Effects of maternal age and environmental enrichment on learning ability and brain size.

It is well known that maternal age at reproduction affects offspring lifespan and some other fitness-related traits, but it remains understudied whether maternal senescence affects how offspring respond to their environments. Early environment often plays a significant role in the development of an animal's behavioral phenotype. For example, complex environments can promote changes in cognitive ability and brain morphology in young animals. Here, we study whether and how maternal effect senescence influences offspring plasticity in cognition, group behavior, and brain morphology in response to environmental complexity. For this, juvenile 3-spined sticklebacks from young and old mothers (i.e. 1-yr and 2-yr-old) were exposed to different levels of environmental enrichment and complexity (i.e. none, simple, and complex), and their behavior, cognitive ability, and brain size were measured. Exposing fish to enriched conditions improved individual learning ability assessed by a repeated detour-reaching task, increased the size of the whole brain, and decreased aggressive interactions in the shoal. Maternal age did not influence the inhibitory control, learning ability, and group behavioral responses of offspring to the experimental environmental change. However, maternal age affected how some brain regions of offspring changed in response to environmental complexity. In offspring from old mothers, those exposed to the complex environment had larger telencephalons and cerebellums than those who experienced simpler environments. Our results suggest that maternal effect senescence may influence how offspring invest in brain functions related to cognition in response to environmental complexity.

Maternal effect senescence via reduced DNA repair ability in the three-spined stickleback.

Maternal effect senescence, a decline in offspring viability with maternal age, has been documented across diverse animals, but its mechanisms remain largely unknown. Here, we test maternal effect senescence and explore its possible molecular mechanisms in a fish. We compared the levels of maternal mRNA transcripts of DNA repair genes and mtDNA copies in eggs and the levels of DNA damage in somatic and germline tissues between young and old female sticklebacks. We also tested, in an in vitro fertilization experiment, whether maternal age and sperm DNA damage level interactively influence the expression of DNA repair genes in early embryos. Old females transferred less mRNA transcripts of DNA repair genes into their eggs than did young females, but maternal age did not influence egg mtDNA density. Despite a higher level of oxidative DNA damage in the skeletal muscle, old females had a similar level of damage in the gonad to young females, suggesting the prioritization for germline maintenance during ageing. The embryos of both old and young mothers increased the expression of DNA repair genes in response to an increased level of oxidative DNA damage in sperm used for their fertilization. The offspring of old mothers showed higher rates of hatching, morphological deformity and post-hatching mortality and had smaller body size at maturity. These results suggest that maternal effect senescence may be mediated by reduced capacity of eggs to detect and repair DNA damages, especially prior to the embryonic genomic activation.

Does the match between individual and group behavior matter in shoaling sticklebacks?

In animals living in groups, the social environment is fundamental to shaping the behaviors and life histories of an individual. A mismatch between individual and group behavior patterns may have disadvantages if the individual is incapable of flexibly changing its state in response to the social environment that influences its energy gain and expenditure. We used different social groups of juvenile three-spined sticklebacks (Gasterosteus aculeatus) with experimentally manipulated compositions of individual sociability to study the feedback between individual and group behaviors and to test how the social environment shapes behavior, metabolic rate, and growth. Experimentally created unsociable groups, containing a high proportion of less sociable fish, showed bolder collective behaviors during feeding than did corresponding sociable groups. Fish within groups where the majority of members had a level of sociability similar to their own gained more mass than did those within mismatched groups. Less sociable individuals within sociable groups tended to have a relatively low mass but a high standard metabolic rate. A mismatch between the sociability of an individual and that of the majority of the group in which it is living confers a growth disadvantage probably due to the expression of nonadaptive behaviors that increase energetic costs.

Mitochondrial DNA content in eggs as a maternal effect.

