Effects of egg albumen removal on yellow-legged gull chick phenotype

Maternal stress effects across generations in a precocial bird

Prenatal maternal stress (PMS) is known to shape the phenotype of the first generation offspring (F1) but according to some studies, it could also shape the phenotype of the offspring of the following generations. We previously showed in the Japanese quail that PMS increased the emotional reactivity of F1 offspring in relation to (i) a variation in the levels of some histone post-translational modification (H3K27me3) in their brains and (ii) a modulation of the hormonal composition of the eggs from which they hatched. Here, we wondered whether PMS could also influence the behaviour of the second (F2) and third (F3) generation offspring due to the persistence of the specific marks we identified. Using a principal component analysis, we found that PMS influenced F2 and F3 quail profiles with subtle differences between generations. It increased F2 neophobia, F3 fearfulness and F3 neophobia but only in females. Interestingly, we did not find any variations in the level of histone post-translational modification in F3 brains and we observed inconsistent modulations of androstenedione levels in F1 and F2 eggs. Although they may vary over generations, our results demonstrate that PMS can have phenotypical effects into the third generation.

Fitness benefits of alternated chick provisioning in cooperatively breeding carrion crows

In most bird species, parents raise offspring cooperatively. In some cases, this cooperation extends to helpers-at-the-nest who assist the breeders with a range of tasks. While cooperative food provisioning might merely arise incidentally, as a result of the efforts of carers that act independently from each other, recent studies suggest that birds may coordinate by taking turns in visiting the nest. However, evidence that such coordination emerges because individuals actively respond to each other's behaviour is controversial, and the potential benefits of carers' alternation remain unknown. We addressed this knowledge gap by analysing a multiyear dataset for cooperatively breeding carrion crows Corvus corone, comprising 8693 nest visits across 50 groups. Our results reveal that turn-taking does occur in this species and that all group members, regardless of their sex and social role (breeder/helper), tend to alternate at the nest with other carers rather than to make repeat visits. Importantly, we found that the body mass of nestlings increased significantly with the degree of carers' alternation, possibly because well-coordinated groups provided food at more regular intervals. Using earlier monitoring data, the observed increase in body mass is predicted to substantially boost postfledging survival rates. Our analyses demonstrate that alternation in nestling provisioning has measurable fitness benefits in this study system. This raises the possibility that cooperatively breeding carrion crows, as well as other bird species with similarly coordinated brood provisioning, exhibit specialized behavioural strategies that enable effective alternation.

Temperature-induced changes in egg white antimicrobial concentrations during pre-incubation do not influence bacterial trans-shell penetration but do affect hatchling phenotype in Mallards

Microbiome formation and assemblage are essential processes influencing proper embryonal and early-life development in neonates. In birds, transmission of microbes from the outer environment into the egg’s interior has been found to shape embryo viability and hatchling phenotype. However, microbial transmission may be affected by egg-white antimicrobial proteins (AMPs), whose concentration and antimicrobial action are temperature-modulated. As both partial incubation and clutch covering with nest-lining feathers during the pre-incubation period can significantly alter temperature conditions acting on eggs, we experimentally investigated the effects of these behavioural mechanisms on concentrations of both the primary and most abundant egg-white AMPs (lysozyme and avidin) using mallard (Anas platyrhychos) eggs. In addition, we assessed whether concentrations of egg-white AMPs altered the probability and intensity of bacterial trans-shell penetration, thereby affecting hatchling morphological traits in vivo. We observed higher concentrations of lysozyme in partially incubated eggs. Clutch covering with nest-lining feathers had no effect on egg-white AMP concentration and we observed no association between concentration of egg-white lysozyme and avidin with either the probability or intensity of bacterial trans-shell penetration. The higher egg-white lysozyme concentration was associated with decreased scaled body mass index of hatchlings. These outcomes demonstrate that incubation prior to clutch completion in precocial birds can alter concentrations of particular egg-white AMPs, though with no effect on bacterial transmission into the egg in vivo. Furthermore, a higher egg white lysozyme concentration compromised hatchling body condition, suggesting a potential growth-regulating role of lysozyme during embryogenesis in precocial birds.

