Origin of evolutionary change in avian clutch size

Why different bird species lay different numbers of eggs is a question that has long been associated with factors external to the organism, that is, factors which operate on inherited variation in clutch size through the action of natural selection. Yet, while external factors are important, the extent of what is evolutionarily possible rests with the mechanisms developed by birds for clutch‐size control. Hitherto neglected, these mechanisms generate factors internal to the organism that are central to the origin of evolutionary change. They are related to the fact that a species‐specific range of clutch size arises from the differential survival of pre‐ovulatory follicles undergoing growth when the signal causing egg laying to end reaches the ovary. Herein, I examine three internal factors that, together with external factors, could impact the evolution of avian clutch size. Each factor acts by changing either the number of pre‐ovulatory follicles present in the ovary at the time of follicular disruption or the timing of this event. These changes to clutch size can be explained by the concept of heterochrony. In light of this, the role of phenotypic plasticity and genes determining clutch size is discussed. Finally, to account for the origin of evolutionary change in clutch size, I detail an hypothesis involving a process similar to Waddington's theory of genetic assimilation.     

Sensory control of clutch size in the Common Swift Apus apus

Clutch size varies among individuals in most bird species. A widespread assumption is that such variation results from variable timing in the disruption of ovarian follicular growth that brings, with a few days' lag, egg‐laying to an end. Currently, there is empirical evidence that this is the case in Blue Tits Cyanistes caeruleus but not in Zebra Finches Taeniopygia guttata, in which the timing of follicular disruption has been shown to be invariant. Here, I investigate clutch size regulation of Common Swifts Apus apus. Using experimental egg removal, I show that, assuming the female gets enough egg contact, the determination of clutch size occurs at noon solar time on the day the first egg of a clutch is laid. In spite of individual variation in clutch size, there was no hint of any variability in the timing of clutch‐size determination. In this species therefore, the timing of the signal disrupting follicular growth appears invariant. The physiological mechanism that controls clutch size is discussed, including the existence of an endogenous circadian clock and potential zeitgeber, the developmental range of clutch size for the species, the sensory nature of the input that triggers follicular disruption, and the stress of laying extra eggs.     

Evolutionary innovations of squamate reproductive and developmental biology in the family Chamaeleonidae

The availability of molecular phylogenies has greatly accelerated our understanding of evolutionary innovations in the context of their origin and rate of evolution. Here, we assess the evolution of reproductive mode, developmental rate and body size in a group of squamate reptiles: the chameleons. Oviparity is ancestral and viviparity has evolved at least twice: Bradypodion and members of the Trioceros bitaeniatus clade are viviparous. Viviparous species are medium‐sized as a result of convergence from either small‐sized ancestors or large‐sized ancestors, respectively, but do not differ from oviparous species in clutch size, hatchling size or the trade‐off between clutch and hatchling size. Basal chameleons (Brookesia, Rhampholeon and Rieppeleon) are small‐sized and have developmental rates comparable with those of other lizards. Derived chameleons (Calumma, Chamaeleo, Trioceros and Furcifer) are mostly large‐sized and all have relatively slow developmental rates. Several clades of derived chameleons also exhibit developmental arrest (embryonic diapause or embryonic diapause plus cold torpor) and incubation periods extend to 6–10 months or more. Developmental arrest is associated with dry, highly seasonal climates in which the period favourable for oviposition and hatching is short. Long incubation periods thus ensure that hatching occurs during the favourable season following egg laying. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 656–668.     

Evolution of skull and body shape in Triturus newts reconstructed from three‐dimensional morphometric data and phylogeny

To explore the relationship between morphological change and species diversification, we reconstructed the evolutionary changes in skull size, skull shape, and body elongation in a monophyletic group of eight species that make up salamander genus Triturus. Their well‐studied phylogenetic relationships and the marked difference in ecological preferences among five species groups makes this genus an excellent model system for the study of morphological evolution. The study involved three‐dimensional imagery of the skull and the number of trunk vertebrae, in material that represents the morphological, spatial, and molecular diversity of the genus. Morphological change largely followed the pattern of descent. The reconstruction of ancestral skull shape indicated that morphological change was mostly confined to two episodes, corresponding to the ancestral lineage that all crested newts have in common and the Triturus dobrogicus lineage. When corrected for common descent, evolution of skull shape was correlated to change in skull size. Also, skull size and shape, as well as body shape, as inferred from the number of trunk vertebrae, were correlated, indicating a marked impact of species' ecological preferences on morphological evolution, accompanied by a series of niche shifts, with the most pronounced one in the T. dobrogicus lineage. The presence of phylogenetic signal and correlated evolutionary changes in skull and body shape suggested complex interplay of niche shifts, natural selection, and constraints by a common developmental system. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 243–255.     

