types imply different costs or constraints in acquiring and/or applying the two types of information, possibly due to variation in, for example, cognitive styles. In conclusion, we demonstrate that personality types have different strategies to cope with environmental uncertainty. • Both social and personal information use are personality related. • Faster explorers tend to sample novel options instead of using personal information. • The use of social information decreases with exploratory tendency. • Slow explorers base their behaviour more on prior knowledge than fast explorers. [ABSTRACT FROM AUTHOR]

In seasonal environments, timing of reproduction is a trait with important fitness consequences, but we know little about the molecular mechanisms that underlie the variation in this trait. Recently, several studies put forward DNA methylation as a mechanism regulating seasonal timing of reproduction in both plants and animals. To understand the involvement of DNA methylation in seasonal timing of reproduction, it is necessary to examine within-individual temporal changes in DNA methylation, but such studies are very rare. Here, we use a temporal sampling approach to examine changes in DNA methylation throughout the breeding season in female great tits (Parus major) that were artificially selected for early timing of breeding. These females were housed in climate-controlled aviaries and subjected to two contrasting temperature treatments. Reduced representation bisulfite sequencing on red blood cell derived DNA showed genome-wide temporal changes in more than 40,000 out of the 522,643 CpG sites examined. Although most of these changes were relatively small (mean within-individual change of 6%), the sites that showed a temporal and treatment-specific response in DNA methylation are candidate sites of interest for future studies trying to understand the link between DNA methylation patterns and timing of reproduction. [ABSTRACT FROM AUTHOR]

1. Wind is an important abiotic factor that influences an array of biological processes, but it is rarely considered in studies on plant--herbivore interactions. 2. Here, we tested whether wind exposure could directly or indirectly affect the performance of two insect herbivores, Plutella xylostella and Pieris brassicae, feeding on Brassica nigra plants. 3. In a greenhouse study using a factorial design, B. nigra plants were exposed to different wind regimes generated by fans before and after caterpillars were introduced on plants in an attempt to separate the effects of direct and indirect wind exposure on herbivores. 4. Wind exposure delayed flowering, decreased plant height and increased leaf concentrations of amino acids and glucosinolates. 5. Plant-mediated effects of wind on herbivores, that is effects of exposure of plants to wind prior to herbivore feeding, were generally small. However, development time of both herbivores was extended and adult body mass of P. xylostella was reduced when they were directly exposed to wind. By contrast, wind-exposed adult P. brassicae butterflies were significantly larger, revealing a trade-off between development time and adult size. 6. Based on these results, we conducted a behavioural experiment to study preference by an avian predator, the great tit (Parus major) for last instar P. brassicae caterpillars on plants that were exposed to either control (no wind) or wind (fan-exposed) treatments. Tits captured significantly more caterpillars on still than on wind-exposed plants. 7. Our results suggest that P. brassicae caterpillars are able to perceive the abiotic environment and to trade off the costs of extended development time against the benefits of increased size depending on the perceived risk of predation mediated by wind exposure. Such adaptive phenotypic plasticity in insects has not yet been described in response to wind exposure. [ABSTRACT FROM AUTHOR]

Understanding how genes shape phenotypes is essential to assess the evolutionary potential of a trait. Identifying the genes underlying quantitative behavioral or life-history traits has, however, proven to be a major challenge. The majority of these traits are phenotypically plastic and different parts of the genome can be involved in shaping the trait under different environmental conditions. These variable genotype-phenotype associations could be one explanation for the limited success of genome-wide association studies in such traits. We here use avian seasonal timing of breeding, a trait that is highly plastic in response to spring temperature, to explore effects of such genotype-by-environment interactions in genome-wide association studies. We genotyped 2045 great tit females for 384081 single nucleotide polymorphisms (SNPs) and recorded their egg-laying dates in the wild. When testing for associations between SNPs and egg-laying dates, no SNP reached genome-wide significance. We then explored whether SNP effects were modified by annual spring temperature by formally testing for an interaction between SNP effect and temperature. The models including the SNP*temperature interaction performed consistently better although no SNP reached genome-wide significance. Our results suggest that the effects of genes shaping seasonal timing depended on annual spring temperature. Such environmentdependent effects are expected for any phenotypically plastic trait. Taking these effects into account will thus improve the success of detecting genes involved in phenotypically plastic traits, thereby leading to a better understanding of their evolutionary potential. [ABSTRACT FROM AUTHOR]