The transmission of detrimental mutations in animal mitochondrial DNA (mtDNA) to the next generation is avoided by a high level of mtDNA content in mature oocytes. Thus, this maternal genetic material has the potential to mediate adaptive maternal effects if mothers change mtDNA level in oocytes in response to their environment or body condition. Here, we show that increased mtDNA abundance in mature oocytes was associated with fast somatic growth during early development but at the cost of increased mortality in three-spined sticklebacks. We also examined whether oocyte mtDNA and sperm DNA damage levels have interacting effects because they can determine the integrity of mitochondrial and nuclear genes in offspring. The level of oxidative DNA damage in sperm negatively affected fertility, but there was no interacting effect of oocyte mtDNA abundance and sperm DNA damage. Oocyte mtDNA level increased towards the end of the breeding season, and the females exposed to warmer temperatures during winter produced eggs with increased mtDNA copies. Our results suggest that oocyte mtDNA level can vary according to the expected energy demands for offspring during embryogenesis and early growth. Thus, mothers can affect offspring development and viability through the context-dependent effects of oocyte mtDNA abundance.

Relationships between male secondary sexual traits, physiological state and offspring viability in the three-spined stickleback.

BACKGROUND: Sexual signals produced by males play a central role in sexual selection, but the relationship between these traits and the quality of the bearer are often ambiguous. Secondary sexual traits may represent genetic quality of the bearer, resulting in positive relationships with physiological state, or may be costly to produce, showing trade-off with physiological state. A number of studies have explored the relationships between secondary sexual traits and other functional traits, but few have studied their fitness consequences. We studied the link between diverse physiological traits and both morphological and behavioural sexual traits and examined how their interplay influences offspring viability in the three-spined stickleback. RESULTS: Male sticklebacks showing nest building and courtship behaviour were smaller than those not investing in reproductive activities. There was no evidence that the expression of red nuptial colouration and the quality of courtship behaviour of males are positively related to their metabolic rates, swim ability, oxidative damage and mtDNA copy number. However, individuals showing larger red nuptial colour areas had higher levels of oxidative DNA damage in their sperm. Male courtship behaviour and aggressiveness, but not red colour area, were good predictors of offspring hatching and survival. CONCLUSIONS: Our results suggest that, in our study population at the southern edge of the species' distribution, sexual colouration of male sticklebacks was not a good indicator of their body state, but both courtship quality and aggressiveness during the courtship are reliable cues of their gamete quality, influencing the viability of their offspring. Thus, females that choose mates based on their courtship behaviour will have high fitness. In the study population, which represents a fast pace-of-life with high reproductive rate and short lifespan, sexual ornaments of males may not honestly signal their physiological and physical state because they invest at maximum in a single reproductive season despite high costs.

Smart mating: the cognitive ability of females influences their preference for male cognitive ability.

Cognitive abilities may be crucial for individuals to respond appropriately to their social and natural environment, thereby increasing fitness. However, the role of cognitive traits in sexual selection has received relatively little attention. Here, we studied 1) whether male secondary sexual traits (colour, courtship, and nest) reflect their cognitive ability, 2) whether females choose mates based on males' and their own cognitive abilities, and 3) how the interplay between secondary sexual traits and cognitive ability determines male attractiveness in the three-spined stickleback (Gasterosteus aculetaus). For this, we first evaluated the cognitive ability of sexually mature males and females in a detour-reaching task. Then, female preference was repeatedly assessed in a dichotomous-choice test, where the female was exposed to two males with contrasting performances (relatively good and bad) in the detour-reaching task. Female preference for better performing males was affected by the female's own cognitive ability. Females with relatively medium-low cognitive ability preferred males with high ability, whereas females with high ability showed no preference. We also found that males with higher cognitive abilities built more elaborated nests, but showed weaker red nuptial colouration. To our knowledge, this is among the first results that illustrate how cognitive traits of both sexes influence female mate preference, which has implications for the strength and direction of sexual selection.

Trap versus net: Behavioural sampling bias caused by capture method in three-spined sticklebacks.

