Larger Sex Research Articles

Sex‐specific effects of altered competition on nestling growth and survival: an experimental manipulation of brood size and sex ratio

1. An increase of competition among adults or nestlings usually negatively affects breeding output. Yet little is known about the differential effects that competition has on the offspring sexes. This could be important because it may influence parental reproductive decisions. 2. In sexual size dimorphic species, two main contradictory mechanisms are proposed regarding sex-specific effects of competition on nestling performance assuming that parents do not feed their chicks differentially: (i) the larger sex requires more resources to grow and is more sensitive to a deterioration of the rearing conditions ('costly sex hypothesis'); (ii) the larger sex has a competitive advantage in intra-brood competition and performs better under adverse conditions ('competitive advantage hypothesis'). 3. In the present study, we manipulated the level of sex-specific sibling competition in a great tit population (Parus major) by altering simultaneously the brood size and the brood sex ratio on two levels: the nest (competition for food among nestlings) and the woodlot where the parents breed (competition for food among adults). We investigated whether altered competition during the nestling phase affected nestling growth traits and survival in the nest and whether the effects differed between males, the larger sex, and females. 4. We found a strong negative and sex-specific effect of experimental brood size on all the nestling traits. In enlarged broods, sexual size dimorphism was smaller which may have resulted from biased mortality towards the less competitive individuals i.e. females of low condition. No effect of brood sex ratio on nestling growth traits was found. 5. Negative brood size effects on nestling traits were stronger in natural high-density areas but we could not confirm this experimentally. 6. Our results did not support the 'costly sex hypothesis' because males did not suffer from higher mortality under harsh conditions. The 'competitive advantage hypothesis' was also not fully supported because females did not suffer more in male-biased broods. 7. We conclude that male nestlings are not likely to be more expensive to raise, yet they have a size-related competitive advantage in large broods, leading to higher mortality of their on average lighter female nest mates.

Size-dependent energetics of metamorphosis in the yellow dung fly, Scathophaga stercoraria

Competing hypotheses: (1) Large body size confers more efficient energy use (relative efficiency hypothesis). (2) Large body size requires more energy to be sustained, a disadvantage when food is limited (absolute energy demand hypothesis). Organism: Yellow dung flies, Scathophaga stercoraria (Diptera: Scathophagidae), artificially selected for large and small body size in the laboratory for 24 generations to augment the available phenotypic body sizes. Methods: Larvae were reared under limited and unlimited food (dung) conditions, and the energy content of pupae was measured at the beginning and the end of the pupal stage in different, size-matched individuals. Conclusions: Over the pupal period, lipids and glycogen decreased whereas sugar content increased. Net energy loss per unit body mass was higher at unlimited food. Contrary to expectation, males (the larger sex) lost less energy than females. Large selection line pupae showed the highest absolute and mass-specific energy loss during metamorphosis, indicating a correlated physiological response to body size selection because phenotypic body sizes do not differ between the lines at limited dung. We conclude that energetic costs due to greater absolute energy demand of larger individuals during the pupal phase outweigh the benefits due to greater metabolic efficiency.

Alcohol Use Trends in South Africa

The purpose of this review is to synthesize the prevalence data from five national surveys and local surveys on alcohol use in South Africa over the past 12 years. Systematic review. Results indicate that life time, current (past month) use and binge drinking remained similar over the years for both adolescents and adults. Binge drinking was between 7-11% and risky drinking also remained stable over the years (6%) with larger sex, geographic and racial differences. Risky drinking in pregnant women was 2.5% nationally but particularly high among urban dwellers (4.1%), Coloureds (11.6%) and in the Northern Cape province (24.9%). Local surveys among adolescents, university students, clinic populations and mine employees seem to all generally indicate higher levels of risky drinking than in the national surveys. The burden of alcohol was found to be high: Hazardous or harmful drinking and binge drinking were associated with multiple/indiscriminate (irregular) sex partners among persons living with HIV; alcohol related death in transport and homicide of 50% and fetal alcohol syndrome from 10-74 per 1000 in local surveys. Alcohol production/per capita remained stable with 8 litres but there is relatively high alcohol consumption considering an additional 3-4 litres unrecorded production/consumption, and that high amounts are consumed by a small population since most abstain from drinking in South Africa.Although no significant increase of alcohol use over the past 12 years was found, a high burden of alcohol abuse was found.