Background: Studies on vertebrate DNA methylomes have revealed a regulatory role of tissue specific DNA methylation in relation to gene expression. However, it is not well known how tissue-specific methylation varies between different functional and structural components of genes and genomes. Using whole-genome bisulfite sequencing data we here describe both CpG and non-CpG methylation profiles of whole blood and brain tissue in relation to gene features, CpG-islands (CGIs), transposable elements (TE), and their functional roles in an ecological model species, the great tit (Parus major). Results: We show that hypomethylation at the transcription start site (TSS) is enriched in genes with functional classes that relate directly to processes specific to each tissue type. We find that 6877 (~21 %) of the CGIs are differentially methylated between blood and brain, of which 1186 and 2055 are annotated to promoter and intragenic regions, respectively. We observe that CGI methylation in promoter regions is more conserved between tissues compared to CGI methylation in intra and inter-genic regions. Differentially methylated CGIs in promoter and intragenic regions are overrepresented in genomic loci linked to development, suggesting a distinct role for CGI methylation in regulating expression during development. Additionally, we find significant non-CpG methylation in brain but not in blood with a strong preference for methylation at CpA dinucleotide sites. Finally, CpG hypermethylation of TEs is significantly stronger in brain compared to blood, but does not correlate with TE activity. Surprisingly, TEs showed significant hypomethylation in non-CpG contexts which was negatively correlated with TE expression. Conclusion: The discovery that TSS methylation levels are directly linked to functional classes related to each tissue provides new insights in the regulatory role of DNA-methylation patterns. The dominant sequence motifs for brain non-CpG methylation, similar to those found in mammals, suggests that a conserved non-CpG regulatory mechanism was already present in the amniote ancestor. The negative correlation between brain non-CpG methylation and TE activity (not found for CpG methylation) suggests that non-CpG is the dominant regulatory form of methylation in TE silencing. [ABSTRACT FROM AUTHOR]

Expression of sexually selected signals in many species varies over time of day and season. A key model system to study this variation in signal expression is birdsong. Yet, despite good ecological understanding of why song varies across time of day and season, much of the individual variation remains unexplained. Although some of the interindividual variation in singing depends on the quality or motivation of an individual, it can also vary with other characteristics. Because singing has been shown to vary with personality traits in specific contexts, personality is thus an important candidate to explain part of the variation in seasonal and daily singing. Using a personality-typed field population of great tits (Parus major), we here show that singing activity peaked at dawn during the fertile period of the females and that the association between male personality and singing activity depended on the reproductive stage of his mate; faster explorers significantly increased in singing activity during main periods of fertility and maternal investment (egg laying and incubation). Moreover, males with higher singing activity tended to raise more fledglings. Increased singing by faster explorers during key periods of female reproductive investment suggests that faster explorers are more responsive to changes in female reproductive stage, contrasting the general view that faster explorers are less responsive to environmental and social changes. Most importantly, these findings highlight that multiple factors including personality need to be integrated when assessing causes of variation of highly variable sexually selected signal traits. [ABSTRACT FROM AUTHOR]

Anthropogenic noise can have serious implications for animals, especially when they communicate acoustically. Yet, the impacts of noise may depend not only on noise characteristics but also on an individual's coping style or personality. We tested whether noise is more disturbing if it masks communication signals, and whether characteristics of both the noise and the individual affect its impact. Using a unique population of personality-typed great tits, Parus major, we tested whether the kind of noise and parental personality affect parental nestbox visits and nestling begging. Nestboxes were exposed to automated noise playbacks, differing in spectral composition (noise masking begging calls, nonmasking noise or no noise). Parental nestbox visits were recorded using RFID transponders. Video and audio recordings were used to quantify nestling begging. Nestlings mainly begged in silence and in the presence of parents. Parents reduced nestbox visits during noise treatments regardless of the kind of noise and initially reacted more strongly to nonmasking noise. Moreover, slower explorers took longer to enter the nestbox during noise than faster explorers. Total visit rates during noise depended on parental sex and personality. In females, bolder individuals, but in males shyer individuals, reduced total visits during noise. These results extend previous findings in showing experimentally that the disturbance effects of noise do not depend on whether or not the noise directly interferes with information exchange by masking signals. Moreover, personality- and sex-specific responses to noise indicate that anthropogenic disturbance can differentially affect individuals within populations, which will influence mitigation strategies. [ABSTRACT FROM AUTHOR]