Wild-caught animals are often used in behavioural or other biological studies. However, different capture methods may target individuals that differ in behaviour, life history and morphology, thereby giving rise to sampling biases. Here, we investigated whether juvenile three-spined sticklebacks caught in a natural population by passive and active sampling methods using frequently used tools (i.e. trap and hand net) differ in behaviours related to cognition and personality. The fish caught by traps were more prone to take risks and shoal (i.e. bolder and more sociable), but smaller in size and mass than the fish caught by hand nets. Individual variation in boldness was greater in the fish caught by hand nets, suggesting that this active sampling method may capture more representative samples of the natural population. Our results show the importance of capture method to avoid sampling bias in behavioural studies using wild-caught animals.

Environment-induced changes in reproductive strategies and their transgenerational effects in the three-spined stickleback.

An organism may increase its fitness by changing its reproductive strategies in response to environmental cues, but the possible consequences of those changes for the next generation have rarely been explored. By using an experiment on the three-spined stickleback (Gasterosteus aculeatus), we studied how changes in the onset of breeding photoperiod (early versus late) affect reproductive strategies of males and females, and life histories of their offspring. We also explored whether telomeres are involved in the within- and transgenerational effects. In response to the late onset of breeding photoperiod, females reduced their investment in the early clutches, but males increased their investment in sexual signals. Costs of increased reproductive investment in terms of telomere loss were evident only in the late females. The environmentally induced changes in reproductive strategies affected offspring growth and survival. Most notably, offspring growth rate was the fastest when both parents experienced a delayed (i.e., late) breeding photoperiod, and survival rate was the highest when both parents experienced an advanced (i.e., early) breeding photoperiod. There was no evidence of transgenerational effects on offspring telomere length despite positive parents-offspring relationships in this trait. Our results highlight that environmental changes may impact more than one generation by altering reproductive strategies of seasonal breeders with consequences for offspring viability.

Attractive male sticklebacks carry more oxidative DNA damage in the soma and germline.

Trade-offs between the expression of sexual signals and the maintenance of somatic and germline tissues are expected when these depend upon the same resources. Despite the importance of sperm DNA integrity, its trade-off with sexual signalling has rarely been explored. We experimentally tested the trade-off between carotenoid-based sexual coloration and oxidative DNA damage in skeletal muscle, testis and sperm by manipulating reproductive schedule (early vs. late onset of breeding) in male three-spined sticklebacks. Oxidative DNA damage was measured as the amount of 8-hydroxy-2-deoxyguanosine in genomic DNA. Irrespective of the experimentally manipulated reproductive schedule, individuals investing more in red coloration showed higher levels of oxidative DNA damage in muscle, testis and sperm during the peak breeding season. Our results show that the expression of red coloration traded off against the level of oxidative DNA damage possibly due to the competing functions of carotenoids as colorants and antioxidants. Thus, female sticklebacks may risk fertility and viability of offspring by choosing redder, more deteriorated partners with decreased sperm DNA integrity. The evolution of sexual signal may be constrained by oxidative DNA damage in the soma and germline.

Redox-regulation and life-history trade-offs: scavenging mitochondrial ROS improves growth in a wild bird.

It has been proposed that animals usually restrain their growth because fast growth leads to an increased production of mitochondrial reactive oxygen species (mtROS), which can damage mitochondrial DNA and promote mitochondrial dysfunction. Here, we explicitly test whether this occurs in a wild bird by supplementing chicks with a mitochondria-targeted ROS scavenger, mitoubiquinone (mitoQ), and examining growth rates and mtDNA damage. In the yellow-legged gull Larus michahellis, mitoQ supplementation increased the early growth rate of chicks but did not reduce mtDNA damage. The level of mtDNA damage was negatively correlated with chick mass, but this relationship was not affected by the mitoQ treatment. We also found that chick growth was positively correlated with both mtDNA copy number and the mitochondrial enzymatic activity of citrate synthase, suggesting a link between mitochondrial content and growth. Additionally, we found that MitoQ supplementation increased mitochondrial content (in males), altered the relationship between mtDNA copy number and damage, and downregulated some transcriptional pathways related to cell rejuvenation, suggesting that scavenging mtROS during development enhanced growth rates but at the expense of cellular turnover. Our study confirms the central role of mitochondria modulating life-history trade-offs during development by other mechanisms than mtROS-inflicted damage.

