Brood desertion in Kentish plover: the value of parental care

To understand the evolution of parental care, one needs to estimatethe payoffs from providing care for the offspring and from terminatingcare and deserting them. In this study we estimated the payofffrom care provision, and in a companion paper we analyze thepayoff from offspring desertion. In the current study we experimentallyinvestigated the influence of the number and sex of attendingparents on growth and survival of offspring in the Kentish ploverCharadrius alexandrinus, in two sites (A and B). Either the maleor the female parent was removed from some broods at hatchingof the chicks (female-only and male-only broods, respectively),whereas in control broods both parents were allowed to attendtheir young. At site A survival of the chicks was lower in uniparental(male-only and female-only) broods than in control broods, whereaswe found no difference in brood survival at site B. Brood survivaldecreased over the season. Removal of either parent did not influencethe growth of the young, although growth varied over the breeding season,and it was significantly different between the sites. Theseresults suggest that the payoff from parental care decreasesover the breeding season and that the value of parental care(i.e., the contribution of parents to the survival of theiryoung) may depend on the environment.     

Sexual conflict over care: antagonistic effects of clutch desertion on reproductive success of male and female penduline tits

A fundamental tenet of sexual conflict theory is that one sex may increase its reproductive success (RS) even if this harms the other sex. Several studies supported this principle by showing that males benefit from reduced paternal care whereas females suffer from it. By investigating penduline tits Remiz pendulinus in nature, we show that parental conflict may be symmetric between sexes. In this small passerine a single female (or male) cares for the offspring, whereas about 30% of clutches are deserted by both parents. Deserting parents enhance their RS by obtaining multiple mates, and they reduce the RS of their mates due to increased nest failure. Unlike most other species, however, the antagonistic interests are symmetric in penduline tits, because both sexes enhance their own RS by deserting, whilst harming the RS of their mates. We argue that the strong antagonistic interests of sexes explain the high frequency of biparental desertion.     

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.     

Differences in parental food allocation rules: evidence for sexual conflict in the blue tit?

Evolutionary conflicts of interest between family members areexpected to influence patterns of parental investment. In altricialbirds, despite providing the same kind of parental care, patternsof investment in different offspring can differ between parents,a situation termed parentally biased favoritism. Previous explanationsfor parentally biased favoritism have received mixed theoreticaland empirical support. Here, we test the prediction that inblue tits, Cyanistes caeruleus, females bias their food allocationrules to favor the smallest offspring during the nestling stage.By doing so, females could increase the subsequent amount ofpaternal care supplied by their partner during the fledgingperiod, as a previous study showed that males feed the largestfledglings. When size differences within the brood are lesspronounced, all offspring will require similar amounts of postfledgingcare, and thus, the male parent will lose the advantage of caringfor the largest offspring that are closest to independence.In this study, we controlled the hunger of the smallest andlargest nestlings in the brood and compared the food allocationrules of the 2 parents. We found that the male parent had astronger preference than the female to feed the closest nestlingsand made no distinction between nestlings based on size, whereasthe female provisioned small hungry nestlings more when theywere at intermediate distances from her. These differences inparental food allocation rules are consistent with predictionsbased on sexual conflict over postfledging parental investment.     

Factors affecting mate desertion by males in free-ranging convict cichlids (Cichlasoma nigrofasciatum)

Convict cichlid fish have biparental care for a period of about6 weeks lasting from egg laying until the young (fry) have grownto about 10 mm. However, the young can sometimes survive withcare from only one parent, and desertion of the mate and offspringby males has been observed. I tested a theoretical model modifiedfrom Lazarus (1990) which predicted that mate and offspringdesertion by male convict cichlids should be promoted by lowpredation pressure on fry, high remating opportunities for males,increasing age of fry, and decreasing number of fry. Males deserted7.8% of 334 broods studied during two breeding seasons in CostaRican streams. As predicted, males deserted their broods mostfrequently at sites with the highest brood survivorship (lowestbrood predation pressure), when fry were close to independenceand when brood size was smaller than average. Sex ratios andinterspawning intervals did not indicate any relationship betweenmate desertion and opportunities for remating for males. Thereuse of spawning caves may favor fidelity to the mate and brood,and defending the young from predators at the same time as defendingthe cave from conspecifics may favor biparental care in thisspecies.     

How is sexual conflict over parental care resolved? A meta‐analysis

Biparental care of offspring is both a form of cooperation and a source of conflict. Parents face a trade‐off between current and future reproduction: caring less for the current brood allows individuals to maintain energy reserves and increase their chances of remating. How can selection maintain biparental care, given this temptation to defect? The answer lies in how parents respond to changes in each other’s effort. Game‐theoretical models predict that biparental care is evolutionarily stable when reduced care by one parent leads its partner to increase care, but not so much that it completely compensates for the lost input. Experiments designed to reveal responses to reduced partner effort have mainly focused on birds. We present a meta‐analysis of 54 such studies, and conclude that the mean response was indeed partial compensation. Males and females responded differently and this was in part mediated by the type of manipulation used.     