Sexual size dimorphism in Orthoptera (sens. str.) — a review

Sexual size dimorphism (SSD) is a common phenomenon in animal taxa. While males are the larger sex in many birds and mammals, female-biased SSD predominates among insects, including Orthoptera. We analyzed size differences of 1503 Orthoptera species, suggesting that SSD is rather uniform in Ensifera, with the females being on average 9% larger than the males (ranging from −20 to 40%). In contrast, SSD is usually much stronger in Caelifera (37%) and also more variable (ranging from −20 to 140%). Caelifera with larger females exhibit stronger size differences than smaller species, whereas in Ensifera SSD decreases with male body size, but is not related to female size. Sexual size differences in Orthoptera are usually associated with a higher number of nymphal instars in females, leading to an earlier emergence of adult males (protandry). Both growth rates and the number of instars seem to be affected by genetic and environmental cues. Two major hypotheses have been proposed to explain the ultimate causes for SSD: the intersexual competition hypothesis and the differential equilibrium hypothesis. The first suggests that sexual dimorphism is a mechanism to reduce intraspecific competition, enabling the sexes to specialize on different food items. The differential equilibrium hypothesis proposes that the different body sizes represent sex-specific fitness optima, which are caused by their specific life-history strategies. Females may maximize their reproductive success by increasing the number (or size) of eggs (fecundity selection), whereas males may maximize their reproduction by being more mobile and fertilizing many females in a short period of time. These fundamental differences in the life-history strategies of the sexes may also lead to sexual selection, which has sometimes been referred to as an additional hypothesis. There is still a need for more empirical research on the ultimate causes for SSD. At present, there is much more support for the differential equilibrium hypothesis, but the intersexual competition hypothesis has rarely been tested. We propose some experimental approaches to test both hypotheses in micro- and macroevolutionary contexts.

Contrasting patterns of sexual size dimorphism in the grasshoppers Dichroplus vittatus and D. pratensis (Acrididae, Melanoplinae)*

Abstract Sexual size dimorphism (SSD) can be the result of sexual selection (SS) or natural selection (NS). Due to male-male competition for access to females, SS could favor an increase in male body size. On the other hand, larger size in females could be favored by NS, since egg production is directly correlated with body size. Rensch`s rule states that SSD increases with increasing body size in animals, where males are the larger sex, and decreases when females are larger than males. Thus, Rensch's rule predicts that in those insects where females are larger than males, SSD should decrease with increasing body size, when comparing populations and species. We analyzed SSD in 19 Argentine populations of the grasshoppers Dichroplus vittatus and 25 of D. pratensis. Both species show latitudinal and altitudinal variation in body size, following the converse to Bergmann's rule: body size decreases with increasing latitude and decreasing ambient temperature. SSD occurs in both species across their geographica...

Morphometrics within dog breeds are highly reproducible and dispute Rensch’s rule