Carry-over effects of early thermal conditions on somatic and germline oxidative damages are mediated by compensatory growth in sticklebacks.

Most studies of climate change impacts focus on the effects of summer temperatures, which can immediately impact fitness of breeders, but winter temperatures are expected to have a greater impact on development and growth of animals with long-lasting consequences. Exposure to warmer temperatures can increase cellular oxidative damage in ectotherms. Yet, it is unknown whether thermal stress during early life has prolonged effects on oxidative status during adulthood. In an experiment using F1 fish originated from a wild three-spined stickleback population at the southern edge of its European distribution, we examined whether experimental thermal conditions experienced in winter had carry-over effects on oxidative status and telomere length, a marker of accumulated stress, in the soma and germline during adulthood. For this, oxidative DNA damage, enzymatic antioxidant activities and telomere length were measured three months after the termination of the temperature manipulation. In addition, we tested whether such delayed effects, if any, were due to individuals' compensatory growth after experiencing unfavourable growth conditions in winter. Warm acclimation during winter induced increased levels of oxidative DNA damage in muscle and sperm and increased enzymatic antioxidant defences in muscle during the breeding season. Telomere length of adult fish was not influenced by thermal conditions experienced during early life. Winter temperature manipulation influenced fish to alter the temporal pattern of growth trajectories across the juvenile and adult stages. Fish reared in warm winter conditions grew at a slower rate than the controls during the period of temperature manipulation then accelerated body mass gain to catch up during the breeding season. Faster somatic growth during the breeding season incurred a higher cost in terms of oxidative damage in the warm-treated individuals. For the first time, we experimentally show the long-lasting detrimental effects of thermal stress on and the positive link between catch-up growth and oxidative DNA damage in the soma and germline. Winter temperature increases due to climate change can reduce fertility and survival of fish by inducing catch-up growth. The detrimental effects of winter climate change may accumulate across generations through the pre-mutagenic DNA damage in the germline.

La mayoría de los estudios sobre los efectos del cambio climático se centran en los efectos de las temperaturas estivales, ya que estas pueden afectar de forma inmediata a la eficacia biológica de los individuos reproductores. No obstante, es esperable que las temperaturas invernales tengan un mayor impacto a largo plazo debido a sus efectos durante el desarrollo y el crecimiento temprano. Aunque la exposición a temperaturas más elevadas puede aumentar el daño oxidativo celular en ectotermos, todavía se desconoce si un estrés térmico durante el desarrollo temprano tiene efectos a largo plazo sobre el estado oxidativo en la edad adulta. En este experimento, en el que usamos la F1 procedente de una población de pez espinoso situada al borde sur de su distribución, examinamos los efectos a largo plazo de las condiciones térmicas invernales sobre los niveles de estrés oxidativo y longitud telomérica de la línea somática y germinal en la edad adulta. Además, evaluamos si tales efectos a largo plazo, si los hubo, se relacionaron con las tasa de crecimiento de los individuos. La aclimatación cálida durante el invierno indujo un aumento de los niveles de daño oxidativo en al ADN del músculo y los espermatozoides durante la estación reproductora, así como un aumento de las defensas antioxidantes enzimáticas en el músculo. La longitud telomérica adulta no se vio influenciada por las condiciones térmicas experimentadas durante el desarrollo temprano. La manipulación de la temperatura invernal alteró la trayectoria de crecimiento de los peces a lo largo de la fase juvenil y adulta. Los peces criados en condiciones invernales cálidas crecieron a un ritmo más lento que los controles durante el período de manipulación pero posteriormente mostraron una mayor tasa de crecimiento. Este crecimiento compensatorio se completó durante la temporada de reproducción. Este crecimiento compensatorio durante la temporada de reproducción tuvo un coste más elevado, en términos de daño oxidativo, en los individuos que experimentaron una condiciones invernales más cálidas. Por primera vez mostramos experimentalmente que el estrés termino temprano tiene efectos perjudiciales a largo plazo, y que existe una relación positiva entre la tasa de crecimiento compensatorio y los niveles de daño oxidativo en el ADN de las línea somática y germinal. El aumento de las temperaturas invernales debido al cambio climático podría reducir la fertilidad y la supervivencia de las poblaciones de peces al inducir cambios en las tasas de crecimiento. Además, los efectos perjudiciales del cambio climático invernal podrían ser trans-generacionales como consecuencia de la acumulación de daños pre-mutagénicos en el ADN de la línea germinal.

