Opportunistic feeding responses of the Yellow-legged Gull Larus michahellis to accessibility of refuse dumps

Capsule The gulls adjust their diet composition and diversity according to refuse dump accessibility.

Aims To examine the influence of the accessibility of open-air refuse dumps on the pre-laying diet of the adult Yellow-legged Gull.

Methods We studied six colonies settled on six rocky islands off the southeastern coast of France. A comparative study of the diet of breeding adults from the six colonies was made through pellet analysis (a total of 848 pellets). We determined the main foraging habitat used (refuse dumps, terrestrial habitats, marine habitat) and the number of foraging habitats used simultaneously (one, two or three), from which we deduced the mean diet diversity.

Results Refuse dumps were consistently the main foraging habitat (evidence in 53–74% of pellets) for the six colonies, even when refuse dump accessibility was low. The majority of pellets contained materials from two simultaneous foraging habitats (evidence in 50–64% of pellets). We demonstrated the influence of a gradient of refuse dump accessibility in terms of adjustment of the pre-laying adult's diet. Indeed, high refuse dump accessibility leads to a poorly diversified diet dominated by refuse. In contrast, when refuse dump accessibility is low, Yellow-legged Gulls broaden their trophic niche, with an increased exploitation of alternative foraging habitats, such as terrestrial habitats.

Conclusion These results show the species' opportunistic feeding and high adaptability, two parameters which need to be known to foresee the consequences on population dynamics, feeding and predatory behaviour of a sudden and severe food shortage, for example due to closure of open-air refuse dumps.     

Sex differences in telomeres and lifespan

Males and females often age at different rates resulting in longevity 'gender gaps', where one sex outlives the other. Why the sexes have different lifespans is an age-old question, still fiercely debated today. One cellular process related to lifespan, which is known to differ according to sex, is the rate at which the protective telomere chromosome caps are lost. In humans, men have shorter lifespans and greater telomere shortening. This has led to speculation in the medical literature that sex-specific telomere shortening is one cause of sex-specific mortality. However, telomere shortening may be a cause for and/or a consequence of the processes that govern survival, and to infer general principles from single-taxon studies may be misleading. Here, we review recent work on telomeres in a variety of animal taxa, including those with reverse sexual lifespan dimorphism (i.e., where males live longer), to establish whether sex-specific survival is generally associated with sex differences in telomere dynamics. By doing this, we attempt to tease apart the potential underlying causes for sex differences in telomere lengths in humans and highlight targets for future research across all taxa.     

Sexual size dimorphism in species with asymptotic growth after maturity

If animals mature at small sizes and then grow to larger asymptotic sizes, many factors can affect male and female size distributions. Standard growth equations can be used to study the processes affecting sexual size dimorphism in species with asymptotic growth after maturity. This paper first outlines the effects of sex differences in growth and maturation patterns on the direction and degree of sexual dimorphism. The next section considers the effects of variation in age structure or growth rates on adult body sizes and sexual size dimorphism. Field data from a crustacean, fish, lizard and mammal show how information on a species' growth and maturation patterns can be used to predict the relationships between male size, female size and sexual size dimorphism expected if a series of samples from the same population simply differed with respect to their ages or growth rates. The last section considers ecological or behavioural factors with different effects on the growth, maturation, survival or movement patterns of the two sexes. This study supports earlier suggestions that information on growth and maturation patterns may be useful, if not essential, for comparative studies of sexual size dimorphism in taxa with asymptotic growth after maturity.     

Sex differences in height and sitting height in the Belgian population

Sex differences in growth were studied in a longitudinal study of 39 boys and 31 girls for sitting height. Individual growth patterns were determined by means of Preece Baines model 1. The results showed no significant bias in the fits of height and sitting height in boys and girls. Girls fits were significantly better than those of the boys for both height and sitting height. Univariate analysis by means of Mann-Whitney test showed significant sex differences for all function and biological parameters of height and sitting height excepted for s₁ parameter (the rate constant controlling pubertal velocity). Linear discriminant analysis revealed that the strongest sex differences for the timing and size parameters at adolescent. Peak velocity at adolescent was slightly less discriminating between the two sexes and velocity at take-off showed the least sex difference. These trends were similar for height and sitting height. Decomposition of sex differences in adult size showed that the major contributor to adult the sex differences is the effect of the later onset of the adolescent growth spurt in boys than in girls. Sex differences in adult phenotypes of height and sitting height are to a slightly lesser extent due to the greater adolescent gain in boys while prepubertal sex differences are almost negligible.     