Using 27 body measurements, we have identified 13 breed-defining metrics for 109 of 159 domestic dog breeds, most of which are recognized by the American Kennel Club (AKC). The data set included 1,155 dogs at least 1 year old (average 5.4 years), and for 53 breed populations, complete measurement data were collected from at least three males and three females. We demonstrate, first, that AKC breed standards are rigorously adhered to for most domestic breeds with little variation observed within breeds. Second, Rensch's rule, which describes a scaling among taxa such that sexual dimorphism is greater among larger species if males are the larger sex, with less pronounced differences in male versus female body size in smaller species, is not maintained in domestic dog breeds because the proportional size difference between males and females of small and large breeds is essentially the same. Finally, principal components (PCs) analysis describes both the overall body size (PC1) and the shape (length versus width) of the skeleton (PC2). That the integrity of the data set is sufficiently rich to discern PCs has strong implications for mapping studies, suggesting that individual measurements may not be needed for genetic studies of morphologic traits, particularly in the case of breed-defining traits that are typically under strong selection. Rather, phenotypes derived from data sets such as these, collected at a fraction of the effort and cost, may be used to direct whole-genome association studies aimed at understanding the genetic basis of fixed morphologic phenotypes defining distinct dog breeds.

Rensch’s Rule in <I>Dichroplus pratensis</I>: A Reply to Wolak

Rensch’s Rule in <I>Dichroplus pratensis</I>: A Reply to Wolak

Rensch’s Rule Applies to Clinal Variation of Body Size in the Argentine Grasshopper <I>Dichroplus pratensis</I>: Correction of Bidau and Martí

In their article entitled “Clinal Variation of Body Size in Dichroplus pratensis (Orthoptera: Acrididae): Inversion of BergmannOs andRenschOs Rules,”Bidau and Martṍ (2007) published results in this journal about clinal population data from 1998 to 2001. In the sexual size dimorphism (SSD) analysis of the grasshopper Dichroplus pratensis Bruner, they incorrectly conclude the 19 populations studied, encompassing 22 of latitude in Argentina, exhibit the inverse of RenschOs rule (Rensch 1960). The authors explain RenschOs rule as stating “that in species where males are larger than females, sexual dimorphism increases with size” (Bidau and Martṍ 2007). Although this is correct, it is an incomplete deÞnition. In his work, Rensch (1960) describes a general pattern to relate SSD to the body size of animals: “In species of birds inwhich themale is larger than the female, the relative sexual difference increases with body size. If by way of exception, the females are larger than the males, as among many species of birds of prey, the opposite correlation applies, i.e., the greater sexual difference is found in the smaller species” (p. 159). RenschOs rule is fully deÞned as the allometry observed when SSD increases with size if males are the larger sex, butwhenSSDdecreaseswith size if females are the larger sex (Rensch 1960, Fairbairn 1990 Abouheif and Fairbairn 1997, Fairbairn 1997). Allometry consistent with RenschOs rule is represented by the slope of a model II regression being 1 ( 1) when log female size is regressed on logmale size, regardless of which sex is larger (Abouheif and Fairbairn 1997). Note that if the axes are reversed (male size regressed on female size),RenschOs rule is representedbya slope of 1 ( 1) (Fairbairn 1997). More generally, RenschOs rule occurs when more variance in male body size than female body size is observed (Fairbairn 1997, Blanckenhorn et al. 2007). The results fromBidau andMartṍ (2007) indicateD. pratensis exhibits female-biased SSD (females larger thanmales). Their data demonstrate “. . . the degree of SSD decreases as mean size of both sexes increase,” thus actually conforming to RenschOs rule. Furthermore, in Fig. 3f they regress log female on log male body length (axes similar to Þg. 1 of Abouheif and Fairbairn 1997) and Þnd a slope of 0.5718. This value is unequivocally 1, but for this study to demonstrate an inverse of RenschOs rule the slope of the regression line would need to be 1 (Abouheif and Fairbairn 1997). The authors did not state whether they used a model I or model II regression in their analyses. If a model I regression was used, reanalyzing thedatawithpropermodel IImethodsmaychange the signiÞcance of the results. The study of Bidau and Martṍ (2007) clearly demonstrates a case where empirical evidence supports RenschOs rule in D. pratensis. Due to a misinterpretation of RenschOs rule, the authors wrongly cite their work as an example of the inverse to RenschOs rule.