Association of social group with both life-history traits and brain size in cooperatively breeding birds

Social group is associated with life-history traits and can predict brain size variation in cooperative primates and some other mammal groups, but such explicit relationships remain enigmatic in cooperatively breeding birds. Indeed, some compositions of social group in cooperative species (e.g., helper number and group size) would affect the fitness of breeders by providing alloparental care. Here, we conducted comparative tests of the relationship between the social group and both life-history traits and brain size across 197 species of cooperatively breeding birds using phylogenetically controlled comparative analyses. We did not find any correlations between helper numbers and both life-history traits and brain size. However, we found that maximum group size was positively associated with clutch size. Moreover, average group size has positive associations with body mass and relative brain size. Our findings suggest that helper numbers cannot promote variation in relative brain size, while larger groups may predict bigger brains in cooperatively breeding birds.

Predation risk affects egg mass but not egg steroid hormone concentrations in yellow-legged gulls

Predators have both direct, consumptive effects on their prey and non-lethal effects on physiology and behavior, including reproductive decisions, with cascading effects on prey ecology and evolution. Here, we experimentally tested such non-lethal effects of exposure to increased predation risk on clutch size, egg mass, and the concentration of yolk steroid hormones in the yellow-legged gull Larus michahellis. We simulated increased predation risk by displaying stuffed predators (adult fox Vulpes vulpes, and adult buzzard Buteo buteo) to breeding adults before egg laying. The concentration of corticosterone, which has been shown to increase under exposure to maternal predation risk in other species, and of testosterone did not differ between eggs from mothers exposed to the predators and eggs from control mothers (i.e., eggs exposed to a novel object of similar size and position to the stuffed predators). The concentration of the two hormones negatively covaried. Clutch size did not vary according to experimental treatment, whereas egg mass was markedly larger in clutches from nests exposed to predators than in clutches from control nests. By increasing egg mass, mothers may reduce the risk of cooling of the eggs when incubation is impeded by predators, boost energy reserves, reduce post-natal detectability caused by food solicitation, and/or enhance development at hatching, thus increasing the chances of offspring survival. In general, our results are inconsistent with most of the few previous studies on similar non-lethal predator effects and suggest that such effects may vary among species according to ecological conditions, social behavior, and developmental mode.

Perinatal variation and covariation of oxidative status and telomere length in yellow-legged gull chicks

The perinatal period is critical to survival and performance of many organisms. In birds, rapid postnatal growth and sudden exposure to aerial oxygen around hatching markedly affect the chick redox status, with potentially negative consequences on physiology mediated by oxidative stress. In addition, telomere length (TL) undergoes reduction during birds’ early life, partly depending on oxidative status. However, relatively few studies have focused specifically on the changes in oxidative status and TL that occur immediately after hatching. In this study of the yellow-legged gull Larus michahellis, we found that chicks undergo a marked increase in plasma total antioxidant capacity and a marked decrease in the concentration of pro-oxidant molecules during the first days after hatching. In addition, TL in erythrocytes decreased by 1 standard deviation over the 4 days post-hatching. Body mass and tarsus length covaried with total antioxidant capacity and concentration of pro-oxidants in a complex way, that partly depended on sex and laying order, suggesting that oxidative status can affect growth. Moreover, TL positively covaried with the concentration of pro-oxidant molecules, possibly because retention of high concentrations of pro-oxidant molecules results from mechanisms of prevention of their negative effects, including reduction in TL. Thus, this study shows that chicks undergo marked variation in oxidative status, which predicts growth and subsequent TL, prompting for more studies of the perinatal changes in the critical post-hatching stages.