Sexual behaviour and evolution of sexual dimorphism in body size in Jaera (Isopoda Asellota)

Individuals of the genus Jaera do not mate at random. In the species from the Mediterranean group, J. italica and. J. nordmanni, large males and medium sized females are at an advantage and their sizes are positively assorted. These effects are attributable to sexual competition between males. In the Ponlo-caspian species J. istri, no advantage of large males exists, but sexual selection could be the cause for a long passive phase prior to copulation and for normalizing selection upon female size at pairing. In the Atlantic species, J. albifrons, no selection can be ascertained.
Differential mating success in males appears as one of the causes of the evolution of sexual dimorphism in body size, which makes males larger, of equal size, or smaller than females according to the species. The reason for this reversal in dimorphism seems to differ in the two sexes. Sexual selection provides an explanation for the evolution of male size, while the interspecific changes in female length are more likely due to ecological factors.     

Optimal age and size at maturity in annuals and perennials with determinate growth

Summary A model predicting optimal age and size at maturity is presented, exploring the conflict between growth and energy allocation to reproduction. According to the model, the factors promoting delayed maturity and large adult body size are as follows: (1) high rate of somatic growth, (2) high percentage increase in reproductive rate with body size increase, (3) long life expectancy at maturity for annuals or large number of expected productive days (when either growth or reproduction is possible) for perennials with growth ceasing at maturity, (4) life expectancy increasing with body size. All these factors are combined in the mathematical formula predicting optimal age and size at maturity, which allows for quantitative predictions. The optimal schedule of growth and reproduction may be achieved by natural selection, developmental plasticity, or when one species replaces another. Sexual size dimorphism is also discussed, resulting from different optimal age at maturity for either sex.     

Timing of ornament growth, phenotypic variation, and size dimorphism in two promiscuous African whydahs (Ploceidae: Vidua)

Variation within populations is a prerequisite for the action of selection on morphological traits. Darwin assumed that there was much greater variation in sexual ornament size than in body size, but this may not be generally true of natural populations. I analyse field data on variation in body size and the length, area and mass of tail ornaments in paradise (Vidua paradisaea) and shaft-tailed whydahs (V. regia). Whydahs are promiscuous, brood parasitic African finches with elaborate tail ornaments in breeding males. The short, unadorned tails of male shaft-tailed whydahs, which carry a wire-like tail ornament, are non-significantly (1%) longer than female tails, but male paradise whydahs, which carry a large, broad ornament, have unadorned tails 10% longer than those of conspecific females. Fully grown ornament length, mass and area vary little more (CVs = 1.8-6.4%) than male or female body size traits (CVs = 1.7-6.1%). Instead, there is high variation in the timing of ornament development during prenuptial moult (CVs = 30.8–39.5% for paradise whydahs and 12.6–23.8% for shaft-tailed whydahs when corrected to a standard date). This temporal variation in development probably has greater significance for sexual selection in whydahs than maximum ornament size.     

Sex differences in behaviour as an indirect consequence of mating system

A considerable literature has been devoted to documenting differences between the sexes. However, relatively little attention has hitherto been directed towards those differences that arise as an indirect consequence of mating system even though they can have profound implications for the daily lives of the animals involved. In this review we focus on differences in the non-reproductive behaviour of fish and relate these to sexual dimorphism in size and morphology, and to variance in fitness between the sexes. In line with our expectation, differences in distributional ecology, schooling, aggression, predator avoidance and foraging are exaggerated in sexually dimorphic species and polygamous mating systems. Nonetheless, the behaviour of males and females may also differ in sexually monomorphic and monogamous species. We conclude by highlighting promising directions for further research.     

Sexual bimaturation and sexual size dimorphism in animals with asymptotic growth after maturity

