Hybridity effect on mortality

Abstract

Several techniques for preselecting the sex of offspring are available now or will be in the near future. Although hundreds of studies indicate that sons are desired by the overwhelming proportion of persons asked, few studies ask whether preselecting of sex of offspring ought to be made available generally, as the technologies become perfected. In northern California, 2,138 respondents indicated widespread acceptance of ongoing biomedical research to perfect preselection methods and of making these procedures available to potential parents. Almost half agreed that they might want to use such techniques. Variation in levels of agreement were assessed by sex, race, marital status, child‐parity, religious affiliation and attendance, level of education, class, and general attitudes toward medical and scientific leaders. The implications of the general acceptability of sex selection go far beyond the freedom of parental choice—to such matters as socialization patterns of first son, second daughter ordering, sex role inflexibilities, sex ratio imbalances, and also include possibilities for curtailing rapid population growth.     

No evidence for adjustment of sex allocation in relation to paternal ornamentation and paternity in barn swallows

Sex allocation theory predicts that parents should adjust investment in sons and daughters according to relative fitness of differently sexed offspring. In species with female preference for highly ornamented males, one advantage potentially accruing to parents from investing more in sons of the most ornamented males is that male offspring will inherit characters ensuring sexual attractiveness or high-quality genes, if ornaments honestly reveal male genetic quality. Furthermore, in species where extra-pair fertilizations occur, offspring sired by an extra-pair male are expected to more frequently be male than those of the legitimate male if the latter is of lower quality than the extra-pair male. We investigated adjustment of sex ratio of offspring in relation to ornamentation of the extra-pair and the social mate of females by direct manipulation of tails of male barn swallows Hirundo rustica . Molecular sexing of the offspring was performed using the W chromosome-linked avian chromo-helicase-DNA-binding protein (CHD) gene while paternity assessment was conducted by typing of hypervariable microsatellite loci. Extra-pair offspring sex ratio was not affected by ornamentation of their biological fathers relative to the experimental ornamentation of the parental male. Experimental ornamentation of the parental males did not affect the sex ratio of nestlings in their broods. Female barn swallows might be unable to bias offspring sex ratio at hatching according to the quality of the biological father. Alternatively, fitness benefits in terms of sexual attractiveness of sons might be balanced by the cost of compensating for little parental care provided by highly ornamented parental males, if sons are more costly to rear than daughters, or the advantage of producing more daughters, if males with large ornaments contribute differentially more to the viability of daughters than sons.     

Intralocus sexual conflict,adaptive sex allocation,and the heritability of fitness

Intralocus sexual conflict arises when selection favours alternative fitness optima in males and females. Unresolved conflict can create negative between‐sex genetic correlations for fitness, such that high‐fitness parents produce high‐fitness progeny of their same sex, but low‐fitness progeny of the opposite sex. This cost of sexual conflict could be mitigated if high‐fitness parents bias sex allocation to produce more offspring of their same sex. Previous studies of the brown anole lizard (Anolis sagrei) show that viability selection on body size is sexually antagonistic, favouring large males and smaller females. However, sexual conflict over body size may be partially mitigated by adaptive sex allocation: large males sire more sons than daughters, whereas small males sire more daughters than sons. We explored the evolutionary implications of these phenomena by assessing the additive genetic (co)variance of fitness within and between sexes in a wild population. We measured two components of fitness: viability of adults over the breeding season, and the number of their progeny that survived to sexual maturity, which includes components of parental reproductive success and offspring viability (RS^V). Viability of parents was not correlated with adult viability of their sons or daughters. RS^V was positively correlated between sires and their offspring, but not between dams and their offspring. Neither component of fitness was significantly heritable, and neither exhibited negative between‐sex genetic correlations that would indicate unresolved sexual conflict. Rather, our results are more consistent with predictions regarding adaptive sex allocation in that, as the number of sons produced by a sire increased, the adult viability of his male progeny increased.     

