Sex-specific growth patterns and effects of hatching condition on growth in the reversed sexually size-dimorphic great skua Stercorarius skua

In many sexually size-dimorphic species of birds and mammals, the larger sex, often the males, show increased environmental sensitivity during ontogeny. This is generally assumed to be due to higher energy requirements, reflected in higher absolute growth rates of the larger sex. Poor early conditions often increase the sex differences in vulnerability. However, it is not clear whether these patterns are equally pervasive in species where females are larger, as males face an additional early disadvantage due to high levels of testosterone. We investigated sex-specific growth patterns of mass, tarsus and wing of the great skua Stercorarius skua , a seabird with reversed size dimorphism. We were particularly interested in sex-specific effects of early conditions on growth. Beside data from unmanipulated nests, we present results from an egg removal experiment, which caused chicks to hatch from smaller eggs and in poorer body condition. Half of the experimental chicks were raised by pairs in which mothers were in poor body condition. At the end of the nesting period, great skua chicks exhibited a comparable degree of size dimorphism as is found in adults, although neither sex had reached final adult size. Despite females reaching larger asymptotic values of mass and tarsus, timing of growth was not different between the sexes. Absolute growth was higher for females around the time of maximum growth, which suggests that daughters face higher energetic demands. We found sex-specific effects of poor early conditions on growth patterns, although not to the extent which we had predicted. Hatching in poor body condition was related to slowed growth in females but not males. However, our experimental manipulations had no additional negative effect on growth. Our results indicate that daughters in the great skua face higher demands during growth than sons, and that early conditions are more important for the development of the larger sex in this reversed dimorphic species.     

Sex-ratio adjustment in response to local mate competition is achieved through an alteration of egg size in a haplodiploid spider mite

Sex-ratio adjustments are commonly observed in haplodiploid species. However, the underlying proximate mechanisms remain elusive. We investigated these mechanisms in Tetranychus urticae, a haplodiploid spider mite known to adjust sex ratio in response to the level of local mate competition (LMC). In this species, egg size determines fertilization probability, with larger eggs being more likely to be fertilized, and thus become female. We explored the hypothesis that sex-ratio adjustment is achieved through adjustment of egg size. By using spider mites from a large population, we found that females produced not only a higher proportion of daughters under high levels of LMC, but also larger eggs. Moreover, in populations experimentally evolving under varying levels of LMC, both the proportion of females and the egg size increased with LMC intensity. These results suggest that sex-ratio adjustment in spider mites is mediated by egg size, although the causal relationship remains to be tested.     

Female body condition and brood sex ratio in Yellow-legged Gulls Larus cachinnans

In the Yellow-legged Gull Larus cachinnans , males are the larger sex, and show more reproductive variance than females. We predicted that the proportion of male chicks in a brood should increase with female body condition. We investigated brood sex ratio by using DNA markers taken from samples of hatchlings or dead embryos, and female body condition using plasma cholesterol concentration as a reliable indicator. The brood sex ratio of females in good condition was male biased and the sex ratio of females in poor condition was female biased. This relationship was also significant in those nests where all the eggs laid were sexed. Thus, manipulation of embryo mortality cannot explain the biases reported in this study, suggesting that the sex ratio of the eggs was biased prior to laying. These results confirm that sex-ratio manipulation in gulls operates under natural conditions, and supports earlier experimental findings.     

Sex allocation in haplodiploids is mediated by egg size: evidence in the spider mite Tetranychus urticae Koch

Haplodiploid species display extraordinary sex ratios. However, a differential investment in male and female offspring might also be achieved by a differential provisioning of eggs, as observed in birds and lizards. We investigated this hypothesis in the haplodiploid spider mite Tetranychus urticae, which displays highly female-biased sex ratios. We show that egg size significantly determines not only larval size, juvenile survival and adult size, but also fertilization probability, as in marine invertebrates with external fertilization, so that female (fertilized) eggs are significantly larger than male (unfertilized) eggs. Moreover, females with on average larger eggs before fertilization produce a more female-biased sex ratio afterwards. Egg size thus mediates sex-specific egg provisioning, sex and offspring sex ratio. Finally, sex-specific egg provisioning has another major consequence: male eggs produced by mated mothers are smaller than male eggs produced by virgins, and this size difference persists in adults. Virgin females might thus have a (male) fitness advantage over mated females.     

