Sex differences in provisioning rules: responses of Manx shearwaters to supplementary chick feeding

Sex differences in food provisioning have been found in a numberof socially monogamous birds with biparental care, but the reasonsremain unclear. In Manx shearwaters, males provide 40–50%more food for chicks than do females, and previous empiricaldata have suggested that this difference could arise becausefemales are able to regulate food delivery by reducing the provisioningof well-nourished chicks, whereas males are not (hypothesis1). Alternatively, however, males may be as capable as femalesof assessing and responding to the variation in the nutritionalrequirements of their chick but have a higher threshold forreducing food delivery to well-nourished chicks (hypothesis2). To test these two hypotheses, we used supplementary feedingto manipulate the nutritional status of chicks and then examinedthe responses of male and female parents and their offspring.Supplementary feeding significantly reduced both the beggingbehavior of chicks and the frequency and sizes of meals deliveredby parents. Males and females reduced their overall provisioningrates to a similar extent (males by 38%, females by 42%), somaintaining the same difference in contributions to provisioningin the control group (males 58%, females 42%) and the experimentaltreatment (males 59%, females 41%). These data strongly supporthypothesis 2. Supplementary feeding of chicks resulted in fewervisits by parents and a higher proportion of long trips in bothsexes (4 days for males, 5–7 days for females). However,maximum trip durations were unchanged, suggesting that supplementaryfeeding of chicks had no effect on the foraging ranges or overallfood-provisioning strategies of parents.     

Negotiation over offspring care--how should parents respond to each other's efforts?

Models of biparental care predict that parents should compensateincompletely for any change in their partner's investment. Experimentaltests have, however, yielded results that range from full compensation,through a lack of any reaction, to a matching response. Herewe suggest a new, adaptive explanation for such variation. Buildingon an approach developed by McNamara et al., we incorporateuncertainty regarding brood need or value into a game-theoreticalmodel of biparental negotiation over offspring care. We showthat when each parent has only partial information, greatereffort invested by one serves as a signal to the other of broodneed. This favors a matching response by the focal parent'smate, whereas the impact of increased effort on the marginalvalue of investment favors a compensatory response. The netoutcome depends on the relative strength of these two effects.The greater the variation in brood need compared with parentalstate, the weaker the predicted level of compensation, and themore likely matching is to occur. Our model also suggests whymales and females might respond differently to each other. Ifthere is an informational asymmetry between them, then the parentthat is better informed about brood need should work harder,respond more strongly to changes in brood need, be less sensitiveto changes in the cost of feeding, and compensate more stronglyfor changes in partner effort. If the asymmetry is very great,the poorly informed parent may even match changes in its partner'swork rate.     

Parental conflict in birds: comparative analyses of offspring development, ecology and mating opportunities

Parents often conflict over how much care to provide to their offspring. This conflict is expected to produce a negative relationship between male and female parental care, the strength of which may be mediated by both ecological and life-history variables. Previous studies have observed such trade-offs, but it is not known how generally they occur. Traditional views of sexual conflict place great importance on ecological factors in determining levels of parental care, whereas alternative views propose that the key determinant is mating opportunity. We carried out a broad-scale comparative study of parental conflict using 193 species from 41 families of birds. Using phylogenetic comparative analysis, we establish the generality of intersexual parental care conflict. We also show that parental conflict, as indicated by the disparity in care between the male and the female, depends on offspring development and mating opportunities, since in precocial species both males and females responded to increased mating opportunities. Altricial birds, however, failed to show these relationships. We also found little influence of breeding climate on parental conflict. Taken together, our results suggest that sexual conflict is a key element in the evolution of parental care systems. They also support the view that the major correlates of the intersexual conflict are mating opportunities for both sexes, rather than the breeding environment.     

Sexual conflict over parental care promotes the evolution of sex differences in care and the ability to care

Strong asymmetries in parental care, with one sex providing more care than the other, are widespread across the animal kingdom. At present, two factors are thought to ultimately cause sex differences in care: certainty of parentage and sexual selection. By contrast, we here show that the coevolution of care and the ability to care can result in strong asymmetries in both the ability to care and the level of care, even in the absence of these factors. While the coevolution of care and the ability to care does not predict which sex evolves to care more than the other, once other factors give rise to even the slightest differences in the cost and benefits of care between the sexes (e.g. differences in certainty in parentage), a clear directionality emerges; the sex with the lower cost or higher benefit of care evolves both to be more able to care and to provide much higher levels of care than the other sex. Our findings suggest that the coevolution of levels of care and the ability to care may be a key factor underlying the evolution of sex differences in care.     