Size dimorphism in Rankinia [Tympanocryptis] diemensis (Family Agamidae): sex-specific patterns and geographic variation

Sexual dimorphism has implications for a range of biological and ecological factors, and intersexual morphological differences within a species provide an ideal opportunity for investigating evolutionary influences on phenotypic variation. We investigated sexual size dimorphism (SSD) in an agamid species, Rankinia [Tympanocryptis] diemensis, to determine whether overall size and/or relative morphological trait size differences exist and whether geographic variation in size dimorphism occurs in this species. Relative morphological trait proportions included a range of head, limb, and inter-limb measurements. We found significant overall intersexual adult size differences; females were the larger sex across all sites but the degree of dimorphism between the sexes did not differ between sites. This female-biased size difference is atypical for agamid lizards, which are usually characterized by large male body size. In this species, large female-biased SSD appears to have evolved as a result of fecundity advantages. The size of relative morphological trait also differed significantly between the sexes, but in the opposite direction: relative head, tail, and limb sizes were significantly larger in males than females. This corresponds to patterns in trait size usually found in this taxonomic group, where male head and limb size is important in contest success such as male–male rivalry. There were site-specific morphological differences in hatchlings, including overall body size, tail, inter-limb, thigh, and hindlimb lengths; however, there were no sex-specific differences indicating the body size differences present in the adult form occur during ontogeny.

Male or female soldiers? An evaluation of several factors which may influence soldier sex ratio in lower termites

In termites, the soldiers’ sex ratio is often biased toward one sex. Unlike in the Hymenoptera, this bias cannot easily be explained by relatedness asymmetries because termites are diploid. Matsuura proposed that when large body size is adaptive for colony defence (e.g. in case of phragmotic defence) then the larger sex (given sexual size dimorphism exists) should be more likely to reach a threshold size and develop into soldiers. This would explain biased sex ratios. Matsuura validated his hypothesis for four Reticulitermes species. Here, we tested his hypothesis for two species of Cryptotermes with phragmotic defence. These drywood termites have a life type that is thought to be ancestral in termite’s evolution, thus giving us potential insights into the evolution of the soldier caste. In one of these species, the sex ratio of soldiers was highly female biased, but we could not support Matsuura’s hypothesis. Both species lacked sexual size dimorphism in all castes. Additionally, in both species, the sex ratio of helpers and sexuals did not deviate from a 1:1 ratio, and hence can also not account for the bias observed in soldiers. However, this study showed that there were behavioural differences between the sexes in both species, which could shed some light on biased sex ratio in soldiers. Our findings also indicate that the developmental pathway taken by individuals reflects a ‘decision’ at the colony level. The discovery of behavioural differences between sexes in termites should open the way to similar studies in other taxa with helpers/ workers of both sexes, as it might reveal more task partitioning in colonies than previously thought and it raises questions concerning the selective pressures that acted on caste evolution in termites.

Variation in the magnitude of sexual size dimorphism in nestling Coal Tits (Periparus ater)

The magnitude of sexual size dimorphism can be affected by sex differences in environmental sensitivity early in ontogeny that result in differential growth rates of male and female nestlings. Here, the larger sex might either be more sensitive because of higher food demands or less sensitive due to greater competitive ability. When environmental conditions deteriorate during the breeding season, this “environmental stress” hypothesis predicts differential seasonal declines in the performance of male and female offspring. Based on a sample of molecularly sexed Coal Tit (Periparus ater) nestlings from 2 years, we investigated sexual size dimorphism in body mass, condition (i.e. size-corrected mass), tarsus and wing length and whether its magnitude changed from early to late broods. Male offspring were heavier, larger (in terms of tarsus and wing length) and had higher size-corrected mass than their female nest mates (the same was evident in adult breeders). In 2002 (the year with the longer effective breeding season), body mass and condition declined with progressing hatching date and this effect was significantly more pronounced in male than in female nestlings. There was also a seasonal decline in male wing length, while female wing length remained relatively constant, which resulted in males having shorter wings than females in late broods. Tarsus length was unaffected by time of breeding, except that the difference between males and females was relatively smaller in late (i.e. second) broods in 2002. While these results are in accordance with the idea of an increased environmental sensitivity of the larger males, confounding effects of sex-differential hatching order cannot be ruled out.