Behavioural and body mass changes before egg laying in the Barn Owl: cues for clutch size determination?

To investigate laying decision and clutch size determination in indeterminate layers, we analysed in-nest activity (nest presence, and copulation, prey deliveries, and entrance frequencies) and female body mass change, as well as their relation to clutch size variation in five Barn Owl pairs (Tyto alba) nesting in eastern France. Body mass of the female and behaviour [copulation frequency, entrance frequency, and prey delivery to the nest by the male (in number and mass)] were monitored using an automated weighing system and a video camera. There was a consistent change of behaviour and foraging activity among pairs ca. 18 days before laying indicating that the females may be tied to the nest at this time. Barn Owls being indeterminate layers have their clutch size determined at the oviposition of the first egg of the clutch. Window correlation analyses between the clutch size and the female body mass gain indicate that the clutch size might be determined no later than a few days before the laying of the first egg. Our results suggest that female Barn Owls may use the pre-laying period to determine the clutch size using cues such as the male food deliveries (a proxy for male quality).     

Natural selection and inheritance of breeding time and clutch size in the collared flycatcher

Many characteristics of organisms in free-living populations appear to be under directional selection, possess additive genetic variance, and yet show no evolutionary response to selection. Avian breeding time and clutch size are often-cited examples of such characters. We report analyses of inheritance of, and selection on, these traits in a long-term study of a wild population of the collared flycatcher Ficedula albicollis. We used mixed model analysis with REML estimation ("animal models") to make full use of the information in complex multigenerational pedigrees. Heritability of laying date, but not clutch size, was lower than that estimated previously using parent-offspring regressions, although for both traits there was evidence of substantial additive genetic variance (h2 = 0.19 and 0.29, respectively). Laying date and clutch size were negatively genetically correlated (rA = -0.41 +/- 0.09), implying that selection on one of the traits would cause a correlated response in the other, but there was little evidence to suggest that evolution of either trait would be constrained by correlations with other phenotypic characters. Analysis of selection on these traits in females revealed consistent strong directional fecundity selection for earlier breeding at the level of the phenotype (beta = -0.28 +/- 0.03), but little evidence for stabilising selection on breeding time. We found no evidence that clutch size was independently under selection. Analysis of fecundity selection on breeding values for laying date, estimated from an animal model, indicated that selection acts directly on additive genetic variance underlying breeding time (beta = -0.20 +/- 0.04), but not on clutch size (beta = 0.03 +/- 0.05). In contrast, selection on laying date via adult female survival fluctuated in sign between years, and was opposite in sign for selection on phenotypes (negative) and breeding values (positive). Our data thus suggest that any evolutionary response to selection on laying date is partially constrained by underlying life-history trade-offs, and illustrate the difficulties in using purely phenotypic measures and incomplete fitness estimates to assess evolution of life-history trade-offs. We discuss some of the difficulties associated with understanding the evolution of laying date and clutch size in natural populations.     

Selection on laying date is connected to breeding density in the pied flycatcher

Timing of reproduction and clutch size are important determinants of breeding success, especially in seasonal environments. Several recent bird population studies have shown changes in breeding time and in natural selection on it. These changes have often been linked with climate change, but few studies have investigated how the traits or natural selection are actually connected with climatic factors. Furthermore, the effect of population density on selection has been rarely considered, despite the potential importance of density in demographic processes. We studied variation in natural selection on laying date and on clutch size in relation to measures of spring phenology and population density in a long-term study of pied flycatchers in SW Finland. The phenological stage of the environment at mean egg-laying did not affect the direction of selection on either laying date or on clutch size. There was, however, stronger selection for earlier laying date when the breeding density of the population was high, suggesting that early breeding is not necessarily beneficial as such, but that its importance is emphasized when high population density increases competition. In addition, early breeding was favoured when the pre-breeding period was cool, which may indicate an increased advantage for the fittest individuals in harsher conditions. In the middle of the twentieth century, there was selection for large clutch size, which subsequently ceased, along with an overall decrease in recruit production. Our results indicate that attention should be paid to demographic factors such as breeding density when studying natural selection and temporal changes in it.     