Many animal taxa exhibit a positive correlation between sexual size dimorphism and sex differences in age at maturity, such that members of the larger sex mature at older ages than members of the smaller sex. Previous workers have suggested that sexual bimaturation is a product of sex differences in growth trajectories, but to date no one has tested this hypothesis. The current study uses growth-based models to study relationships between sexual size dimorphism and sexual bimaturation in species with asymptotic growth after maturity. These models show that sex differences in asymptotic size would produce sexual bimaturation even if both sexes approach their respective asymptotic sizes at the same age, mature at the same proportion of asymptotic size and have otherwise equivalent growth and maturation patterns. Furthermore, our analyses show that there are three ways to reduce sexual bimaturation in sexually size-dimorphic species: (1) higher characteristic growth rates for members of the larger sex, (2) larger size at birth, hatching or metamorphosis for members of the larger sex or (3) smaller ratio of size at maturity to asymptotic size (relative size at maturity) for members of the larger sex. Of these three options, sex differences in relative size at maturity are most common in size-dimorphic species and, in both male-larger and female-larger species, members of the larger sex frequently mature at a smaller proportion of their asymptotic size than do members of the smaller sex. Information about the growth and maturation patterns of a taxon can be used to determine relationships between sexual size dimorphism and sexual bimaturation for the members of that taxon. This process is illustrated for Anolis lizards, a genus in which both sexes exhibit the same strong correlation (r 0.97) between size at maturity and asymptotic size, and in which the relative size at maturity is inversely related to asymptotic size for both sexes. As a result, sexually size-dimorphic species of anoles exhibit the expected pattern of a smaller relative size at maturity for members of the larger sex. However, for species in this genus, sex differences in the relative size at maturity are not strong enough to produce the same age at maturity for both sexes in sexually size-dimorphic species. Members of the larger sex (usually males) are still expected to mature at older ages than members of the smaller sex in Anolis lizards.     

Spatiotemporal variation in selection on body size in the dung fly Sepsis cynipsea

Standardized measures of the strength of selection on a character allow quantitative comparisons across populations in time and space. Spatiotemporal variation in selection influences patterns of adaptation and the evolution of characters and must therefore be documented. For the dung-breeding fly Sepsis cynipsea, we document patterns of variation in sexual, fecundity and larval and adult viability selection on body size at several spatiotemporal scales: between-populations, over the season, over the day and between dung pats. Adult viability selection based on residual physiological survivorship in the laboratory was nil or weakly negative. In contrast, larval viability selection in two laboratory environments was weakly positive for males at low competition and females at high competition. Fecundity selection was positive and strong at all times and in all populations. Sexual selection reflecting pairing success was overall strongly positive (about three times stronger than fecundity selection), while selection reflecting male reproductive success via the clutch size of his mate (i.e. assortative mating) was essentially nil. Only sexual selection varied significantly at coarse (between populations and seasonally) but not at fine (within a day or between pats on a pasture) spatial and temporal scales. Quadratic and correlational selection differentials were low and inconsistent in all episodes except for fecundity selection, where there was some evidence that clutch size reaches an asymptote at large body sizes, implying weaker selection for large size as females get bigger. Implications of these results for the evolution of body size and body size dimorphism are discussed.     

Macroevolutionary patterns of bumblebee body size: detecting the interplay between natural and sexual selection

Bumblebees and other eusocial bees offer a unique opportunity to analyze the evolution of body size differences between sexes. The workers, being sterile females, are not subject to selection for reproductive function and thus provide a natural control for parsing the effects of selection on reproductive function (i.e., sexual and fecundity selection) from other natural selection. Using a phylogenetic comparative approach, we explored the allometric relationships among queens, males, and workers in 70 species of bumblebees (Bombus sp.). We found hyperallometry in thorax width for males relative to workers, indicating greater evolutionary divergence of body size in males than in sterile females. This is consistent with the hypothesis that selection for reproductive function, most probably sexual selection, has caused divergence in male size among species. The slope for males on workers was significantly steeper than that for queens on workers and the latter did not depart from isometry, providing further evidence of greater evolutionary divergence in male size than female size, and no evidence that reproductive selection has accelerated divergence of females. We did not detect significant hyperallometry when male size was regressed directly on queen size and our results thus add the genus Bombus to the increasing list of clades that have female-larger sexual size dimorphism and do not conform to Rensch's rule when analyzed according to standard methodology. Nevertheless, by using worker size as a common control, we were able to demonstrate that bumblee species do show the evolutionary pattern underlying Rensch's rule, that being correlated evolution of body size in males and females, but with greater evolutionary divergence in males.     

Correlates of sexual dimorphism in primates: Ecological and size variables

The effects of a series of ecological and size factors on the degree of sexual dimorphism in body weight and canine size were studied among subsets of 70 primate species. Variation in body-weight dimorphism can be almost entirely attributed to body weight (83% of variance R² of weight dimorphism). Much smaller amounts of the variation can be attributed to mating system (R² =6.8%,polygynous species being more dimorphic than monogamous ones) and diet (R² = 2.5%,frugivorous species being more dimorphic than folivorous ones). Habitat (arboreal vs. terrestrial) and activity rhythm (nocturnal vs. diurnal) have only an indirect effect on weight dimorphism. Variation in canine-size dimorphism can be explained in terms of canine size (R² =49%),activity rhythm (R² = 20%,diurnal species being more dimorphic than nocturnal ones), and mating system (R² = 10%).Habitat and diet do not play a significant role in canine-size dimorphism. The unexpectedly high contribution of size to sexual dimorphism coupled with the observation of increased sexual dimorphism with increased size leads us to formulate a new selection model for the evolution of sexual dimorphism. We suggest that if there is selection for size increase, whatever its cause, directional selection in both males and females will lead to an increase in sexual dimorphism based on differences in genetic variance between the sexes. Sexual selection, resource division between the sexes, or lopsided reproductive selection need not play a role in such a model.     

