Patterns of reproductive skew in the polygynandrous acorn woodpecker

We compared observed levels of reproductive skew in the cooperatively breeding acorn woodpecker (Melanerpes formicivorus) with those predicted by two alternative transactional models. "Concession" models predict the degree to which parentage is shared assuming that a single dominant is in complete control of reproduction. Alternatively, "restraint" models predict reproductive sharing assuming that the dominant controls only whether subordinates remain in the group but does not control its share of reproduction. Reproductive skew is high among males: on average, the most successful male sires more than three times as many offspring as the next most successful male. Females share parentage equally and have lower constraints on dispersal and lower survival rates compared with males, which is consistent with predictions from the concessions model. Also as predicted by the concessions model, yearly variation in opportunities for dispersal before the breeding season correlates positively with skew. However, in contrast to concessions but consistent with the restraint model, skew decreases with relatedness. Thus, neither model consistently predicts patterns of reproductive skew in this species. We suggest that models of reproductive skew will need to include competitive interactions among potential breeders and mate choice before they will adequately predict patterns of reproductive partitioning in most vertebrate societies.     

Sex-biases in the hatching sequence of cooperatively breeding apostlebirds <Emphasis Type="Italic">Struthidea cinerea</Emphasis>

In the cooperatively breeding apostlebird (Struthidea cinerea, Corcoracidae) both sexes are philopatric and help to raise offspring. However, male helpers provision nestlings more often than females, an activity associated with reduced nestling starvation and enhanced fledgling production. Presuming that males are the more helpful sex, we examined the helper repayment hypothesis by testing the predictions that offspring sex ratio should be skewed toward the production of males (a) among breeding groups with relatively few helpers, and (b) in the population as a whole. The relationship between sex and hatching order was examined as a potential mechanism of biasing sex allocation. The sex ratio of all sexed offspring was male biased (57.9%; n = 171) as was the mean brood sex ratio (0.579; n = 70 broods). These biases were less pronounced in the subset of clutches/broods in which all offspring were sexed. This overall bias appeared to result from two distinct patterns of skew in the hatching order. First, mothers in small breeding groups produced significantly more males among the first-hatching pair. This is consistent with the helper repayment hypothesis given that later hatching chicks were less likely to survive, particularly in small groups. Second, almost all fourth-hatching chicks, usually the last in the brood, were male (91.7%, n = 12). This bias is difficult to interpret but demonstrates the value of examining hatching sequences when evaluating specific predictions of sex allocation theory in birds.     

Reproductive parasitism of broodcare helpers in a cooperatively breeding fish

Large male helpers in the cooperatively breeding cichlid Neolamprologus pulchergain reproductive success by parasitizing the reproductive effort ofmale territory owners. Under controlled, experimental conditionswe examined the genetic relatedness between the members of broodpairs (n = 14), their male helpers (n = 8), and offspring (n= 292) in seven families. We used multilocus DNA fingerprintingto check for potential reproductive parasitism by male helpersand to assess their fertilization success. Of offspring producedin these families, 10.3% were sired by helpers. In parasitizedbroods, helper fertilization success varied between 12.5% and35.8%. Male helpers parasitized parental reproduction when theirbody size exceeded 4.5 cm standard length (SL), even thoughsexual maturity may be reached much earlier (3.5 cm SL). Twoof three parasitic helpers were punished by severe aggressiveattacks when parasitizing the reproduction of breeders, whichled to their expulsion from the territory. This study demonstratesa potential fitness benefit to broodcare helpers that is oftenneglected. It also points to the delicate balance that may exist betweencooperative and competitive behavior in cooperatively breeding species.     

Smart is the new sexy: female mountain chickadees increase reproductive investment when mated to males with better spatial cognition

Understanding the evolution of inter and intraspecific variation in cognitive abilities is one of the main goals in cognitive ecology. In scatter‐caching species, spatial memory is critical for the recovery of food caches and overwinter survival, but its effects on reproduction are less clear. Better spatial cognition may improve pre‐breeding condition allowing for earlier reproduction. Alternatively, when mated to males with better spatial memory, females may be able to invest more in reproduction which may allow increased offspring survival and hence higher fitness. Using wild food‐caching mountain chickadees, we found that when environmental conditions were favourable for breeding, females mated to males with better spatial cognition laid larger clutches and fledged larger broods than females mated to males with worse cognitive performance. Our results support the hypothesis that females may increase their reproductive investment to gain indirect, genetic benefits when mated to high‐quality males with better spatial cognitive abilities.     