Contrasting effects of increased yolk testosterone content on development and oxidative status in gull embryos

Hormone-mediated maternal effects generate variation in offspring phenotype. In birds, maternal egg testosterone (T) exerts differential effects on offspring traits after hatching, suggesting that mothers experience a trade-off between contrasting T effects. However, there is very little information on T pre-natal effects. In the yellow-legged gull (Larus michahellis), we increased yolk T concentration within physiological limits and measured the effects on development and oxidative status of late-stage embryos. T-treated embryos had a larger body size but a smaller brain than controls. Males had a larger brain than females, controlling for overall size. T treatment differentially affected brain mass and total amount of pro-oxidants in the brain depending on laying order. T-treatment effects were not sex dependent. For the first time in the wild, we show contrasting T pre-natal effects on body mass and brain size. Hence, T may enforce trade-offs between different embryonic traits, but also within the same trait during different developmental periods.

Energetic Effects of Pre-hatch Albumen Removal on Embryonic Development and Early Ontogeny in Gallus gallus

Studies on the yolk and albumen content in bird eggs, and the effects of variations in their relative loads in the phenotype of the birds, have revealed multiple consequences at different levels of biological organization, from biochemical traits to behavior. However, little is known about the effect of albumen variation on energetics performance during development and early ontogeny, despite the fact that variation in energy expenditure may have consequences in terms of fitness for both feral and domestic species. In this work, we evaluated experimentally whether variations in the content of albumen of Gallus gallus eggs could generate differences in metabolic rates during embryonic development. Additionally, we assessed changes in the activity of mitochondrial enzymes (cytochrome c oxidase and citrate synthase) in skeletal muscles and liver. Finally, we evaluated the success of hatching of these embryos and their metabolic rates (MR) post-hatching. The results revealed a significant reduction in MR in the last fifth of embryonic life, and reduced catabolic activities in the skeletal muscle of chicks hatched from albumen-removed eggs. However, the same group demonstrated an increase in catabolic activity in the liver, suggesting the existence of changes in energy allocation between tissues. Besides, we found a decrease in hatching success in the albumen-removed group, suggesting a negative effect of the lower albumen content on eggs, possibly due to lower catabolic activities in skeletal muscle. We also found a compensatory phenomenon in the first week after hatching, i.e., birds from albumen-removed eggs did not show a decrease in MR either at thermoneutral temperatures or at 10°C, compared to the control group. Collectively, our data suggest that a reduction in albumen may generate a trade-off between tissue metabolic activities, and may explain the differences in metabolic rates and hatching success, supporting the immediate adaptive response (IAR) hypothesis.

Extreme intra-clutch egg size dimorphism is not coupled with corresponding differences in antioxidant capacity and stable isotopes between eggs

Oviparous females need to allocate resources optimally to their eggs in order to maximize their fitness. Among these resources, dietary antioxidants, acquired by females and transferred to the eggs during egg formation, can greatly affect the development and survival of the embryo and chick. In crested penguins, incubation starts after the second and last egg is laid and, as opposed to many other bird species, this egg hatches first, thereby enhancing the survival of the chick. Here, we assessed whether antioxidant and isotopic composition could underlie these differences between eggs within clutches of southern rockhopper penguins (Eudyptes chrysocome chrysocome). The second-laid egg had higher total antioxidant capacity than the first-laid egg, although this was not due to higher antioxidant concentration but to its higher mass. This suggests that resources are allocated by females at a constant rate in both eggs within clutches. Accordingly, we found a strong correlation for isotopic compositions between eggs suggesting that resources were allocated similarly to each egg within the clutch. Overall, we found little evidence for a significant role of antioxidant and isotopic compositions to explain differences in terms of embryo/chick development between eggs in crested penguins. However, since our results suggest a constant rate of antioxidant transfer from females to eggs, limiting the mass of the first-laid egg might represent a strategy for females to spare antioxidant defences and preserve self-maintenance.