Birth sex ratio in captive mammals: Patterns,biases, and the implications for management and conservation

Sex allocation theory predicts that a female should produce the offspring of the sex that most increases her own fitness. For polygynous species, this means that females in superior condition should bias offspring production toward the sex with greater variation in lifetime reproductive success, which is typically males. Captive mammal populations are generally kept in good nutritional condition with low levels of stress, and thus populations of polygynous species might be expected to have birth sex ratios biased toward males. Sex allocation theory also predicts that when competition reduces reproductive success of the mother, she should bias offspring toward whichever sex disperses. These predicted biases would have a large impact on captive breeding programs because unbalanced sex ratios may compromise use of limited space in zoos. We examined 66 species of mammals from three taxonomic orders (primates, ungulates, and carnivores) maintained in North American zoos for evidence of birth sex ratio bias. Contrary to our expectations, we found no evidence of bias toward male births in polygynous populations. We did find evidence that birth sex ratios of primates are male biased and that, within primates, offspring sex was biased toward the naturally dispersing sex. We also found that most species experienced long contiguous periods of at least 7 years with either male‐ or female‐biased sex ratios, owing in part to patterns of dispersal (for primates) and/or to stochastic causes. Population managers must be ready to compensate for significant biases in birth sex ratio based on dispersal and stochasticity. Zoo Biol 19:11–25, 2000. © 2000 Wiley‐Liss, Inc.     

Sexual Imprinting on a Novel Adornment Influences Mate Preferences in the Javanese Mannikin Lonchura leucogastroides

We investigated whether sexual imprinting on an artificial novel adornment in the Javanese Mannikin Lonchura leucogastroides , a monomorphic estrildid finch, can occur and might provide a mechanism for the evolution of novel traits. We introduced a red feather on the forehead as a novel adornment. Young were raised by parents which were both adorned, which were both unadorned, or only one of which was adorned with the red feather. We tested the female and male offspring of those parents in mate choice tests with an adorned and unadorned conspecific of the opposite sex. Males raised by an adorned mother or adorned parents preferred adorned females significantly more often than males raised by unadorned parents. We conclude that males were sexually imprinted on the red feather. Females raised by an adorned mother or raised by adorned parents significantly preferred adorned males, whereas females raised by unadorned parents showed no preference for adorned males. Thus, females also became imprinted on the red feather. Males might learn the novel adornment in combination with the parent's sex or learn just the most conspicuous sex, whereas females showed a preference for the adornment independent of which sex bore the feather. Our study shows that sexual imprinting might be an effective mechanism for the evolution of a novel trait and that males and females might become imprinted on a novel trait in different ways.     

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.     

Facultative Adjustment of Brood Sex Ratio in Response to Indirect Manipulation of Behaviour

Sex allocation theory states that parents should adjust their offspring sex ratio according to the expected fitness returns from sons and daughters. Several recent studies indicate that such adaptive manipulation of offspring sex ratio is achievable, and that it may be influenced by e.g. morphological characters. Here we manipulate behaviour through interspecific cross-fostering of great tits ( Parus major ) and blue tits ( Cyanistes caeruleus ), and investigate its effect on the offspring sex ratio of adults that were themselves cross-fostered as chicks. The experience of being raised by a different species has previously been shown to result in aberrant species assortative behaviour and song, and a lowered dominance status during winter. Brood sex ratios of conspecifically breeding pairs with and without cross-fostered members were compared. Broods with at least one cross-fostered parent contained significantly more males than did control broods. Sex of cross-fostered parents did not influence the brood sex ratio. We conclude that female great tits and blue tits seem to be able to adjust the sex ratio of their broods, and that changes in their own or their partners' behaviour may elicit such adjustments.     