Thermal conditions during early life influence seasonal maternal strategies in the three-spined stickleback.

BACKGROUND: Conditions experienced by a female during early life may affect her reproductive strategies and maternal investment later in life. This effect of early environmental conditions is a potentially important mechanism by which animals can compensate for the negative impacts of climate change. In this study, we experimentally tested whether three-spined sticklebacks (Gasterosteus aculeatus) change their maternal strategy according to environmental temperatures experienced earlier in life. We studied maternal investment from a life-history perspective because females are expected to adjust their reproductive strategy in relation to their current and future reproductive returns as well as offspring fitness. RESULTS: F1 families were reared in control and elevated winter temperatures and their reproductive trajectories were studied when returned to common conditions. Females that had experienced the warm winter treatment (n = 141) had a lower fecundity and reduced breeding and total lifespan compared to the control individuals (n = 159). Whereas the control females tended to produce their heaviest and largest clutches in their first reproductive attempt, the warm-acclimated females invested less in their first clutch, but then produced increasingly heavy clutches over the course of the breeding season. Egg mass increased with clutch number at a similar rate in the two groups. The warm-acclimated females increased the investment of carotenoids in the first and last clutches of the season. Thus, any transgenerational effects of the maternal thermal environment on offspring phenotype may be mediated by the allocation of antioxidants into eggs but not by egg size. CONCLUSIONS: Our results indicate that conditions experienced by females during juvenile life have a profound effect on life-time maternal reproductive strategies. The temperature-induced changes in maternal strategy may be due to constraints imposed by the higher energetic costs of a warm environment, but it is possible that they allow the offspring to compensate for higher energetic costs and damage when they face the same thermal stress as did their mothers.

Transcriptional mechanisms underlying life-history responses to climate change in the three-spined stickleback.

Phenotypic plasticity, the ability of an organism to express different phenotypes depending on the environment, provides an important mechanism by which an animal population can persist under rapid climate change. We experimentally tested both life-history and transcriptional responses of an ecological model species, the three-spined stickleback, to warm acclimation at the southern edge of its European range. We explored cross-environment genetic correlations of key life-history traits in male sticklebacks exposed to long-term temperature changes to examine whether the plasticity pattern was variable among genotypes by using a character-state approach. We also studied gene expression plasticity by analysing both whole-transcriptome and candidate gene expression in brain and liver. Male sticklebacks that developed under warmer conditions during winter were smaller in size and invested less in nuptial coloration at the beginning of the breeding season, showing similar responses across different genotypes. The lack of genetic variation in life-history responses may limit any future evolution of the thermal reaction norm in the study population. After long-term exposure to increased winter temperatures, genes responsible for several metabolic and oxidation-reduction processes were upregulated, and some hormone genes involved in growth and reproduction were downregulated in the brain. In the liver, there was no significantly represented gene ontology by the differentially expressed genes. Since a higher temperature leads to a higher resting metabolic rate, living in warmer environments may incur higher energetic costs for ectotherms to maintain cellular homoeostasis, resulting in negative consequences for life-history traits. The expression of genes related to metabolism, cellular homoeostasis and regulatory signalling may underlie temperature-induced changes in life history.