Sexual dimorphism of heads and abdomens: Different approaches to ‘being large’ in female and male lizards

Sexual‐size dimorphism (SSD) is widespread in animals. Body length is the most common trait used in the study of SSD in reptiles. However, body length combines lengths of different body parts, notably heads and abdomens. Focusing on body length ignores possible differential selection pressures on such body parts. We collected the head and abdomen lengths of 610 lizard species (Reptilia: Squamata: Sauria). Across species, males have relatively larger heads, whereas females have relatively larger abdomens. This consistent difference points to body length being an imperfect measure of lizard SSD because it comprises both abdomen and head lengths, which often differ between the sexes. We infer that female lizards of many species are under fecundity selection to increase abdomen size, consequently enhancing their reproductive output (enlarging either clutch or offspring size). In support of this, abdomens of lizards laying large clutches are longer than those of lizards with small clutches. In some analyses, viviparous lizards have longer abdomens than oviparous lizards with similar head lengths. Our data also suggest that male lizards are under sexual selection to increase head size, which is positively related to winning male–male combats and to faster grasping of females. Thus, larger heads could translate into higher probability to mate. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 665–673.     

Evolution of clutch size in cavity-excavating birds: the nest site limitation hypothesis revisited

There are two major competing hypotheses for variation in clutch size among cavity-nesting species. The nest site limitation hypothesis postulates that nesting opportunities are more limited for weak excavators, which consequently invest more in each breeding attempt by laying larger clutches. Alternatively, clutch size may be determined by diet; the clutch sizes of strong excavators may be smaller because they are able to specialize on a more seasonally stable prey. We built a conceptual model that integrated hypotheses for interspecific variation in clutch size and tested it with comparative data on life-history traits of woodpeckers (Picidae) and nuthatches (Sittidae). In most analyses, diet explained more variation in clutch size among species than did propensity to excavate. Migratory status was positively associated with clutch size but was difficult to distinguish from diet since resident species consumed more bark beetles (a prey available in winter) and had smaller clutches than migratory species. The literature suggests that cavities are not limited in natural, old-growth forests. Although our data do not rule out nest site limitation, we conclude that annual stability of food resources has a larger impact on the evolution of clutch sizes in excavators than does limitation of nest sites.     

COSTS OF REPRODUCTION IN THE WILD: PATH ANALYSIS OF NATURAL SELECTION AND EXPERIMENTAL TESTS OF CAUSATION

During 1991 through 1994, natural selection on reproductive effort in side-blotched lizards was indexed by measuring total clutch mass produced on the first clutch of the reproductive season and assessing how such effort in current reproduction affects subsequent survival and clutch production. In addition, selection was also experimentally assessed in free-ranging female side-blotched lizards by (1) surgically decreasing total clutch mass (direct ovarian manipulation) and enhancing clutch mass using (2) exogenous gonadotropin, and (3) exogenous corticosterone. Surgical reduction of clutch mass uniformly enhanced survival. However, increasing clutch mass had more complex effects depending on year. Experimentally enhanced clutch mass enhanced survival in 1991, had no effect on survival in 1992, and decreased survival in 1993. Despite the complexity of these experimental results, they are corroborated by our comparative data. It is important to note that local environmental effects can obscure detection of costs arising from natural variation in reproductive effort, and we removed such effects using path analysis. The striking shift in natural selection favoring females laying a large clutch mass (1991) to selection against females laying a large clutch mass (1993) is associated with an end of a severe multiyear drought. Our natural-history observations suggest that the correlated increase in predatory snake activity on our study site, coincident with the end of the drought, is the agent of natural selection. Although the actual agents of selection (e.g., snake predation versus drought-related effects) are not resolved, the patterns of natural selection measured in our comparative and experimental data are also consistent with year-to-year changes in clutch mass and egg size that would be indicative of rapid short-term evolution in these traits.     