Female blue tits adjust parental effort to manipulated male UV attractiveness

The differential allocation hypothesis predicts that parents should adjust their current investment in relation to perceived mate attractiveness if this affects offspring fitness. It should be selectively advantageous to risk more of their future reproductive success by investing heavily in current offspring of high reproductive value but to decrease investment if offspring value is low. If the benefits of mate attractiveness are limited to a particular offspring sex we would instead expect relative investment in male versus female offspring to vary with mate attractiveness, referred to as 'differential sex allocation'. We present strong evidence for differential allocation of parental feeding effort in the wild and show an immediate effect on a component of offspring fitness. By experimentally reducing male UV crown coloration, a trait known to indicate attractiveness and viability in wild-breeding blue tits (Parus caeruleus), we show that females, but not males, reduce parental feeding rates and that this reduces the skeletal growth of offspring. However, differential sex allocation does not occur. We conclude that blue tit females use male UV coloration as an indicator of expected offspring fitness and adjust their investment accordingly.     

Sex differences in biparental care as offspring develop: a field study of convict cichlids (<Emphasis Type="Italic">Amatitlania siquia</Emphasis>)

Parental investment theory states that parents should contribute more to older offspring. Differences between the sexes also influence how each parent contributes to offspring in biparental species. Here, we examined a naturally occurring population of biparental convict cichlids in Costa Rica to determine how each parent cared for offspring during two distinct offspring development stages. Consistent with the predictions of the reproductive value hypothesis, we hypothesized that the levels of parental contribution would be relative to the value that each parent places on a brood. We predicted that female parents would contribute more than male parents because female convict cichlids have lower future reproductive success than males. Additionally, we predicted that both parents should contribute more to older offspring, either due to the young’s increased susceptibility to predation (i.e., the vulnerability hypothesis) or because of the longer period of time parents have been interacting with older offspring (i.e., feedback hypotheses). This increase in investment by males should coincide with a change in the coordination of care between parents. Detailed observations of parental pairs in their natural habitat supported these predictions. Females contributed more to broods than males and were relatively unaffected by offspring age while males spent significantly more time with older, free-swimming fry. Additionally, males tended to leave younger offspring more than females did, and were more likely to do so consecutively with younger offspring. This suggests that the coordination of duties between parents changes as parental investment changes. Overall, these data support both the reproductive value and the vulnerability hypotheses, but not necessarily the feedback hypothesis.     

Manipulation of parental nutritional condition reveals competition among family members

Parental care is thought to be costly, as it consumes time and energy. Such costs might be reduced in animal parents that raise their young on valuable food sources such as dung or carcasses, as parents are able to invest in self‐maintenance by feeding from the same resource. However, this might lower the nutritional value for other family members and, as a consequence, food competition might arise. To promote our understanding of the outcome of such competition, we manipulated the necessity of parents to feed from the resource. Using a full factorial design, we paired food‐deprived or well‐fed males with food‐deprived or well‐fed females of burying beetles, which are known to raise their young on vertebrate cadavers. We found that food‐deprived parents consumed more of the carrion than those that were well‐fed and this had a negative impact on other family members. However, the outcome of the competition depended on the sex of the parents, with females suffering when males fed more and offspring suffering when females fed more. Thus, family life involves selfish elements, as both parents remove resources for the purpose of self‐maintenance. However, females show altruistic aspects, as they appear to restrict their food consumption for the benefit of their offspring when paired with a food‐deprived male. Interestingly, males extend their stay with the brood when having faced food scarcity prior to reproduction, presumably to replenish their energy reserves. Our study therefore reveals that breeding on shared resources can promote family living, but also results in competition.     

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.     