Sex-ratio optimization with helpers at the nest

In many cooperatively breeding animals, offspring produced earlier in life assist their parents in raising subsequent broods. Such helping behaviour is often confined to offspring of one sex. Sex-allocation theory predicts that parents overproduce offspring of the helping sex, but the expected degree of sex-ratio bias was thought to depend on specific details of female and male life histories, hampering empirical tests of the theory. Here we demonstrate the following two theories. (i) If all parents produce the same sex ratio, the evolutionarily stable sex ratio obeys a very simple rule that is valid for a general class of life histories. The rule predicts that the expected sex-ratio bias depends on the product of only two parameters which are relatively easily measured: the average number of helping offspring per nest and the relative contribution to offspring production per helper. (ii) If the benefit of helping varies between parents, and parents facultatively adjust the sex ratio accordingly, then the population sex ratio is not necessarily biased towards the helping sex. For example, in line with empirical evidence, if helpers are produced under favourable conditions and parents do not adjust their clutch size to the number of helpers, then a surplus of the non-helping sex is expected.     

Sex-ratio variation and reproductive costs in relation to density in a forest-dwelling population of red deer (Cervus elaphus)

For dimorphic species in which the variance in reproductivesuccess of males is more pronounced than that of females, theoriesof adaptive variation in sex ratio predict that mothers shouldinvest more heavily in sons than in daughters. By using harvestdata from a forest-dwelling red deer population that experienceda marked reduction in population density we tested the hypothesisthat adaptive sex-ratio variation should occur only when populationsare much below carrying capacity. More specifically, we testedwhether at low density, females in better than average conditionwere more likely to produce male offspring and to invest inindividual sons rather than were females in poorer than averagecondition. We also investigated female reproductive costs arisingfrom a decrease either in body mass or in reproduction. We didnot find any support for a biased sex ratio or investment towardmale calves by high-quality mothers at any population density.Costs of reproduction in terms of body mass and pregnancy rateswere only detectable for females that reproduced as yearlingsand not for those that reproduced as adults. Our results thereforedo not support the hypothesis of adaptive sex-ratio variationin a population living below carrying capacity. The four-folddifference in party sizes (defined as the number of deer aggregatein the same party in which no individual was more than 50 mfrom any other) observed in our population living in a closedforest habitat compared with populations living in more openhabitats previously studied might account for such a discrepancy.We suggest that a smaller party size may decrease the intensityof sexual selection and could be the proximal cause for thelack of adaptive sex-ratio variation we report for the populationstudied here.     

Sex allocation in yellow-legged gulls (Larus michahellis) depends on nutritional constraints on production of large last eggs

Male and female offspring can differ in their susceptibility to pre-natal (e.g. egg quality) and post-natal (e.g. sib–sib competition) conditions, and parents can therefore increase their individual fitness by adjusting these maternal effects according to offspring sex. In birds, egg mass and laying/hatching order are the main determinants of offspring viability, but these effects can act differently on each sex. In a previous study, relatively large last-laid (c-)eggs of yellow-legged gulls (Larus michahellis) were more likely to carry a female embryo. This suggests compensatory allocation of maternal resources to daughters from c-eggs, which suffer reduced viability. In the present study, we supplemented yellow-legged gulls with food during the laying period to experimentally test whether their nutritional conditions were responsible for the observed covariation between c-egg sex and mass. As predicted, food supplementation enhanced female c-eggs'' mass more than that of male c-eggs. Thus, this experiment indicates that mothers strategically allocated their resources to c-eggs, possibly in order to compensate for the larger susceptibility of daughters to hatching (and laying) order. The results also suggested that mothers decided on resource allocation depending on the sex of already ovulated c-eggs, rather than ovulating ova of either sex depending on food availability.     

SEXUAL SIZE DIMORPHISM AND EGG-SIZE ALLOMETRY IN BIRDS

We tested the hypothesis that egg size should evolve in sexually dimorphic birds to reduce costs associated with more rapid growth by nestlings of the larger sex. Consistent with this hypothesis, we found that in species in which males were larger, females laid proportionately larger eggs as sexual size dimorphism increased. However, this result was also consistent with the hypothesis that egg size varied allometrically with both male and female body size. Furthermore we found that in species in which females were larger, relative egg size decreased as size dimorphism increased, which is consistent with the “allometry hypothesis” but not the “cost-reduction hypothesis. That male body size contributes to the allometric relationship between egg size and body size suggests that the basis for the allometric relationship is not wholly a mechanical one stemming from the physical requirements of developing, transporting, and laying an egg of a particular size. Rather, the relationship seems likely to be tied more directly to body size itself the tact that male body size influences a female trait suggests that egg size–body size relationships otter some scope for investigating the basis for allometric relationships in general.     