The effects of environmental heterogeneity on multivariate selection on reproductive traits in female great tits

Describing natural selection on phenotypic traits under varying environmental conditions is essential for a quantitative assessment of the scale at which adaptation might occur and of the impact of environmental variability on evolution. Here we analyzed patterns of multivariate selection via fecundity and viability on three reproductive traits (laying date, clutch size, and egg weight) in a population of great tits (Parus major). We quantified selection under different environmental conditions using (1) local variation in breeding density and (2) distinct areas of the population's habitat. We found that selection gradients were generally stronger for fecundity than for viability selection. We also found correlational selection acting on the combination of laying date and clutch size; this is the first documented evidence of such selection acting on these two traits in a passerine bird. Our analyses showed that both local breeding density and habitat significantly influenced selection patterns, hence favoring different patterns of reproductive investment at a small-scale relative to typical dispersal distances in this species. Canonical rotation of the nonlinear selection matrices yielded similar conclusions as traditional nonlinear selection analyses, and also showed that the main axes of selection and fitness surfaces varied over space within the population. Our results emphasize the importance of quantifying different forms of selection, and of including variation in environmental conditions at small scales to gain a better understanding of potential evolutionary dynamics in wild populations. This study suggests that the fitness landscape for this species is relatively rugged at scales relevant to the life histories of individual birds and their close relatives.     

Female coloration indicates female reproductive capacity in blue tits

It is poorly understood whether female morphological and behavioural traits can be used as 'signals'. In particular, experimental tests of the hypothesis that female ornaments reflect quality are scarce. Here, we experimentally examine whether female plumage coloration might signal maternal quality in the blue tit, Cyanistes caeruleus by forcing half of the females breeding in our population to produce a replacement clutch. Using statistical models that controlled for the effects of male coloration, and the effects of age and condition of both parents, we found that carotenoid-based female coloration was positively linked to key proxies of bird lifetime reproductive success: clutch size, fledgling success and recruitment. Importantly, the relationships between maternal yellow carotenoid coloration and both clutch size and recruitment were stronger in the experimental group than in the control group, indicating that breeding females with higher values of yellow coloration were better able to handle the cost of producing a second clutch. Finally, UV-blue female coloration was positively linked to female survival and marginally linked to laying date. Taken together, these results show for the first time in a natural population that female coloration can indicate individual and maternal quality under natural and adverse reproductive conditions. They highlight the potential for the evolution of female ornamental traits through sexual selection.     

Clutch size manipulation, hatching success and offspring phenotype in the ball python (Python regius)

In a diverse array of avian and mammalian species, experimental manipulations of clutch size have tested the hypothesis that natural selection should adjust numbers of neonates produced so as to maximize the number of viable offspring at the end of the period of parental care. Reptiles have not been studied in this respect, probably because they rarely display parental care. However, females of all python species brood their eggs until hatching, but they do not care for their neonates. This feature provides a straightforward way to experimentally increase or reduce clutch size to see whether the mean clutch size observed in nature does indeed maximize hatching success and/or optimize offspring phenotypes. Eggs were removed or added to newly laid clutches of Ball Pythons ( Python regius ) in tropical Africa (nine control clutches, eight with 50% more eggs added, six with 42% of eggs removed). All clutches were brooded by females throughout the 2-month incubation period. Experimental manipulation of clutch-size did not significantly affect the phenotypes (morphology, locomotor ability) of hatchlings, but eggs in 'enlarged' clutches hatched later, and embryos were more likely to die before hatching. This mortality was due to desiccation of the eggs, with females being unable to cover 'enlarged' clutches sufficiently to retard water loss. Our results support the notion of an optimal clutch size, driven by limitations on parental ability to care for the offspring. However, the proximate mechanisms that generate this optimum value differ from those previously described in other kinds of animals. © 2003 The Linnean Society of London . Biological Journal of the Linnean Society 2003, 78 , 263–272.     