A test of Rensch’s rule in dwarf chameleons (Bradypodion spp.), a group with female-biased sexual size dimorphism

Rensch’s rule describes a pattern of allometry in sexual size dimorphism (SSD): when males are the larger sex (male-biased SSD), SSD increases with increasing body size, and when females are the larger sex (female-biased SSD), SSD decreases with increasing body size. While this expectation generally holds for taxa with male-biased or mixed SSD, examples of allometry for SSD consistent with Rensch’s rule in groups with primarily female-biased SSD are remarkably rare. Here, I show that the majority of dwarf chameleons (Bradypodion spp.) have female-biased SSD. In accordance with Rensch’s rule, the group exhibits an allometric slope of log(female size) on log(male size) less than one, although statistical significance is dependent on the phylogenetic comparative method used. In this system, this pattern is likely due to natural selection on both male and female body size, combined with fecundity selection on female body size. In addition to quantifying SSD and testing Rensch’s rule in dwarf chameleons, I discuss reasons why Rensch’s rule may only rarely apply to taxa with female-biased SSD.

Manipulating rearing conditions reveals developmental sensitivity in the smaller sex of a passerine bird, the European starling Sturnus vulgaris

Traditionally, studies of sexually size‐dimorphic birds and mammals report that the larger sex is more sensitive to adverse environmental conditions during ontogeny. However, recent studies in avian species that exhibit moderate size‐dimorphism indicate that the smaller sex may be more sensitive to poor rearing conditions. To better understand sex‐specific sensitivity in a passerine exhibiting moderate size‐dimorphism, we examined growth, cell‐mediated immunity (CMI) and survival of European starling Sturnus vulgaris nestlings following an experimental reduction of maternal rearing ability (via a feather‐clipping manipulation). Contrary to conventional theory, daughters showed reduced growth in both body mass and measures of structural size in response to the maternal treatment. In contrast, sons showed no reductions in any of these traits in relation to the treatment. No sex‐specific differences in nestling CMI were found for either group, although CMI of nestlings raised by manipulated mothers were higher than those of control nestlings. Finally, fledging sex ratios did not change from those at hatching indicating that neither sex appeared differentially sensitive to the maternal treatment in terms of mortality. These results reveal that variation in the quality of the rearing environment can have significant effects on the smaller sex of a passerine exhibiting moderate dimorphism and as such support recent studies of species with small‐moderate sexual size‐dimorphism. Combined results suggest that sex‐specific effects of environmental variation on nestling development may be both context‐ (i.e., brood size, resource level, hatching order) and temporally‐ (when during development they occur) specific. Furthermore, more studies are needed that examine multiple traits at several developmental stages and then follow the sexes over the longer‐term to examine potential effects on fitness.

Sex-specific development of avian flight performance under experimentally altered rearing conditions

Numerous studies have examined predation risk resulting from the costs of impaired flight performance associated with many key life-history stages such as reproduction and migration. Interestingly, although avian nestlings experience multiple resourcebased physiological trade-offs and undergo considerable morphological and physiological changes during postnatal development, there is no data available on how nestlings manage the competing demands of growth and the development of flight ability at this critical life-history stage. We examined numerous morphological traits to determine which are responsible for variation in flight performance in juvenile European starlings (Sturnus vulgaris), a sexually size-dimorphic passerine. We then manipulated maternal quality during chick rearing (via feather clipping) to examine sex-specific sensitivity of fledgling flight performance to the quality of the rearing environment. Results suggest that the mechanics underlying variation in juvenile flight performance are relatively simple, being principally determined by the ratio of pectoral muscle mass to body mass (BM) and the surface area of the wings. Interestingly, although the maternal quality manipulation decreased BM and structural size in daughters, only the flight performance of sons was negatively affected. Our results suggest that a survival-related trait can be significantly affected in the larger sex when raised under stressful conditions. Furthermore, measuring only BM and structural size may not be sufficient in understanding how the sexes are affected by stressful rearing conditions in sexually size-dimorphic species. Key words: development, escape performance, european starling, flight ability, predation risk, sexual size dimorphism. [Behav Ecol]