Yolk testosterone affects growth and promotes individual-level consistency in behavioral lateralization of yellow-legged gull chicks

Behavioral lateralization is common in animals and may be expressed at the individual- and at the population-level. The ontogenetic processes that control lateralization, however, are largely unknown. Well-established sex-dependence in androgen physiology and sex-dependent variation in lateralization have led to the hypothesis that testosterone (T) has organizational effects on lateralization. The effects of T exposure in early life on lateralization can be efficiently investigated by manipulating T levels in the cleidoic eggs of birds, because the embryo is isolated from maternal and sibling physiological interference, but this approach has been adopted very rarely. In the yellow-legged gull (Larus michahellis) we increased yolk T concentration within the physiological limits and tested the effects on the direction of lateralization in two functionally fundamental behaviors (begging for parental care and escape to cover) of molecularly sexed hatchlings. We also speculated that T may intervene in regulating consistency, rather than direction of lateralization, and therefore tested if T affected the 'repeatability' of lateral preference in consecutive behavioral trials. T treatment had no effect on the direction of lateralization, but enhanced the consistency of lateral preference in escape responses. Sex did not predict lateralization. Neither behavior was lateralized at the population-level. We therefore showed for the first time in any species an effect of egg T on consistency in lateralization. The implications of the effect of T for the evolution of trade-offs in maternal allocation of egg hormones, and the evolutionary interpretations of findings from our studies on lateralization among unmanipulated birds are discussed.

Effects of experimentally increased in ovo lysozyme on egg hatchability, chicks complement activity, and phenotype in a precocial bird

In birds, spectrum of egg white proteins deposited into the egg during its formation are thought to be essential maternal effects. Particularly, egg white lysozyme (LSM), exhibiting great between and within species variability, is considered to be essential for developing avian embryos due to its physiological, antimicrobial, and innate immune defense functions. However, there have been few studies investigating effects of LSM on early post-hatching phenotype, despite its broad physiological and protective role during embryogenesis. Here, we test how experimentally increased concentrations of egg white LSM affect hatchability in Japanese quail (Coturnix japonica) and chick phenotype immediately after hatching (particularly body weight, tarsus length, plasma LSM concentration, and plasma complement activity). Chicks from eggs with increased LSM concentration displayed reduced tarsus length compared to chicks from control eggs while hatchability, body weight and plasma LSM concentration were unaffected. It is worth noting that no effect of increased in ovo lysozyme on eggs hatchability could be related to pathogen-free environment during artificial incubation of experimental eggs causing minimal pressure on embryo viability. While tangible in vivo mechanisms during avian embryogenesis remain to be tested, our study is the first to document experimentally that egg white LSM appears to have growth-regulation role during embryo development, with possible underlying phenotypic consequences in the early post-hatching period in precocial birds.

The effect of albumen removal before incubation (embryonic protein under-nutrition) on the post-hatch performance, regulators of protein translation activation and proteolysis in neonatal broilers

Albumen was removed from broiler eggs before the start of incubation to induce prenatal protein under-nutrition in chicken embryos. With this method, the direct effect of protein deficiency was investigated, differing from mammalian models manipulating the maternal diet where indirect, hormonal effects can interfere. Based on the estimated albumen/egg weight ratio, 10 % of albumen was removed with an 18G needle, after making a hole at the sharp end of the egg with another 18G needle. Eggs were taped thereafter. The sham group underwent the same procedure, except that no albumen was removed. Control eggs did not receive any treatment. The removal of albumen decreased both embryonic and post-hatch body weight up to day 7 compared with the control group. On embryonic day 18, embryos from the albumen-deprived group had higher plasma uric acid levels compared with the sham (P= 0·016) and control (P= 0·009) groups. Moreover, a lower plasma amino acid concentration was observed at hatch compared with the sham (P= 0·038) and control (P= 0·152) groups. These findings indicate an altered protein metabolism. At hatch, a higher mRNA expression of muscle ring finger-1 (MuRF1), a gene related to proteolysis, was observed in albumen-deprived chicks compared with the control and sham chicks, together with an up-regulated expression of atrogin-1 (another atrogene) at this time point in the male protein-deficient chicks. These findings suggest that muscle proteolysis is transiently increased by the removal of albumen before the start of incubation. No evidence was found for altered protein synthesis capacity and translational efficiency in albumen-deprived chicks.