Evolutionary dynamics of the Trivers–Willard effect: A nonparametric approach

The Trivers–Willard hypothesis (TWH) states that parents in good condition tend to bias their offspring sex ratio toward the sex with a higher variation in reproductive value, whereas parents in bad condition favor the opposite sex. Although the TWH has been generalized to predict various Trivers–Willard effects (TWE) depending on the life cycle of a species, existing work does not sufficiently acknowledge that sex‐specific reproductive values depend on the relative abundances of males and females in the population. If parents adjust their offspring sex ratio according to the TWE, offspring reproductive values will also change. This should affect the long‐term evolutionary dynamics and might lead to considerable deviations from the original predictions.In this paper, I model the full evolutionary dynamics of the TWE, using a published two‐sex integral projection model for the Columbian ground squirrel (Urocitellus columbianus). Offspring sex ratio is treated as a nonparametric continuous function of maternal condition. Evolutionary change is treated as the successive invasion of mutant strategies. The simulation is performed with varying starting conditions until an evolutionarily stable strategy (ESS) is reached.The results show that the magnitude of the evolving TWE can be far greater than previously predicted. Furthermore, evolutionary dynamics show considerable nonlinearities before settling at an ESS. The nonlinear effects depend on the starting conditions and indicate that evolutionary change is fastest when starting at an extremely biased sex ratio and that evolutionary change is weaker for parents of high condition. The results show neither a tendency to maximize average population fitness nor to minimize the deviation between offspring sex ratio and offspring reproductive value ratio.The study highlights the importance of dynamic feedback in models of natural selection and provides a new methodological framework for analyzing the evolution of continuous strategies in structured populations.     

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.     

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 differences in begging vocalizations of nestling barn swallows, Hirundo rustica

Parents of sexually reproducing species should adjust their investment in production of sons and daughters in relation to the relative costs and reproductive value of offspring of either sex. Sex allocation mediated by differential allocation of care such as food provisioning, however, requires that parents can identify offspring sex. We analysed sex differences in offspring begging calls that may serve as a cue for parents to discriminate between sons and daughters. A combination of three sonagraphic variables of begging calls of nestling barn swallows allowed us to classify them according to sex at day 16, but not at day 12 after hatching, suggesting that sex differences in begging calls arise during the nestling period as the time of fledging approaches. Hence, parents may be able to discriminate between sons and daughters by auditory cues, which would enable differential allocation of food between offspring during the late nestling and early fledging stages. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.      

Female Control of Offspring Sex Ratios Based on Male Attractiveness in the Guppy

The sex allocation hypothesis predicts that females manipulate the offspring sex ratios according to mate attractiveness. Although there is increasing evidence to support this prediction, it is possible that paternal effects may often obscure the relationship between female control of offspring sex ratios and male attractiveness. In the present study, we examined whether females played a primary role in the manipulation their offspring sex ratios based on male attractiveness, in the guppy Poecilia reticulata, a live‐bearing fish. We excluded the paternal effects by controlling the relative sexual attractiveness of the male by presenting them to the females along with a more attractive or less attractive stimulus male. The test male was perceived to be relatively more attractive by females when it was presented along with a less attractive stimulus male, or vice versa. Subsequently, test male was mated in two different roles (relatively more and less attractive) with two females. If females were responsible for offspring sex ratio manipulation, the sex ratio of the brood would be altered on the basis of the relative attractiveness of the test male. On the other hand, if males play a primary role in offspring sex ratio manipulation, the sex ratios would not differ with the relative attractiveness of the test male. We found that females gave birth to more male‐biased broods when they mated with test males in the attractive role than when they mated with males in the less attractive role. This finding suggests that females are responsible for the manipulation of offspring sex ratios based on the attractiveness of their mates.     

Mating systems and sexual selection in male-pregnant pipefishes and seahorses: insights from microsatellite-based studies of maternity