Male brood care without paternity increases mating success

We investigate under which conditions we can expect the evolutionof costly male care for unrelated offspring, when the benefitof such care is in the form of increased mating success. Thisapplies to male helping behavior that cannot be explained aspaternal care because the male's own offspring does not benefitfrom his behavior. Our model shows that caring for others' offspringcan be a stable strategy for males, if a male that does not"help" loses mating opportunities, for example if females discriminateagainst non-helping males as mating partners. This is possiblewhen females are polyandrous. Increasing population densitydecreases the parameter region where male care is stable. Malecare is also more likely to be stable when male mortality rateis higher than that of females. We discuss the results withspecial reference to the golden egg bug Phyllomorpha laciniata,where females lay eggs on conspecifics, often on males beforemating. Males therefore carry mostly unrelated eggs. We investigatehow oviposition rate and female mating rate influences whenegg carrying is an evolutionary stable strategy. We concludethat in the golden egg bug, male egg carrying could be explainedas a form of mating investment.     

Should I stay or should I go? Female brood desertion and male counterstrategy in rock sparrows

Brood desertion involves a series of interactions between themembers of a pair. This process is likely to be based on eithermember's perception of the other's propensity to desert. Wemanipulated this perception in males by experimentally increasingfemale body mass in the rock sparrow (Petronia petronia), aspecies in which females can desert their first brood beforethe nestlings from the first brood leave the nest. We predictedthat the male would either desert the brood first or stay evenif this implied the risk of caring for the brood alone. We foundthat males mated to loaded females did not leave but stayedand significantly increased their courtship rate and mate guarding.Unexpectedly, they also increased their food provisioning tothe nestlings, even though loaded females did not reduce theirnestling-feeding rate. The increase in male feeding rate maybe explained as a way for the male to reduce the female's propensityto switch mate and desert or to increase her propensity to copulatewith the male to obtain paternity in her next brood. Altogether,our results demonstrate that the perception of the risk of beingdeserted by the female does not necessarily induce males todesert first, contrary to what is generally assumed by theoreticalmodels.     

Factors affecting the duration of nest defense and reproductive lifespan of female sockeye salmon, Oncorhynchus nerka

Reproductive success of female animals is often affected by a combination of fecundity and parental care. In female salmonid fishes, acquisition of nest (redd) sites and prevention of their use by other females are critical to reproductive success. These factors are particularly important for stocks that spawn at high densities. Body size is positively correlated with fecundity and egg size, and has been hypothesized to control the outcome of intrasexual competition and longevity. We tested this hypothesis by evaluating the influences of body size, intrasexual aggression and arrival date on duration and success of redd guarding by female sockeye salmon, Oncorhynchus nerka, in a small Alaskan creek. Contrary to the hypothesis, larger females guarded their redds no longer than smaller females, and did not live as long in the stream. Aggression was not related to body size or overall longevity but was positively correlated with residence period on the redd. Females that entered the creek earlier lived longer, spent longer on their redds, and spent more time guarding their redds after spawning than females that entered the creek later. However, despite their longevity, early-arriving fish were more likely to have their redds reused by another female because they died before all the females had selected redd sites. The small average body size in this stock is consistent with weak selection for large size, and with our evidence that size provided little if any advantage in nest guarding.     

Dynamic games and field experiments examining intra- and intersexual conflict: explaining counterintuitive mating behavior in a Mediterranean wrasse, Symphodus ocellatus

Intersexual conflict and intrasexual competition are widelyrecognized as playing critical roles in determining mating systems.Although they occur simultaneously in populations, these processesare usually treated independently. In reality, the fitness ofreproductive strategies will depend on the outcome of both within-and between-sex conflicts. Using a modeling approach based onmultiple, linked, dynamic state variable models, we examined thereproductive behavior of a Mediterranean wrasse, Symphodus ocellatus.We compared the predictions of models that examine only a singleconflict interaction with those that consider multiple within-and between-sex conflicts simultaneously. The observed distributionof sneaker males and females among nests was compared with thosepredicted by the models. We found that the closest fit withempirical observations and experiments is given by the modelthat examines conflict between females, sneakers, and nestingmales simultaneously. Removal of successful nests indicatedthat females join nests with few or no sneakers present, whereassneakers join these nests only later, even though this leadsto lower sneaker mating success. This behavior can be explainedby observing that although sneakers would have higher fitnessat nests where the spawning rate is greater, females would notbe willing to spawn at these nests in the presence of sneakers. Presumably,once the nests have achieved high past success, females are willingto spawn in the presence of sneakers because of the associated decreasedchance of nesting male desertion.     