Evaluation of Darwin's fecundity advantage hypothesis in Parthenium beetle,Zygogramma bicolorata

In the Parthenium beetle, Zygogramma bicolorata Pallister (Coleoptera: Chrysomelidae), variation in body size exists between and within the sexes. The females are larger than the males. Darwin (1874) proposed the fecundity advantage hypothesis, that is, large‐sized females produce more progeny, with subsequent studies supporting, as well as, refuting the hypothesis. Thus, in order to evaluate whether this hypothesis stands in Z. bicolorata we performed experiments to investigate the role of body size in influencing: (i) assortative mating; (ii) reproductive attributes; and (iii) growth, development and survival of offspring. It is the first attempt in this beetle. We found that size influenced assortative mating, reproductive output and offspring fitness. Larger males and females were preferred as mates over smaller ones. The pairs, having larger adults as mates, had higher fecundity, while the egg viability was influenced by the male size only. The offspring of larger parents had fast development and higher survival, indicating thereby possible better nutrient allotment by the female and supply of accessory gland proteins by the male in addition to better quality of genes.     

Adaptive Offspring Sex Ratio Depends on Male Tail Length in the Guppy

A biased sex ratio in a brood is considered to be an adaptive strategy under certain circumstances. For example, if the expected reproductive success of one sex is greater than that of the other, parents should produce more offspring of the former sex than the latter. A previous study has documented that in the guppy, Poecilia reticulata, the female offspring of males possessing proportionally longer tails exhibit smaller body sizes and show decreased reproductive outputs than those of males having shorter tails. On the other hand, the total lengths of the male offspring of the long‐tailed males are larger because of their longer tails; consequently, they exhibit greater sexual attractiveness to females. Therefore, it has been hypothesized that this asymmetry in the expected reproductive success between the male and female offspring of long‐tailed males may result in a biased sex ratio that is dependent on the tail lengths of their fathers. This hypothesis was tested in the present study. The results showed that the females that mated with long‐tailed males produced more male offspring than those that mated with short‐tailed males. Logistic regression analysis showed that the ratio of tail length to the standard length of the fathers is a determinant factor of the sex of their offspring. These results suggest that the manipulation of the offspring sex ratios by parents enhances the overall fitness of the offspring.     

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.     

The effects of the minimum threshold condition for breeding on offspring sex-ratio adjustment in the lesser kestrel

We propose a model for sex-ratio adjustment complementary to that of Trivers and Willard. In addition to the three basic assumptions of the Trivers-Willard model, our model assumes that the sex with more variable reproductive success (normally male) is also the sex less constrained for reproduction. This assumption seems realistic, because several studies have demonstrated that poor-condition males may adopt alternative mating strategies and sire some offspring, whereas females have physiological constraints for gestation or egg production that cannot be avoided. Thus, under these circumstances, sons of both poor and good condition would be more valuable for parents than daughters, whereas daughters would be relatively more valuable than sons at intermediate condition. This model predicts, therefore, a U-shaped relationship between parental condition and offspring sex ratio. We present a case study for the monogamous lesser kestrel (Falco naumanni) that fulfills the assumptions and predictions of the model. The minimum body condition for breeding, measured as pectoral thickness, was lower for sons than for daughters. Below this minimum, males had a higher chance of breeding than females. Above this minimum, however, the lifetime reproductive success was condition dependent in males but not in females. Thus, males in better body condition attain, on average, higher reproductive success than females. Offspring sex ratio varied with the size of the father's ornaments and mother condition according to the U-shaped pattern predicted by the model.     

The Role of Sex-specific Plasticity in Shaping Sexual Dimorphism in a Long-lived Vertebrate,the Snapping Turtle <Emphasis Type="Italic">Chelydra serpentina</Emphasis>

Sex-specific plasticity, the differential response that the genome of males and females may have to different environments, is a mechanism that can affect the degree of sexual dimorphism. Two adaptive hypotheses have been proposed to explain how sex-specific plasticity affects the evolution of sexual size dimorphism. The adaptive canalization hypothesis states that the larger sex exhibits lesser plasticity compared to the smaller sex due to strong directional selection for a large body size, which penalizes individuals attaining sub-optimal body sizes. The condition-dependence hypothesis states that the larger sex exhibits greater plasticity than the smaller sex due to strong directional selection for a large body size favoring a greater sensitivity as an opportunistic mechanism for growth enhancement under favorable conditions. While the relationship between sex-specific plasticity and sexual dimorphism has been studied mainly in invertebrates, its role in long-lived vertebrates has received little attention. In this study we tested the predictions derived from these two hypotheses by comparing the plastic responses of body size and shape of males and females of the snapping turtle (Chelydra serpentina) raised under common garden conditions. Body size was plastic, sexually dimorphic, and the plasticity was also sex-specific, with males exhibiting greater body size plasticity relative to females. Because snapping turtle males are larger than females, sexual size dimorphism in this species appears to be driven by an increased plasticity of the larger sex over the smaller sex as predicted by the condition-dependent hypothesis. However, male body size was enhanced under relatively limited resources, in contrast to expectations from this model. Body shape was also plastic and sexually dimorphic, however no sex by environment interaction was found in this case. Instead, plasticity of sexual shape dimorphism seems to evolve in parallel for males and females as both sexes responded similarly to different environments.     