Measuring temporal variation in reproductive output reveals optimal resource allocation to reproduction in the northern grass lizard, Takydromus septentrionalis

We measured the reproductive output of Takydromus septentrionalis collected over 5 years between 1997 and 2005 to test the hypothesis that reproductive females should allocate an optimal fraction of accessible resources in a particular clutch and to individual eggs. Females laid 1–7 clutches per breeding season, with large females producing more, as well as larger clutches, than did small females. Clutch size, clutch mass, annual fecundity, and annual reproductive output were all positively related to female size (snout–vent length). Females switched from producing more, but smaller eggs in the first clutch to fewer, but larger eggs in the subsequent clutches. The mass-specific clutch mass was greater in the first clutch than in the subsequent clutches, but it did not differ among the subsequent clutches. Post-oviposition body mass, clutch size, and egg size showed differing degrees of annual variation, but clutch mass of either the first or the second clutch remained unchanged across the sampling years. The regression line describing the size–number trade-off was higher in the subsequent clutch than in the first clutch, but neither the line for first clutch, nor the line for the second clutch varied among years. Reproduction retarded growth more markedly in small females than in large ones. Our data show that: (1) trade-offs between size and number of eggs and between reproduction and growth (and thus, future reproduction) are evident in T. septentrionalis ; (2) females allocate an optimal fraction of accessible resources in current reproduction and to individual eggs; and (3) seasonal shifts in reproductive output and egg size are determined ultimately by natural selection.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 315–324.     

Divergence in the shell morphology of the land snail genus Aegista (Pulmonata: Bradybaenidae) under phylogenetic constraints

The role of natural selection in phenotypic evolution is central to evolutionary biology. Phenotypic evolution is affected by various factors other than adaptation, and recent focus has been placed on the effects of phylogenetic constraints and niche conservatism on phenotypic evolution. Here, we investigate the relationship between the shell morphology and habitat use of bradybaenid land snails of the genus Aegista and clarify the causes of the divergence in shell morphology among phylogenetically related species. The results of ancestral state reconstruction showed that arboreal species have evolved independently from ground‐dwelling species at least four times. A significant association was found between shell shape and habitat use, despite the existence of a certain degree of phylogenetic constraint between these traits. A principal component analysis showed that arboreal species tend to have a relatively high‐spired shell with a narrow umbilicus. By contrast, ground‐dwelling species have a low‐spired shell with a wide umbilicus. Although the latitude and elevation of the sampling locations showed no relationship with shell morphology, the geology of the sampling locations affected the shell size of arboreal species. The development of a well‐balanced shell shape is one effective method for reducing the cost of locomotion under the force of gravity in each life habitat, resulting in the divergence in shell morphology and the independent evolution of morphologically similar species among different lineages. The present study suggests that ecological divergence is probably the cause of shell morphology divergence in land snails. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 114 , 229–241.     

Egg production and individual genetic diversity in lesser kestrels

Fecundity is an important component of individual fitness and has major consequences on population dynamics. Despite this, the influence of individual genetic variability on egg production traits is poorly known. Here, we use two microsatellite-based measures, homozygosity by loci and internal relatedness, to analyse the influence of female genotypic variation at 11 highly variable microsatellite loci on both clutch size and egg volume in a wild population of lesser kestrels (Falco naumanni). Genetic diversity was associated with clutch size, with more heterozygous females laying larger clutches, and this effect was statistically independent of other nongenetic variables such as female age and laying date, which were also associated with fecundity in this species. However, egg volume was not affected by female heterozygosity, confirming previous studies from pedigree-based breeding experiments which suggest that this trait is scarcely subjected to inbreeding depression. Finally, we explored whether the association between heterozygosity and clutch size was due to a genome-wide effect (general effect) or to single locus heterozygosity (local effect). Two loci showed a stronger influence but the correlation was not fully explained by these two loci alone, suggesting that a main general effect underlies the association observed. Overall, our results underscore the importance of individual genetic variation for egg production in wild bird populations, a fact that could have important implications for conservation research and provides insights into the study of clutch size evolution and genetic variability maintenance in natural populations.     