Testis size depends on social status and the presence of male helpers in the cooperatively breeding cichlid Julidochromis ornatus

Cooperatively breeding animals, in which helpers may participatein reproduction with dominant breeders, are ideal species forexamining intraspecific variation in testis size because theyoften exhibit both monogamous breeding (low risk of sperm competition)and polyandrous breeding (high risk) within a population. However,little is known about testis investment as a result of spermcompetition in these animals. The substrate-brooding cichlidfish Julidochromis ornatus has a cooperatively breeding system,in which some males mate monogamously and other males reproduceas dominant breeders or helpers within cooperatively breedinggroups, in which male helpers frequently sire young. We examinedthe relationship between testis investment and male social statusin relation to the risk of sperm competition. As predicted fromsperm competition models, in groups with male helpers, boththe male breeders and the male helpers invested more in testesmass, compared to breeding males without male helpers. We alsofound a positive relationship between the testes mass of malebreeders and their male helpers, suggesting that males increasetheir investment in reproductive capability under the risk ofsperm competition. Sperm competition models also predict thatlarger testes are associated with increased siring success.Our paternity analysis supported this prediction; we found apositive relationship between testis investment by male helpersand the number of offspring they sired.     

Helpers increase the reproductive potential of offspring in cooperative meerkats

In both animal and human societies, individuals may forego personal reproduction and provide care to the offspring of others. Studies aimed at investigating the adaptive nature of such cooperative breeding systems in vertebrates typically calculate helper 'fitness' from relationships of helper numbers and offspring survival to independence. The aim of this study is to use observations and supplemental feeding experiments in cooperatively breeding meerkats, Suricata suricatta, to investigate whether helpers influence the long-term reproductive potential of offspring during adulthood. We show that helpers have a significant and positive influence on the probability that offspring gain direct reproductive success in their lifetimes. This effect arises because helpers both reduce the age at which offspring begin to reproduce as subordinates and increase the probability that they will compete successfully for alpha rank. Supplemental feeding experiments confirm the causality of these results. Our results suggest that one can neither discount the significance of helper effects when none is found nor necessarily estimate accurately the fitness benefit that helpers accrue, unless their effects on offspring are considered in the long term.     

COOPERATIVE BREEDING FAVORS MATERNAL INVESTMENT IN SIZE OVER NUMBER OF EGGS IN SPIDERS

The transition to cooperative breeding may alter maternal investment strategies depending on density of breeders, extent of reproductive skew, and allo‐maternal care. Change in optimal investment from solitary to cooperative breeding can be investigated by comparing social species with nonsocial congeners. We tested two hypotheses in a mainly semelparous system: that social, cooperative breeders, compared to subsocial, solitarily breeding congeners, (1) lay fewer and larger eggs because larger offspring compete better for limited resources and become reproducers; (2) induce egg size variation within clutches as a bet‐hedging strategy to ensure that some offspring become reproducers. Within two spider genera, Anelosimus and Stegodyphus, we compared species from similar habitats and augmented the results with a mini‐meta‐analysis of egg numbers depicted in phylogenies. We found that social species indeed laid fewer, larger eggs than subsocials, while egg size variation was low overall, giving no support for bet‐hedging. We propose that the transition to cooperative breeding selects for producing few, large offspring because reproductive skew and high density of breeders and young create competition for resources and reproduction. Convergent evolution has shaped maternal strategies similarly in phylogenetically distant species and directed cooperatively breeding spiders to invest in quality rather than quantity of offspring.     