Family-transmitted stress in a wild bird.

Recent data suggest that, in animals living in social groups, stress-induced changes in behavior have the potential to act as a source of information, so that stressed individuals could themselves act as stressful stimuli for other individuals with whom they interact repeatedly. Such form of cross-over of stress may be beneficial if it enhances adaptive responses to ecological stressors in the shared environment. However, whether stress can be transferred among individuals during early life in natural populations remains unknown. Here we tested the effect of living with stressed siblings in a gull species where, as in many vertebrates, family represents the basic social unit during development. By experimentally modifying the level of stress hormones (corticosterone) in brood mates, we demonstrate that the social transfer of stress level triggers similar stress responses (corticosterone secretion) in brood bystanders. Corticosterone-implanted chicks and their siblings were faster in responding to a potential predator attack than control chicks. In gulls, fast and coordinated reactions to predators may increase the chances of survival of the whole brood, suggesting a beneficial fitness value of cross-over of stress. However, our data also indicate that living with stressed brood mates early in life entails some long-term costs. Near independence, fledglings that grew up with stressed siblings showed reduced body size, high levels of oxidative damage in lipids and proteins, and a fragile juvenile plumage. Overall, our results indicate that stress cross-over occurs in animal populations and may have important fitness consequences.

Genetic conflict between sexual signalling and juvenile survival in the three-spined stickleback.

BACKGROUND: Secondary sexual traits and mating preferences may evolve in part because the offspring of attractive males inherit attractiveness and other genetically correlated traits such as fecundity and viability. A problem regarding these indirect genetic mechanisms is how sufficient genetic variation in the traits subject to sexual selection is maintained within a population. Here we explored the additive genetic correlations between carotenoid-based male ornament colouration, female fecundity and juvenile survival rate in the three-spined stickleback (Gasterosteus aculeatus) to test the possibility that attractiveness genes reduce important fitness components in the bearers not expressing the sexual trait. RESULTS: Male sexual attractiveness (i.e., red nuptial colouration) as well as female fecundity and juvenile viability showed heritable variations in the three-spined stickleback. Thus, females can gain indirect benefits by mating with an attractive male. There was a strong positive genetic correlation between female fecundity and juvenile viability. However, red sexual signal of male sticklebacks was negatively genetically correlated with juvenile survival, suggesting genetic conflict between attractiveness and viability. There was no significant correlation between attractiveness of brothers and fecundity of sisters, suggesting no intra-locus sexual conflict. CONCLUSIONS: The negative effects of mating with a colourful male on offspring viability may contribute to maintaining the heritable variation under strong directional sexual selection. The strength of indirect sexual selection may be weaker than previously thought due to the hidden genetic conflicts.

A benign juvenile environment reduces the strength of antagonistic pleiotropy and genetic variation in the rate of senescence.

The environment can play an important role in the evolution of senescence because the optimal allocation between somatic maintenance and reproduction depends on external factors influencing life expectancy. The aims of this study were to experimentally test whether environmental conditions during early life can shape senescence schedules, and if so, to examine whether variation among individuals or genotypes with respect to the degree of ageing differs across environments. We tested life-history plasticity and quantified genetic effects on the pattern of senescence across different environments within a reaction norm framework by using an experiment on the three-spined stickleback (Gasterosteus aculeatus, Linnaeus) in which F1 families originating from a wild annual population experienced different temperature regimes. Male sticklebacks that had experienced a more benign environment earlier in life subsequently reduced their investment in carotenoid-based sexual signals early in the breeding season, and consequently senesced at a slower rate later in the season, compared to those that had developed under harsher conditions. This plasticity of ageing was genetically determined. Both antagonistic pleiotropy and genetic variation in the rate of senescence were evident only in the individuals raised in the harsher environment. The experimental demonstration of genotype-by-environment interactions influencing the rate of reproductive senescence provides interesting insights into the role of the environment in the evolution of life histories. The results suggest that benign conditions weaken the scope for senescence to evolve and that the dependence on the environment may maintain genetic variation under selection.