Body size and sexual size dimorphism in calanoid copepods

Patterns of sexual size dimorphism and body size in calanoid copepods are examined. We hypothesize that favorable conditions for development will result in large body size and high sexual size dimorphism among populations of a given species and that differences in this allometric relationship among species is governed by the male's role in insemination. We confirm that there is a greater advantage to large female size, normally the larger sex, when compared to males, hence leading to selection for developmental patterns favoring high size dimorphism. Individuals from populations of four centropagid copepod species were measured; other sizes were obtained from published sources. In the four species we examined, the relationships between prosome length and both clutch size and the ability to produce multiple clutches with one insemination were determined. Results show a trend toward hyperallometry in all centropagid species examined: sexual size dimorphism increases with increasing size. Large females produce larger clutches and more additional clutches on one insemination. That hyperallometry is not observed in diaptomid copepods may result from the greater role the male plays in reproduction. Males are needed for each clutch produced, hence the selective pressure to be larger is greater than that in the centropagidae.     

A developmental perspective on the evolution of sexual size dimorphism of a moth

Males and females of almost all organisms exhibit sexual differences in body size, a phenomenon called sexual size dimorphism (SSD). How the sexes evolve to be different sizes, despite sharing the same genes that control growth and development, and hence a common genetic architecture, has remained elusive. Here, we show that the genetic architecture (heritabilities and genetic correlations) of the physiological mechanism that regulates size during the last stage of larval development of a moth, differs between the sexes, and thus probably facilitates, rather than hinders, the evolution of SSD. We further show that the endocrine system plays a critical role in generating SSD. Our results demonstrate that knowledge of the genetic architecture underlying the physiological process during development that ultimately produces SSD in adults can elucidate how males and females of organisms evolve to be of different sizes.     

Sex differences in metabolic rates in field crickets and their dipteran parasitoids

Sex differences in metabolic rate (MR) can result from dimorphism in the performance of energetically demanding activities. Male crickets (Teleogryllus oceanicus) engage in costly calling and aggressive activity not performed by females. Consistent with this difference, we found higher maximal MR, factorial scope, and fat content in males than females. T. oceanicus song is also costly because it attracts the parasitoid fly Ormia ochracea. Parasitized crickets had reduced maximal MR consistent with a metabolic cost to harboring larvae. This cost was greater for females, either because females invest more heavily into reproduction at the expense of metabolic capacity, or because males are under stronger selection to respond to infection. Little is known about O. ochracea outside of its auditory system and parasitic lifestyle. We observed greater resting MR in male flies, possibly reflecting a sex difference in the requirement for metabolic power output, because male flies perform potentially costly mating behavior not seen in females. We found a positive relationship between larval density within a cricket and pupal resting MR, suggesting that crickets in good condition are able to both harbor more larvae and produce larvae with higher resting MR. These results reveal a complex interplay between the metabolism of crickets and their fly parasitoids.Communicated by G. Heldmaier     

Early maternal, genetic and environmental components of antioxidant protection, morphology and immunity of yellow-legged gull (Larus michahellis) chicks

Maternal effects mediated by egg quality are important sources of offspring phenotypic variation and can influence the course of evolutionary processes. Mothers allocate to the eggs diverse antioxidants that protect the embryo from oxidative stress. In the yellow-legged gull (Larus michahellis), yolk antioxidant capacity varied markedly among clutches and declined considerably with egg laying date. Analysis of bioptic yolk samples from clutches that were subsequently partially cross-fostered revealed a positive effect of yolk antioxidant capacity on embryonic development and chick growth, but not on immunity and begging behaviour, while controlling for parentage and common environment effects. Chick plasma antioxidant capacity varied according to rearing environment, after statistically partitioning out maternal influences mediated by egg quality. Thus, the results of this study indicate that egg antioxidants are important mediators of maternal effects also in wild bird populations, especially during the critical early post-hatching phase.