Animals constantly need to cope with changes in their environment. Coping with changes in cues that are associated with the location and abundance of food is essential for being able to adjust behaviourally to a variable environment. The use of cues in decision making requires appropriate levels of attention and learning ability, which may be affected by the personality of an individual. The relationship between personality, attention and learning as essential mechanisms for behavioural adaptation, however, is not well understood. We studied the relationship between attention to environmental cues, behavioural flexibility in learning and exploratory behaviour, a proxy for personality, in great tits, Parus major. We used a dimensional shift learning paradigm; a learning task involving several stages differing in complexity and requiring attention to changes in relevant cues. The results show personality differences in performance in learning flexibility in only the apparently most difficult stage, yet in opposite directions for males and females. Fast-exploring males showed more flexible learning abilities than slow males, whereas in females slow explorers outperformed fast explorers. These context-dependent and sex-specific personality effects reveal behavioural and cognitive mechanisms that may underlie observed sex- and personality-dependent fitness differences in natural populations. [Copyright &y& Elsevier]

The article provides information on a study which analyzed whether Great Tits Parus major are able to use the odor of mustelids to assess predation risk when selecting roosting cavities. The experiment was carried out during February and March 2007 using 20 hand reared captive Great Tits, in two outdoor Y-shaped aviaries. There were no differences in the behavior of males and females in any of the variables considered. The researchers discovered that more birds slept outside a nestbox when there was predator scent in one of the nestboxes. The predator inspection behavior noticed in the birds may allow them to identify the odor source and assess predator presence.

Individuals within species differ in their behavioural reactions to the environment. Consistent individual differences in these behaviours (personality traits) are often correlated and known to be under natural selection. These differences are frequently associated with variation in physiological traits, such as endocrine profiles. For example, variation in circulating testosterone levels is associated with variation in several personality traits and has been hypothesized to be a marker for personality in humans and rodents. The importance of testosterone in controlling both behavioural strategies and individual physiological differences suggests that direct selection on personality traits might cause pleiotropic selection on the physiological mechanisms underlying these traits. To test this hypothesis, we quantified levels of plasma testosterone levels and measured phytohaemagglutinin (PHA)-induced immune responses of male great tits, Parus major, in lines artificially selected for diverging levels of avian personality (‘fast’ and ‘slow’ exploratory behaviour). We found that testosterone levels were highly repeatable within individuals and fluctuated predictably over the season. Contrary to our expectations, ‘slow’ explorers had consistently higher levels of baseline testosterone and higher immune responses than ‘fast’ explorers. These results show that phenotypic selection for variation in personality traits corresponds to consistent differences in hormone profile and immune function, but that higher aggression levels do not need to be associated with higher baseline testosterone levels. Our results confirm that personality traits have evolved as a result of selection on both the underlying controlling physiological mechanisms and the phenotypic traits. [ABSTRACT FROM AUTHOR]