DOES LARGE BODY SIZE IN MALES EVOLVE TO FACILITATE FORCIBLE INSEMINATION? A STUDY ON GARTER SNAKES

A trend for larger males to obtain a disproportionately high number of matings, as occurs in many animal populations, typically is attributed either to female choice or success in male-male rivalry; an alternative mechanism, that larger males are better able to coercively inseminate females, has received much less attention. For example, previous studies on garter snakes (Thamnophis sirtalis parietalis) at communal dens in Manitoba have shown that the mating benefit to larger body size in males is due to size-dependent advantages in male-male rivalry. However, this previous work ignored the possibility that larger males may obtain more matings because of male-female interactions. In staged trials within outdoor arenas, larger body size enhanced male mating success regardless of whether a rival male was present. The mechanism involved was coercion rather than female choice, because mating occurred most often (and soonest) in females that were least able to resist courtship-induced hypoxic stress. Males do physically displace rivals from optimal positions in the mating ball, and larger males are better able to resist such displacement. Nonetheless, larger body size enhances male mating success even in the absence of such male-male interactions. Thus, even in mating systems where males compete physically and where larger body size confers a significant advantage in male-male competition, the actual selective force for larger body size in males may relate to forcible insemination of unreceptive females. Experimental studies are needed to determine whether the same situation occurs in other organisms in which body-size advantages have been attributed to male-male rather than male-female interactions.     

Ecology and evolution of extravagant feather ornaments

The ancestral conditions that permit the evolution of extravagant secondary sexual characters are of considerable theoretical and empirical interest because they allow identification of necessary ecological conditions, but also allow empirical tests of models of female mate preferences. We investigated the ancestral and derived state of a range of ecological and evolutionary variables that might have been implicated in the evolution of secondary sexual characters. Extravagant feather ornaments have evolved independently at least 70 times in birds, and the context of these evolutionary events was investigated statistically. The acquisition of feather ornaments was significantly associated with a change in social mating system from monogamy to polygyny or lekking. This association is consistent with the Fisherian mechanism of sexual selection. However, very often also the acquisition of feather ornaments occurred without change in mating system. Therefore, ornamentation can develop for reasons other than polygyny. We did not find any indication of male parental care, kind of food, foraging mode, coloniality, nest site, migration or body mass being significantly associated with a change in the state of ornamentation.     

Parental investment, sexual selection and sex ratios

Conventional sex roles imply caring females and competitive males. The evolution of sex role divergence is widely attributed to anisogamy initiating a self‐reinforcing process. The initial asymmetry in pre‐mating parental investment (eggs vs. sperm) is assumed to promote even greater divergence in post‐mating parental investment (parental care). But do we really understand the process? Trivers [Sexual Selection and the Descent of Man 1871–1971 (1972), Aldine Press, Chicago] introduced two arguments with a female and male perspective on whether to care for offspring that try to link pre‐mating and post‐mating investment. Here we review their merits and subsequent theoretical developments. The first argument is that females are more committed than males to providing care because they stand to lose a greater initial investment. This, however, commits the ‘Concorde Fallacy’ as optimal decisions should depend on future pay‐offs not past costs. Although the argument can be rephrased in terms of residual reproductive value when past investment affects future pay‐offs, it remains weak. The factors likely to change future pay‐offs seem to work against females providing more care than males. The second argument takes the reasonable premise that anisogamy produces a male‐biased operational sex ratio (OSR) leading to males competing for mates. Male care is then predicted to be less likely to evolve as it consumes resources that could otherwise be used to increase competitiveness. However, given each offspring has precisely two genetic parents (the Fisher condition), a biased OSR generates frequency‐dependent selection, analogous to Fisherian sex ratio selection, that favours increased parental investment by whichever sex faces more intense competition. Sex role divergence is therefore still an evolutionary conundrum. Here we review some possible solutions. Factors that promote conventional sex roles are sexual selection on males (but non‐random variance in male mating success must be high to override the Fisher condition), loss of paternity because of female multiple mating or group spawning and patterns of mortality that generate female‐biased adult sex ratios (ASR). We present an integrative model that shows how these factors interact to generate sex roles. We emphasize the need to distinguish between the ASR and the operational sex ratio (OSR). If mortality is higher when caring than competing this diminishes the likelihood of sex role divergence because this strongly limits the mating success of the earlier deserting sex. We illustrate this in a model where a change in relative mortality rates while caring and competing generates a shift from a mammalian type breeding system (female‐only care, male‐biased OSR and female‐biased ASR) to an avian type system (biparental care and a male‐biased OSR and ASR).     

Sexual conflict over parental investment in repeated bouts: negotiation reduces overall care

Understanding the evolution of parental care is complicated by the occurrence of evolutionary conflicts of interest within the family, variation in the quality and state of family members, and repeated bouts of investment in a family of offspring. As a result, family members are expected to negotiate over care. We present a model for the resolution of sexual conflict in which parents negotiate over repeated bouts of care. Negotiation is mediated by parents deciding at the start of each bout how much care to give on the basis of the state (mass) of offspring, which reflects the amount of care previously received. The evolutionarily stable pattern of care depends on whether the parents care together for the whole family, or each cares alone for part of the divided family. When they care together, they provide less care in the first bout, more in the last bout, and less care overall, resulting in lower parental and offspring fitness. Our results emphasize that negotiation over parental care may occur as a means of avoiding exploitation owing to sexual conflict, even in the absence of variation in the quality of either sex of parent, and lead to a reduction in fitness.