Male attractiveness regulates daughter fecundity non-genetically via maternal investment

Mothers can non-genetically influence offspring phenotype in response to environmental conditions, including mate attractiveness. If such 'maternal effects' influence the offspring's reproduction and F2 generation, there is a mechanism for non-genetic trans-generational effects on phenotype, including epigenetic phenomena, with implications for evolution and population dynamics. We demonstrate in the zebra finch Taeniopygia guttata such non-genetic effects on offspring fecundity and the size of early stage F2 (eggs) in response to experimentally manipulated father's attractiveness. Our experimental design allowed us to deduce that the mechanism for this non-genetic paternal effect was via maternal investment in eggs. This affected female offspring size and, consequently, fecundity and F2 (egg) size. This demonstrates that female perception of mate attractiveness can have non-genetic, trans-generational fitness consequences and this may have important implications for the evolution of sexually selected traits and population dynamics.

Helpers at the nest compensate for reduced maternal investment in egg size in carrion crows

Life history theory predicts that mothers should trade off current and future reproductive attempts to maximize lifetime fitness. When breeding conditions are favourable, mothers may either increase investment in the eggs to improve the quality of the offspring or save resources for future reproduction as the good raising environment is likely to compensate for a 'bad start'. In cooperatively breeding birds, the presence of helpers improves breeding conditions so that mothers may vary the number, size and quality of the eggs in response to the composition of the group. Here, we show that in cooperatively breeding carrion crows Corvus corone corone, where nonbreeding males are more philopatric and more helpful at the nest than females, breeding females decreased egg size as the number of subordinate males in the group increased. However, despite the smaller investment in egg size, fledglings' weight increased in groups with more male subordinates, improving post-fledging survival and indicating that helpers fully compensated for the initial 'bad start'. These results highlight a 'hidden effect' of helpers that bears profound implications for understanding the ultimate function of helping.

Maternal investment and nutrient use affect phenotype of American alligator and domestic chicken hatchlings

Maternal investment by oviparous amniotes, in the form of yolk and albumen, and the mechanisms by which embryos use available energy and nutrients have a profound effect on embryo and, consequently, hatchling phenotype. Nutrient provisioning and uptake vary within and among oviparous taxa, avian and non-avian reptiles, due to differences and similarities in environment, behavior, and phylogeny. Eggs of crocodilians, the closest extant relatives to modern birds, are ideal models for examining modes of embryonic development, especially with regard to nutrient uptake, in non-avian reptiles and comparing them with those of birds. In this study, we investigated egg composition, embryo growth, and nutrient use in the domestic chicken (Gallus gallus) and American alligator (Alligator mississippiensis). We explored egg composition by separating and weighing components of fresh eggs. We measured embryo growth and nutrient usage by dissecting embryos and by obtaining samples of liquid from the amnion, digestive tract, and yolk sac throughout the last half of incubation. Variation in albumen mass contributed most to egg mass variation in chicken eggs, whereas alligator eggs were composed almost equally of yolk and albumen, although larger eggs contained proportionally more albumen and less yolk than smaller eggs. Both chicken and alligator albumen were mostly water (87% and 96%, respectively) although chicken albumen contained over three times more solid mass per gram than alligator albumen. In both species, yolk contained a high proportion of solids. Larger eggs produced larger hatchlings in both chickens and alligators, but albumen solids contributed to embryo mass only in chicken embryos. However, intact albumen proteins appeared in the stomach in embryos of both species. While the final disposition of albumen in alligators is unclear, variation in maternal investment of yolk at oviposition was responsible for nearly all of the variation in alligator hatchling phenotype, while both yolk and albumen contributed to chicken hatchling mass.