Identifying genes that underlie ecological traits will open exiting possibilities to study gene–environment interactions in shaping phenotypes and in measuring natural selection on genes. Evolutionary ecology has been pursuing these objectives for decades, but they come into reach now that next generation sequencing technologies have dramatically lowered the costs to obtain the genomic sequence information that is currently lacking for most ecologically important species. Here we describe how we generated over 2 billion basepairs of novel sequence information for an ecological model species, the great tit Parus major. We used over 16 million short sequence reads for the de novo assembly of a reference sequence consisting of 550 000 contigs, covering 2.5% of the genome of the great tit. This reference sequence was used as the scaffold for mapping of the sequence reads, which allowed for the detection of over 20 000 novel single nucleotide polymorphisms. Contigs harbouring 4272 of the single nucleotide polymorphisms could be mapped to a unique location on the recently sequenced zebra finch genome. Of all the great tit contigs, significantly more were mapped to the microchromosomes than to the intermediate and the macrochromosomes of the zebra finch, indicating a higher overall level of sequence conservation on the microchromosomes than on the other types of chromosomes. The large number of great tit contigs that can be aligned to the zebra finch genome shows that this genome provides a valuable framework for large scale genetics, e.g. QTL mapping or whole genome association studies, in passerines. [ABSTRACT FROM AUTHOR]

Animals differ in their behaviour comparable to how humans differ in personality: individuals consistently differ in suites of correlated traits. Relationships between ‘personality traits’ and fitness imply that personality traits can evolve by means of natural selection. We studied whether animal personality is also involved in sexual selection. We investigated whether exploratory behaviour (an aspect of animal personality, ranging from ‘slow’ to ‘fast’) correlated with the occurrence of extrapair paternity (EPP) in broods of wild great tits. We expected that EPP rates should be highest for females mated with social partners of the same personality type (i.e. for slow–slow or fast–fast pairs, but not other pair combinations). We found that the likelihood of EPP was highest for these pairs. Disassortative extrapair mating with respect to personality can be the consequence of several non-mutually exclusive processes. It might be caused by adaptive mate choice, which allows assortatively paired females to produce offspring with either more variable or more intermediate phenotypes, but it could also be the consequence of behavioural incompatibility between extreme behavioural phenotypes. Our findings indicate that personality differences play a role in the mechanism behind extrapair behaviours and we therefore conclude that it is now plausible that partner preference is based not only on morphological characteristics, but also on consistent behavioural traits or personality. [Copyright &y& Elsevier]

Adapting to environmental change is a major challenge faced by animals and the role of individual behavioural differences in facilitating this process is currently the focus of much research. Innovation, the generation of a novel behaviour or use of a known behaviour in a novel context, is one form of behaviour that enables animals to respond to change. By deciphering the mechanisms underlying innovativeness, especially those that explain consistent differences between individuals, we can further understand the consequences of this behavioural variation. We tested whether motivation, experience, inhibitory control and personality were linked to different stages of sequential innovative problem-solving performance among great tits, Parus major , and of their overall innovativeness across tasks. We gave animals originating from lines bidirectionally selected for fast or slow early exploratory behaviour, a multiaccess problem-solving device. Diverse motor skills and behavioural flexibility were required to solve all three different access points sequentially over trials. Food-deprived, highly motivated birds had shorter latency to touch the device, were more likely to solve an access point within a trial, and solved a greater diversity of them, than their less motivated counterparts. Solving success increased with accuracy when interacting with the device (proportion of touches to functional components of the device compared to all touches to the device per trial), and with previous experience. Personality selection lines and inhibitory control had little effect. Repeatability analysis showed that between-individual differences in problem-solving performance were explained by: (1) pseudorepeatable effects (upward bias) linked to hunger-induced motivation, (2) repeatable differences in accuracy when interacting with devices, and (3) a feedback loop caused by experience gained over successive trials. Our results highlight the challenges of characterizing consistent individual differences in behaviour generally and support the idea that complex sources of variation play an important role in problem-solving performance. • We decomposed sequential innovative problem-solving success in a multiaccess task. • Individuals showed consistent differences in problem-solving performance. • Exploratory behaviour and inhibitory control played no role. • Differences were mostly explained by motivation and accuracy interacting with devices. • Positive feedback through experience also played a role. [ABSTRACT FROM AUTHOR]