The evolutionary genetics of sexual size dimorphism in the cricket Allonemobius socius

In recent years, investigations into the evolution of sexual size dimorphism have moved from a simple single trait, single sex perspective, to the more robust view of multivariate selection acting on both males and females. However, more accurate predictions regarding selection response may be possible if some knowledge of the underlying sex-specific genetic architecture exists. In the striped ground cricket, Allonemobius socius, females are the larger sex. Furthermore, body size appears to be closely associated with fitness in both males and females. Here, we investigate the role that genetic architecture may play in affecting this pattern. Employing a quantitative genetic approach, we estimated the sex-specific selection gradients and the (co)variance matrix for body size and wing morphology (that is, either a long-winged flight-capable phenotype or a short-winged flightless phenotype) to predict phenotypic change in the next generation. We found that the sexes differed significantly in their selection gradients as well as several of their genetic parameters. Our predictions of next-generation change indicated that the within-sex genetic correlations, as well as the between-sex genetic correlations, should play a significant role in sexually dimorphic evolution in this system. Specifically, the female size response was increased by approximately 178% when the between-sex genetic correlations were considered. Thus, our predictions reinforce the notion that genetic architecture can produce counterintuitive responses to selection, and suggest that even a complete knowledge of the selection pressures acting on a trait may misrepresent the trajectory of trait evolution.

Altitudinal variation of the sex ratio and segregation by gender in the dioecious plant Mercurialis perennis L. (Euphorbiaceae) in Serbia

The sex ratio is one of the most intriguing problems for evolutionary biologists. Spatio-temporal variation of male frequency in sexually dimorphic plant species, spatial segregation, and differential investment of the two sex functions have attracted much research interest. In this study, we examined altitudinal variation of the sex ratio and segregation by gender in Mercurialis perennis (dog?s mercury), a dioecious anemophilous species with wide distribution. The eight studied populations from Serbia represented an altitudinal range of 196 to 1480 m. Sex ratio was significantly biased in seven of the eight populations, with males outnumbering females 3.91:1 in the lowest-altitude population. Our results support the notion of spatial segregation of sexes along on altitudinal gradient: the frequency of males decreased with altitude, from 79.6% to 41.0%. The sex ratio was not significantly correlated with population density. We also examined intersexual differences of plant height in two populations from the extremes of the studied altitudinal range. Males were the larger sex in both populations, though the difference was not significant in the high-altitude population. .

Sexual dimorphism, female reproduction and egg incubation in the oriental leaf-toed gecko ( Hemidactylus bowringii) from southern China

We studied sexual dimorphism, female reproduction and egg incubation of the oriental leaf-toed gecko ( Hemidactylus bowringii) from a population in southern China. The largest male and female in our sample were 60 and 57 mm snout-vent length (SVL), respectively. Males are the larger sex; sexual dimorphism in head size and tail length (TL) is evident in juveniles and adults, with males having larger heads as well as longer tails than females. Oviposition occurred between late May and late July. Females switched from laying two eggs early in the breeding season to 1–2 eggs later in the season. Clutch mass and egg mass were both independent of female SVL, whereas relative clutch mass was negatively correlated with female SVL. The previous conclusion that female H. bowringii lay a single clutch of eggs per breeding season is unlikely to be true. Thermal environments experienced by H. bowringii eggs affect incubation length as well as morphological and locomotor phenotypes of hatchlings. Hatchlings from eggs incubated at 30 °C were larger (SVL, tail length and body mass) and performed better in the racetrack than their counterparts from eggs incubated at 24 °C. Temperatures suitable for embryonic development are relatively high in H. bowringii, primarily as a consequence of the adaptive response to warm environments in southern China.