Patterns of sexual size dimorphism in Chelonia

Macroevolutionary patterns of sexual size dimorphism (SSD) indicate how sexual selection, natural selection, and genetic and developmental constraints mold sex differences in body size. One putative pattern, known as Rensch's rule, posits that, among species with female‐larger SSD, the relative degree of SSD declines with species' body size, whereas, among male‐larger SSD species, relative SSD increases with size. Using a dataset of 196 chelonian species from all fourteen families, we investigated the correlation in body size evolution between male and female Chelonia and the validity of Rensch's rule for the taxon and within its major clades. We conclude that male–female correlations in body size evolution are high, although these correlations differ among chelonian families. Overall, SSD scales isometrically with body size; Rensch's rule is valid for only one family, Testudinidae (tortoises). Because macroevolutionary patterns of SSD can vary markedly among clades, even in a taxon as morphologically conservative as Testudines, one must guard against inappropriately pooling clades in comparative studies of SSD. The results of the present study also indicate that regression models that assume the x‐variable (e.g. male body size) is measured without statistical error, although frequently reported, will result in erroneous conclusions about phylogenetic trends in sexual size dimorphism. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 108 , 396–413.     

Effect of food abundance on laying date and clutch size in the White Stork Ciconia ciconia

CapsuleFood independently affects both laying date and clutch size, suggesting that seasonal decline in clutch size is related to a decrease in food availability.

Aim To test the effect of food abundance on laying date and clutch size of the White Stork and identify the cause of seasonal decline in the number of eggs laid.

Methods During 1991 and 1996 we recorded clutch size and laying date of pairs breeding next to rubbish dumps (food abundant and constant throughout the breeding season) and birds breeding far from rubbish dumps (using natural food sources).

Results In 1991 there was no difference in mean laying date between pairs nesting at rubbish dumps and control pairs. Clutch size was significantly larger at rubbish dump nests. In contrast, mean laying date was earlier in control pairs in 1996 and there was no significant differences in clutch sizes, even when controlling for laying date effect.

Conclusion The results support the hypothesis that food availability independently affects both laying date and clutch size. The seasonal decline in clutch size close to rubbish dumps was negligible (1991) or much smaller than in the control zone (1996) suggesting that a progressive deterioration of natural food sources is the most probable reason for a decline in clutch size as the season advances.     

Differences in size between first and replacement clutches match the seasonal decline in single clutches in Tree Swallows Tachycineta bicolor

The seasonal decline in clutch size in birds can be a response to the environmentally conditioned decrease in prospects for offspring or a consequence of a lower physical ability of late‐breeding females. To find out which of the explanations apply in Tree Swallows Tachycineta bicolor, we assessed whether replacement clutch size in this species is affected by an individual female's ability to lay a certain number of eggs. To do this, we measured the decline in clutch size as a function of laying date between first and replacement clutches in individuals that re‐nested following natural failure, and compared this with the rate of decline in clutch size with laying date for Tree Swallows that laid only a single clutch in that season. Additionally, we assessed whether the clutch size and the rate of its seasonal decline varied across years. We accounted for the truncated and under‐dispersed nature of clutch size data by using a Bayesian approach in the analysis. We found little variation in the rate of clutch size decline across years at our breeding site. Accounting for this seasonal decline in clutch size, mean clutch size was similar between single‐time breeding females and those that laid replacement clutches, implying that the number of eggs laid on the second attempt by female Tree Swallows is determined by laying date, rather than by the female's physical ability to produce a clutch of a certain size.     

Maturation increases early reproductive investment in Adélie Penguins Pygoscelis adeliae

Correlations between female investment in egg production and age, breeding experience and laying date have been reported in several seabird species. In general, clutch and egg sizes increase with female age and breeding experience but decrease with laying date. Positive correlations of clutch and egg size with age and breeding experience can be caused by an increase in reproductive investment with maturation or they may be an artefact of lower survival rates for individuals with poor-quality phenotypes. Negative correlations of clutch and egg size with laying date might signal an adaptive reduction in egg production or be due in part to variation among individuals. We examined the interactions of female age, breeding experience, laying date and clutch and egg size in Adélie Penguins Pygoscelis adeliae . Breeding experience strongly affected clutch size with 87.3% of all one-egg clutches laid by first-time breeders. In addition, increasing age had a positive influence on egg size and was associated with earlier laying dates. However, there was little evidence to suggest that either clutch or egg sizes are influenced by laying date. Laying dates and clutch and egg sizes did not affect a female's probability of returning to breed in the following year, indicating that increased investment is a product of maturation and not of the loss of poor-quality breeders from the population. Our results suggest that as female Adélie Penguins gain foraging and breeding experience they are able to initiate breeding earlier, to lay complete clutches of two eggs and to lay larger eggs.