Stress physiology is thought to contribute to individual differences in behaviour. In part this reflects the fact that canonical personality measures consist of responses to challenges, including novel objects and environments. Exposure to novelty is typically assumed to induce a moderate increase in glucocorticoids (CORT), although this has rarely been tested. We tested this assumption using great tits, Parus major , selected for divergent personalities (bold-fast and shy-slow explorers), predicting that the shy birds would exhibit higher CORT following exposure to a novel object. We also scored behavioural responses to the novel object, predicting that bold birds would more frequently approach the novel object and exhibit more abnormal repetitive behaviours. We found that the presence of a novel object did induce a moderate CORT response, but selection lines did not differ in the magnitude of this response. Furthermore, although both selection lines showed a robust CORT elevation to a subsequent restraint stressor, the CORT response was stronger in bold birds and this effect was specific to novel object exposure. Shy birds showed a strong positive phenotypic correlation between CORT concentrations following the novel object exposure and the subsequent restraint stress. Behaviourally, the selection lines differed in their response during novel object exposure: as predicted, bold birds more frequently approached the novel object and shy birds more strongly decreased overall locomotion during the novel object trial, but birds from both selection lines showed significant and similar frequencies of abnormal repetitive behaviours during novel object exposure. Our findings support the hypothesis that personality emerges as a result of correlated selection on behaviour and underlying endocrine mechanisms and suggest that the relationship between endocrine stress physiology and personality is context dependent. [ABSTRACT FROM AUTHOR]

In many seasonally breeding birds, female and male testosterone (T) levels peak at the start of the breeding season, coinciding with pair bonding and nesting activities. Shortly after the onset of egg laying, T levels slowly decline to baseline levels in both sexes, but more rapidly so in females. During this period, T in males may still function to facilitate territorial behaviour, mate guarding and extra pair copulations, either via short lasting peaks or elevated basal levels of the hormone. In some species, however, males become insensitive to increased T after the onset of egg laying. It has been postulated that in these species bi-parental care is essential for offspring survival, as T is known to inhibit paternal care. However, only very few studies have analysed this for females. As females are heavily involved in parental care, they too might become insensitive to T after egg laying. Alternatively, because territorial defence, mate guarding and extra pair copulations are expected to be less important for females than for males, they may not have had the need to evolve a mechanism to become insensitive to T during the period of maternal care, because their natural T levels are never elevated during this part of the breeding season anyway. We tested these alternative hypotheses in female great tits ( Parus major ). Male great tits have previously been shown to be insensitive to T after egg laying with regard to nestling feeding behaviour (but not song rate). When females had started nest building, we experimentally elevated their T levels up to the nestling feeding phase, and measured incubation behaviour (only females incubate) and reproductive success. T did not significantly affect nest building or egg laying behaviour, although egg laying tended to be delayed in T females. Females with experimentally enhanced T maintained lower temperature during incubation but did not spend less time incubating. This might explain the reduced hatching success of their eggs, smaller brood size and lower number of fledglings we found in this study. As in this species T-dependent behaviour by females during the phase of parental care is not needed, the results support the hypothesis that in this species the need for selection in favour of T-insensitivity did not occur. [ABSTRACT FROM AUTHOR]

Abstract: Animal ‘personality’ describes consistent individual differences in suites of behaviors, a phenomenon exhibited in diverse animal taxa and shown to be under natural and sexual selection. It has been suggested that variation in personality reflects underlying physiological variation; however there is limited empirical evidence to test this hypothesis in wild animals. The hypothalamic–pituitary–adrenal axis is hypothesized to play a central role in personality variation. Here we tested whether in great tits Parus major variation in personality traits is related to plasma concentrations of corticosterone (CORT). Using a capture-restraint protocol we examined baseline and stress-induced CORT levels in two captive experimental groups: (1) birds selected for divergent personalities (‘fast-bold’ and ‘slow-shy’ explorers); and (2) non-selected offspring of wild parents. We first tested for differences in CORT between selection lines, and second examined the relationship between responses in a canonical personality test and CORT concentrations in non-selected birds. We found support for our prediction that the slow-shy line would exhibit a higher acute stress response than the fast-bold line, indicating a genetic correlation between exploratory behavior and stress physiology. We did not, however, find that continuous variation in exploratory behavior co-varies with CORT concentrations in non-selected birds. While our results provide support for the idea that personality emerges as a result of correlated selection on behavior and underlying physiological mechanisms, they also indicate that this link may be particularly evident when composite personality traits are the target of selection. [Copyright &y& Elsevier]