Parental food conditions affect sex-specific embryo mortality in the yellow-legged gull (Larus michahellis)

Different mortality of males and females during early post-hatching development in sexually size-dimorphic bird species is usually attributed to different nutritional requirements of the sexes, because mortality is mostly biassed toward the larger sex. We investigated whether sex-specific embryo mortality in the yellow-legged gull (Larus michahellis), a size-dimorphic seabird, depends on parental condition. To test this, we experimentally modified parental nutritional conditions by supplementary feeding of yellow-legged gulls during egg formation, to evaluate sex-biassed environmental sensitivity of gull embryos. We found that eggs were larger in supplemented clutches, but egg size did not affect embryo survival. Survival of male gull embryos was more related to parental food conditions than was survival of female embryos. Survival of male embryos in supplemented clutches was greater than in unsupplemented clutches whereas survival of female embryos was similar in both groups. Because size at hatching was similar in both sexes our results suggest that male phenotype disadvantage is not exclusively linked to the energy demands of size-dimorphic development at the embryo stage.

Long‐term paternity skew and the opportunity for selection in a mammal with reversed sexual size dimorphism

Most mammalian groups are characterized by male-biased sexual size dimorphism, in which size-dependent male-male competition and reproductive skew are tightly linked. By comparison, little is known about the opportunity for sexual selection in mammalian systems without male-biased dimorphism, where the traits under sexual selection might be less obvious. We examined 10 years of parentage data in a colony of greater horseshoe bats (Rhinolophus ferrumequinum) to determine the magnitude of male reproductive skew and the opportunity for sexual selection in a mammal in which females are the larger sex. Annual paternity success was weakly skewed but consistent patterns led to strong longitudinal paternity skew among breeders. Just three males accounted for a third of all paternity assignments, representing at least a fifth of all colony offspring born in a decade. Paternity success was in part determined by age but was not influenced by dispersal status. Our results show that paternity skew and the opportunity for sexual selection in a species with reversed sexual size dimorphism can approach levels reported for classical examples of species with polygyny and male-biased dimorphism, even where the traits under sexual selection are not known.

Hatching order and size-dependent mortality in relation to brood sex ratio composition in chinstrap penguins

The differential environmental sensitivity of the sexes has strong implications in the evolutionary history of species as it can alter sexual size dimorphism, population sex ratios, and the faculty of parents to manipulate offspring sex in relation to environmental conditions. We studied sexual differences in hatching patterns and evaluated sex- and size-related mortality in relation to hatching order and brood sex ratios in the chinstrap penguin Pygoscelis antarctica, a moderately size-dimorphic species, with a modal clutch size of 2 eggs. We found that male, second-hatched, and large eggs showed shorter hatching periods than female, first-hatched, and small eggs. We also found a male-biased mortality of nestlings in the colony. However, male mortality patterns differed depending on the brood sex ratio composition. Mortality of male chicks in all-male broods was higher than in mixed broods and higher than female mortality in all-female broods. Contrary, females from mixed brood showed higher mortality than their male nest mates and higher too than females in all-female broods. Second-hatched chicks also suffered from higher mortality than first-hatched chicks. Our results indicate that both the superior competitive capacity and the higher energy demand of the larger sex constitute 2 causal factors explaining patterns of sex-biased mortality. Both factors occur in the same species and in different situations of sibling competition shaped by brood sex ratio composition. This study constitutes a good example of how patterns of sex-related mortality can vary depending on nest environmental circumstances. Furthermore, our study suggests that hatching period can be a mechanism underlying sexual differences in the embryonic period of birds.