Testing the predictions of sex allocation hypotheses in dimorphic,cooperatively breeding riflemen

Evolutionary theory predicts that parents should invest equally in the two sexes. If one sex is more costly, a production bias is predicted in favour of the other. Two well‐studied causes of differential costs are size dimorphism, in which the larger sex should be more costly, and sex‐biased helping in cooperative breeders, in which the more helpful sex should be less costly because future helping “repays” some of its parents’ investment. We studied a bird species in which both processes should favor production of males. Female riflemen Acanthisitta chloris are larger than males, and we documented greater provisioning effort in more female‐biased broods indicating they are likely costlier to raise. Riflemen are also cooperative breeders, and males provide more help than females. Contrary to expectations, we observed no male bias in brood sex ratios, which did not differ significantly from parity. We tested whether the lack of a population‐wide pattern was a result of facultative sex allocation by individual females, but this hypothesis was not supported either. Our results show an absence of adaptive patterns despite a clear directional hypothesis derived from theory. This appears to be associated with a suboptimal female‐biased investment ratio. We conclude that predictions of adaptive sex allocation may falter because of mechanistic constraint, unrecognized costs and benefits, or weak selection.     

Cross‐fostering eggs reveals that female collared flycatchers adjust clutch sex ratios according to parental ability to invest in offspring

Across animal taxa, reproductive success is generally more variable and more strongly dependent upon body condition for males than for females; in such cases, parents able to produce offspring in above‐average condition are predicted to produce sons, whereas parents unable to produce offspring in good condition should produce daughters. We tested this hypothesis in the collared flycatcher (Ficedula albicollis) by cross‐fostering eggs among nests and using the condition of foster young that parents raised to fledging as a functional measure of their ability to produce fit offspring. As predicted, females raising heavier‐than‐average foster fledglings with their social mate initially produced male‐biased primary sex ratios, whereas those raising lighter‐than‐average foster fledglings produced female‐biased primary sex ratios. Females also produced male‐biased clutches when mated to males with large secondary sexual characters (wing patches), and tended to produce male‐biased clutches earlier within breeding seasons relative to females breeding later. However, females did not adjust the sex of individuals within their clutches; sex was distributed randomly with respect to egg size, laying order and paternity. Future research investigating the proximate mechanisms linking ecological contexts and the quality of offspring parents are able to produce with primary sex‐ratio variation could provide fundamental insight into the evolution of context‐dependent sex‐ratio adjustment.     

SEX RATIO,PARENTAL INVESTMENT,AND INTERPARENT VARIABILITY IN NESTING SUCCESS IN A SOLITARY BEE

Observations of uniquely marked females of the solitary, twig-nesting bee, Osmia bruneri, were conducted under greenhouse conditions to test several predictions of sex-ratio and parental-investment theory. In support of Fisher's (1958) theory, we found that the observed sex-ratio of progeny in this dimorphic species did not differ from that expected on the basis of average male and female weights. Investment patterns also exhibited a seasonal component: female parents produced more female than male offspring early in the nesting season but reversed this pattern later. Interfemale variability was large for all nesting parameters examined. Neither female-parent size nor the rate at which females completed cells was significantly related to several estimates of parent fitness. Parent-offspring heritability for size was also low. We found no evidence to support the hypothesis that progeny sex-ratios are influenced by maternal condition. Variance in progeny sex-ratios was large, but the population sex-ratio probably departs frequently from the equilibrium value. The results marginally support Kolman's (1960) prediction of large variance in progeny sex-ratios in large panmictic populations. We conclude that variability among females in investment patterns and variability in size among progenies are probably maintained by such factors as resource heterogeneity and the shape of the adult survivorship curve.     