Females increase reproductive investment in response to helper-mediated improvements in allo-feeding, nest survival, nestling provisioning and post-fledging survival in the Karoo scrub-robin Cercotrichas coryphaeus

In many cooperatively-breeding species, the presence of one or more helpers improves the reproductive performance of the breeding pair receiving help. Helper contributions can take many different forms, including allo-feeding, offspring provisioning, and offspring guarding or defence. Yet, most studies have focussed on single forms of helper contribution, particularly offspring provisioning, and few have evaluated the relative importance of a broader range of helper contributions to group reproductive performance. We examined helper contributions to multiple components of breeding performance in the Karoo scrub-robin Cercotrichas coryphaeus , a facultative cooperative breeder. We also tested a prediction of increased female investment in reproduction when helpers improve conditions for rearing young. Helpers assisted the breeding male in allo-feeding the incubating female, increasing allo-feeding rates. Greater allo-feeding correlated with greater female nest attentiveness during incubation. Nest predation was substantially lower among pairs breeding with a helper, resulting in a 74% increase in the probability of nest survival. Helper contributions to offspring provisioning increased nestling feeding rates, resulting in a reduced incidence of nestling starvation and increased nestling mass. Nestling mass had a strong, positive effect on post-fledging survival. Controlling for female age and habitat effects, annual production of fledged young was 130% greater among pairs breeding with a helper, and was influenced most strongly by helper correlates with nest survival, despite important helper effects on offspring provisioning. Females breeding with a helper increased clutch size, supporting the prediction of increased female investment in reproduction in response to helper benefits.     

Multiple paternity in an aggregate breeding amphibian: the effect of reproductive skew on estimates of male reproductive success

Aggregate, or explosive, breeding is widespread among vertebrates and likely increases the probability of multiple paternity. We assessed paternity in seven field-collected clutches of the explosively breeding spotted salamander (Ambystoma maculatum) using 10 microsatellite loci to determine the frequency of multiple paternity and the number of males contributing to a female's clutch. Using the Minimum Method of allele counts, multiple paternity was evident in 70% of these egg masses. Simple allele counts underestimate the number of contributing males because this method cannot distinguish multiple fathers with common or similar alleles. Therefore, we used computer simulations to estimate from the offspring genotypes the most likely number of contributing fathers given the distributions of allele frequencies in this population. We determined that two to eight males may contribute to A. maculatum clutches; therefore, multiple paternity is a common strategy in this aggregate breeding species. In aggregate mating systems competition for mates can be intense, thus differential reproductive success (reproductive skew) among males contributing to a female's clutch could be a probable outcome. We use our data to evaluate the potential effect of reproductive skew on estimates of the number of contributing males. We simulated varying scenarios of differential male reproductive success, ranging from equal contribution to high reproductive skew among contributing sires in multiply sired clutches. Our data suggest that even intermediate levels of reproductive skew decrease confidence substantially in estimates of the number of contributing sires when parental genotypes are unknown.     

Breeding success in cooperative meerkats: effects of helper number and maternal state

Studies of cooperatively breeding birds and mammals generallyconcentrate on the effects that helpers have on the number ofreproductive attempts females have per year or on the numberand size of offspring that survive from hatching/weaning toindependence. However, helpers may also influence breeding successbefore hatching or weaning. In the present study, we used anultrasound imager to determine litter sizes close to birth,and multivariate statistics to investigate whether helpers influencefemale fecundity, offspring survival to weaning, and offspringsize at weaning in cooperative meerkats, Suricata suricatta.We found that the number of helpers in a group was correlatedwith the number of litters that females delivered each year,probably because females in large groups gave birth earlierand had shorter interbirth intervals. In addition, althoughpup survival between birth and weaning was primarily influencedby maternal dominance status, helper number may also have asignificant positive effect. By contrast, we found no evidenceto suggest that helpers have a direct effect on either littersizes at birth or pup weights at weaning, which were both significantlyinfluenced by maternal weight at conception. However, becausedifferences in maternal weight were associated with differencesin helper number, helpers have the potential to influence maternalfecundity and offspring size within reproductive attempts indirectly.These results suggest that future studies may need to considerdirect and indirect helper effects on female fecundity and investmentbefore assessing helper effects on reproductive success in societiesof cooperatively breeding vertebrates.