Model parents: is full compensation for reduced partner nest attendance compatible with stable biparental care?

Previous models have suggested that biparental care will beevolutionarily stable when each parent only partially compensatesfor decreases in effort by their partner. We investigated asystem where breeding success is an accelerating function ofparental effort. This could occur in species with a high predationlevel—for example, in a dense sea-bird colony or in specieswhere eggs or young are very prone to cooling. In these caseswe found that parents will fully compensate for decreased partnereffort, or else they will abandon the breeding attempt altogether.We use a second graphical model to show that biparental carecan exist under a situation of full compensation for reducedpartner effort if neither parent can do all the care alone.Each parent will abandon the breeding attempt if his or hercondition falls below a certain threshold. If the participationof both parents is necessary for the breeding attempt to besuccessful, then neither parent will want to force their partnerto abandon by making them work so hard that they fall belowthe condition threshold. Because abandonment by oen partnermeans the failure of the breeding attempt, each individualwill do at least enough work so that the partner will not abandon,resulting in biparental care. There will be a region of conflictbetween the parents, within which the conflict can be resolvedin various ways. Possible resolutions of this conflict, andthe consequences and applications of the model, are discussed.     

Behavioral dynamics between caring males and females in a beetle with facultative biparental care

In families in which both parents care for multiple offspring,the amount of care a parent provides can be simultaneously influencedby multiple social interactions (i.e., parent-parent and parent-offspring).In this study, we first tested for sex differences in the parents'contribution to care and then used path analysis to addressthe simultaneous impact of parent-parent and parent-offspringinteractions on male and female care in the burying beetle,Nicrophorus vespilloides. In this species, both parents provisiontheir offspring predigested carrion from a vertebrate carcass,and the larvae beg for food from their parents. We found thatfemales were more involved in direct care for the larvae andspent more time than did males provisioning the larvae withfood. By using path analysis, we found a negative relationshipbetween male and female provisioning, suggesting that parentsadjust their behavior to that of their mate. Furthermore, wefound that both social interactions (i.e., larval begging) andnonsocial factors (i.e., brood size) significantly influencedmale provisioning, but had no significant effect on female provisioning.We suggest that the difference in the relative contributionof the two sexes to the care of the offspring explains why onlymales seemed to adjust their care to variation in social andnonsocial factors. For example, females may be less able toadjust their care to variation in larval begging and brood sizebecause they were already working near their maximum capacity.     

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.     

Characteristics of the alula in relation to wing and body size in the Laridae and Sternidae

The alula is a small structure present on the leading edge of bird wings and is known to enhance lift by creating a small vortex at its tip. Alula size vary among birds, but how this variation is associated with the function of the alula remains unclear. In this study, we investigated the relationship between the size and shape of the alula and the features of the wing in the Laridae and Sternidae. Laridae birds have generally longer wings and greater loadings than Sternidae birds. The two families differed in the relationships between body size or wing length and the size or shape of the alula. In the Laridae, the aspect ratio of the alula was smaller in the species that have relatively longer wings, but the pattern was opposite in the Sternidae. The aspect ratio of the alula was greater in the species that are relatively heavier in the Sternidae but not in the Laridae. Combined, these results suggest that the species with high loading potential and long wings exhibit long alula. We hypothesize that heavier species may benefit from having longer alula if they perform flights with higher attack angles than lighter species, as longer alula would better suppress flow separation at higher attack angles. Our results suggest that the size and shape of the alula can be explained in one allometric landscape defined by wing length and loading in these two closely related families of birds with similar wing shapes.     

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.     

Biparental care in house sparrows: negotiation or sealed bid?

We explored the responses of monogamous house sparrow parentsto deviations in their mates' contributions to nestling provisioning.Following 1-2 days of baseline measurement of parental fooddelivery rates, we applied small lead fishing weights to thetail feathers of either male or female parents. Weighting hadmuch greater immediate impact on male parental care than on female care, but the handicapping had little long-term effecton either male or female provisioning behavior. When parentalperformance of handicapped males was most impaired, their matesdid not show significant increases in parental care as compensation,nor did females mated to handicapped males reduce their provisioningas their mates recovered from weighting. Similarly, males matedto handicapped females did not respond to their partners' recovery with declines in their own efforts; paradoxically, these malesshowed a sustained elevation of provisioning throughout thepost-treatment interval, despite no significant reduction inprovisioning by weighted females. The apparent insensitivityof both males and females to changes in their mates' parentalbehavior, and the ineffectiveness of current partner behaviorat predicting an individual's provisioning effort, fail toconform to assumptions of biparental care models that requirefacultative responses to partner deviations in effort. Instead,the remarkable consistency of each individual's behavior supportsthe notion of "sealed bids" and suggests that variation innestling provisioning is largely attributable to factors thatare independent of the mate's current behavior, such as differencesin individual quality.