Sex ratio variation in female-biased populations of Notostracans

Females from female-biased populations of the notostracan Triops newberryi produce viable eggs when reared in isolation. Clutches produced under such conditions exhibit sex-ratio polymorphism. One category of females (monogenics) produces only female offspring; the second category (amphigenics) produces clutches with a sex ratio of 3 females: 1 male. The relative proportions of the two categories of females varies significantly between populations and is correlated with population sex ratio. This correlation, and the pattern of offspring distribution in amphigenics, suggests that sex is determined by an autosomal Mendelian gene locus for which the male-determining allele is recessive. Limited pedigree analysis of lineages under selfing support the genetic model.     

Costs of rearing and sex‐ratio variation in southern grey shrike Lanius meridionalis broods

Several non‐mutually exclusive hypotheses predict adaptive variation in the offspring sex ratio. When conditions for breeding are adverse, parents are predicted to produce more offspring of the less costly sex to rear (‘the cost‐of‐reproduction hypothesis’). Moreover, they also should produce the more dispersing sex in order to diminish future competition (‘the local‐resource‐competition hypothesis’). Here, we analyse brood sex ratio according to rearing conditions in the southern shrike Lanius meridionalis, a species with moderately reversed sexual dimorphism. Our results suggest that females are more costly to rear than males in this species. Adult females proved heavier than males, and female nestling tended to be heavier than male nestlings. Moreover, the greater brood reduction, the more male‐biased was the brood, suggesting that brood reduction implied higher mortality in female nestlings. Consistent with these findings, the brood sex ratio was biased to the less costly sex (males) when breeding conditions were adverse (bad years or low‐quality male parents), supporting the cost‐of‐reproduction hypothesis. By contrast, these findings did not support the local‐resource‐competition hypothesis, which predicted female‐biased brood sex ratio under adverse conditions. As a whole, our results support the idea that birds adaptively modulate sex ratio in order to minimize reproduction costs.     

Sex-biased environmental sensitivity: natural and experimental evidence from a bird species with larger females

The larger sex is often more vulnerable, in terms of developmentand survival, to poor conditions during early life. Differentialvulnerability has implications for parental investment strategiessuch as sex ratio theory. When males are larger, it is not possibleto separate the effects of larger size per se and other aspectsof the male phenotype on vulnerability. Furthermore, offspringcompetition might favor the larger sex and thereby mask intrinsic,size-related effects. We studied sex-specific mortality in abird species with reversed size dimorphism, the great skua Stercorariusskua, under natural and experimentally created poor conditions.Small eggs from extended laying sequences were used to createpoor early conditions for the offspring, which were raised assingletons. Daughters had a lower survival in all treatmentgroups. Survival in natural broods was additionally affectedby hatch date and position. Hatch weight was not different forsons and daughters but was lower in experimental than in naturalnests. In natural nests, daughters fledged 10% heavier thansons, but in experimental nests, they did not reach a highermass. The average survival difference between sons and daughterswas not increased in experimental broods. However, hatch weighthad a strong sex-specific effect. Very light females never survived,and survival probability of daughters increased with increasinghatch weight. By contrast, survival of sons over the same rangeof hatch weights was not related to weight. These findings supportthe hypothesis that larger (final) size per se is related tosex-specific offspring vulnerability during early life.     

Can the limited marsupium space be a limiting factor for Syngnathus abaster females? Insights from a population with size-assortative mating

1. Some syngnathid species show varying degrees of sex role reversal aside from male pregnancy, with females competing for access to mates and sometimes presenting conspicuous secondary sexual characters. Among other variables, brooding space constraints are usually considered a key element in female reproductive success, contributing strongly to the observed morphological and behavioural sexual differences. Nevertheless, a close relationship between sex role reversal and male brooding space limitation has not yet been accurately demonstrated in field studies. 2. The present work, conducted over two consecutive breeding seasons in a wild population of the sex role-reversed pipefish Syngnathus abaster, simultaneously analysed egg number and occupied space, as well as the free area in the male's marsupium. The number of eggs that would fit in the observed unoccupied space was estimated. 3. Contrary to what would be expected, given the marked sexual dimorphism observed in the population studied, where females were larger and more colourful, male brooding space did not appear to limit female reproduction as neither large nor small individuals presented a fully occupied pouch. Interestingly, the largest unoccupied areas of marsupium were found in the larger individuals, although they received more and larger eggs. Laboratory data also showed that larger females lay larger eggs. 4. Together, these results suggest the existence of assortative mating, which may result from: (i) the reluctance of larger males (which tend not to receive small eggs usually laid by small females) to mate with lower quality females, even at the expense of a smaller number of offspring; or (ii) female-female competition, which might strongly reduce the hypothesis of a small female mating with a large male. The potential impact of temperature on reproduction and population dynamics is also discussed in the light of ongoing climatic changes.