In pipefishes and seahorses (family Syngnathidae), the males provide all postzygotic care of offspring by brooding embryos on their ventral surfaces. In some species, this phenomenon of male "pregnancy" results in a reversal of the usual direction of sexual selection, such that females compete more than males for access to mates, and secondary sexual characteristics evolve in females. Thus the syngnathids can provide critical tests of theories related to the evolution of sex differences and sexual selection. Microsatellite-based studies of the genetic mating systems of several species of pipefishes and seahorses have provided insights into important aspects of the natural history and evolution of these fishes. First, males of species with completely enclosed pouches have complete confidence of paternity, as might be predicted from parental investment theory for species in which males invest so heavily in offspring. Second, a wide range of genetic mating systems have been documented in nature, including genetic monogamy in a seahorse, polygynandry in two species of pipefish, and polyandry in a third pipefish species. The genetic mating systems appear to be causally related to the intensity of sexual selection, with secondary sex characters evolving most often in females of the more polyandrous species. Third, genetic studies of captive-breeding pipefish suggest that the sexual selection gradient (or Bateman gradient) may be a substantially better method for characterizing the mating system than previously available techniques. Finally, these genetic studies of syngnathid mating systems have led to some general insights into the occurrence of clustered mutations at microsatellite loci, the utility of linked loci in studies of parentage, and the use of parentage data for direct estimation of adult population size.     

Temporal but not spatial environmental variation drives adaptive offspring sex allocation in a plural cooperative breeder

Cooperatively breeding birds have been used frequently to study sex allocation because the adaptive value of the sexes partly depends upon the costs and benefits for parents of receiving help. I examined patterns of directional sex allocation in relation to maternal condition (Trivers-Willard hypothesis), territory quality (helper competition hypothesis), and the number of available helpers (helper repayment hypothesis) in the superb starling, Lamprotornis superbus, a plural cooperative breeder with helpers of both sexes. Superb starlings biased their offspring sex ratio in relation to prebreeding rainfall, which was correlated with maternal condition. Mothers produced relatively more female offspring in wetter years, when they were in better condition, and more male offspring in drier years, when they were in poorer condition. There was no relationship between offspring sex ratio and territory quality or the number of available helpers. Although helping was male biased, females had a greater variance in reproductive success than males. These results are consistent with the Trivers-Willard hypothesis and suggest that although females in most cooperatively breeding species make sex allocation decisions to increase their future direct reproductive success, female superb starlings appear to base this decision on their current body condition to increase their own inclusive fitness.     

The Trivers–Willard hypothesis of parental investment: No effect in the contemporary United States

The Trivers–Willard hypothesis (TWH) predicts that parents will bias their sex ratio toward sons when in good condition and toward daughters when in poor condition. Many human studies have tested the related hypothesis that parents' bias allocation of resources to existing sons and daughters according to the same principle. The present study used time diary and self-report data from the parents of 3200 children in the US to test the hypothesis that as status increases, parents will allocate more resources to sons vs. daughters. It finds no evidence that higher-status parents invest more in sons or that lower status parents invest more in daughters. This finding illustrates the specificity of situations in which the TWH effects should be expected. Only certain types of parental investment — such as protection and a bias in the sex ratio — may have been selected to vary according to parental condition. Optimal allocation of resources after the child is born, however, is achieved not by the simple bias predicted by the TWH, but by allocating resources among offspring in ways that yield the largest marginal inclusive fitness gains.     

Sex ratio manipulation within broods of house wrens, Troglodytes aedon

Trivers & Willard (1973, Science, 179, 90-92) developed an economic theory of parental investment to explain how the relative profitability of sons and daughters varies under specific ecological conditions. In their maternal condition hypothesis they proposed that in polygynous species, the sex of an offspring should be associated with the amount of parental care likely to be made available to it. In these species, the amount of parental investment directed towards offspring may differentially influence the fitness of male and female offspring because males in better than average condition as adults may enjoy larger fitness gains than a female would if she were in better than average condition, while the reverse may be true when conditions are poor. I tested this hypothesis by determining the sex of specific offspring within house wren broods. Because hatching is asynchronous and fledging is synchronous in this polygynous species, last-hatched young fledge having received less parental care than their broodmates. I predicted that last-hatched offspring would be more likely to be female. I found that these young were indeed more likely to be females, were more likely to have hatched from last-laid eggs and were fledging in poor condition relative to their broodmates. I propose that female house wrens behave in a manner consistent with the predictions of the Trivers & Willard hypothesis by producing female offspring last in the laying sequence of their clutches. Copyright 2000 The Association for the Study of Animal Behaviour.