Antioxidants safeguard telomeres in bold chicks.

Telomeres are sensitive to damage induced by oxidative stress, and thus it is expected that dietary antioxidants may support the maintenance of telomere length in animals, particularly those with a fast rate of life (e.g. fast metabolism, activity and growth). We tested experimentally the effect of antioxidant supplements on telomere length during early development in wild gull chicks with natural individual variations in behaviour pattern and growth rate. Proactive chicks had shorter telomeres than reactive chicks, but the penalty for the bold behaviour pattern was reduced by antioxidant supplementation. Chicks growing faster had longer telomeres during early growth, suggesting that inherited quality supports a fast life history.

Phenotypic integration between antipredator behavior and camouflage pattern in juvenile sticklebacks.

Predation is a strong selective force that promotes the evolution of antipredator behaviors and camouflage in prey animals. However, the independent evolution of single traits cannot explain how observed phenotypic variations of these traits are maintained within populations. We studied genetic and phenotypic correlations between antipredator behaviors (shoaling and risk-taking) and morphology traits (pigmentation and size) in juvenile three-spined sticklebacks by using pedigree-based quantitative genetic analysis to test phenotypic integration (or complex phenotype) as an evolutionary response to predation risk. Individuals with strongly melanized (i.e., camouflaged) phenotype and genotype were less sociable to conspecifics, but bolder during foraging under predation risk. Individuals with faster growing phenotype and genotype were bolder, and those with lager eyes were more fearful. These phenotypic integrations were not confounded with correlated plastic responses to predation risk because the phenotypes were measured in naïve fish born in the laboratory, but originated from a natural population with predation pressure. Consistent selection for particular combinations of traits under predation pressure or pleiotropic genes might influence the maintenance of the genetic (co)variations and polymorphism in melanin color, growth trajectory, and behavior patterns.

Maternal testosterone influences a begging component that makes fathers work harder in chick provisioning.

In species with biparental care, parents disagree evolutionarily over the amount of care that each of them is willing to provide to offspring. It has recently been hypothesised that females may try to manipulate their mates by modifying offspring begging behaviour through yolk hormone deposition, shifting the division of labour in their own favour. To test this hypothesis we first investigated how yellow-legged gull (Larus michaellis) parents feed offspring in relation to each component of complex begging behaviour and if feeding behaviour varies between sexes. Then we investigated the effect of yolk testosterone on chicks' begging by experimentally increasing yolk testosterone levels. Our results revealed that yolk testosterone has a component-specific effect on chicks' begging, specifically increasing the number of chatter calls. Parental feeding effort was influenced by the number of chatter calls emitted by chicks, but most importantly, the influence was stronger in male than in female parents. Moreover, chick body mass increased with the number of paternal feeds. In conclusion, these results show that female gulls may use yolk testosterone deposition to exploit their partners as predicted by the 'Manipulating Androgen Hypothesis (MAH)'.

Pre-fledgling oxidative damage predicts recruitment in a long-lived bird.

Empirical evidence has shown that stressful conditions experienced during development may exert long-term negative effects on life-history traits. Although it has been suggested that oxidative stress has long-term effects, little is known about delayed consequences of oxidative stress experienced early in life in fitness-related traits. Here, we tested whether oxidative stress during development has long-term effects on a life-history trait directly related to fitness in three colonies of European shags Phalacrocorax aristotelis. Our results revealed that recruitment probability decreased with oxidative damage during the nestling period; oxidative damage, in turn, was related to the level of antioxidant capacity. Our results suggest a link between oxidative stress during development and survival to adulthood, a key element of population dynamics.