Extra-pair paternity in waved albatrosses

Genetic monogamy despite frequent extrapair copulations in "strictly monogamous" wild jackdaws

"Monogamy" refers to different components of pair exclusiveness: the social pair, sexual partners, and the genetic outcome of sexual encounters. Avian monogamy is usually defined socially or genetically, whereas quantifications of sexual behavior remain scarce. Jackdaws (Corvus monedula) are considered a rare example of strict monogamy in songbirds, with lifelong pair bonds and little genetic evidence for extrapair (EP) offspring. Yet jackdaw copulations, although accompanied by loud copulation calls, are rarely observed because they occur visually concealed inside nest cavities. Using full-day nest-box video surveillance and on-bird acoustic bio-logging, we directly observed jackdaw sexual behavior and compared it to the corresponding genetic outcome obtained via molecular parentage analysis. In the video-observed nests, we found genetic monogamy but frequently detected forced EP sexual behavior, accompanied by characteristic male copulation calls. We, thus, challenge the long-held notion of strict jackdaw monogamy at the sexual level. Our data suggest that male mate guarding and frequent intrapair copulations during the female fertile phase, as well as the forced nature of the copulations, could explain the absence of EP offspring. Because EP copulation behavior appeared to be costly for both sexes, we suggest that immediate fitness benefits are an unlikely explanation for its prevalence. Instead, sexual conflict and dominance effects could interact to shape the spatiotemporal pattern of EP sexual behavior in this species. Our results call for larger-scale investigations of jackdaw sexual behavior and parentage and highlight the importance of combining social, sexual, and genetic data sets for a more complete understanding of mating systems.

Extra-pair paternity in the black-headed gull: is it exceptional among colonial waterbirds?

Although nesting at high densities promotes frequent interactions with individuals other than social mates, the rate of extra-pair paternity (EPP) in colonial waterbirds is generally low. So far, the highest EPP rate within this group has been reported in the black-headed gull Chroicocephalus ridibundus breeding in a colony in Czech Republic (33% broods with extra-pair offspring). In this study, we used a set of microsatellite loci to analyse parentage in 34 gull families (102 nestlings) from northern Poland. We recorded intra-specific brood parasitism in 10.8% of broods, but no evidence for EPP was found. The striking discrepancy between our results and previous estimates of EPP rate in the black-headed gull strongly indicate that promiscuity is a context-dependent strategy in this species and that its prevalence may greatly vary between populations. We recommend that a special care should be taken while using single-population data on EPP in the inter-specific comparative analyses.

Rethinking Bateman's Principles: Challenging Persistent Myths of Sexually Reluctant Females and Promiscuous Males

In 1948, Angus Bateman published a paper on fruit flies that tested Charles Darwin's ideas of sexual selection. Based on this one fruit fly study, Bateman concluded that because males are able to produce millions of small sperm, males are likely to behave promiscuously, mating with as many females as possible. On the other hand, because females produce relatively fewer, larger, and presumably more expensive eggs, females are likely to be very discriminating in selecting only one high-quality sexual partner. He also posited that a male's reproductive success increases linearly with the number of females he is able to mate with, but that a female's reproductive success peaks after she mates with only one male. Consequently, in almost all organisms, sexual selection acts most strongly on males. These ideas became a recurring theme in attempts to explain wide-ranging differences in male and female behavior not only in nonhuman animals but also in humans. As such, Bateman's conclusions and predictions have become axiomatic and, at times, have gone unquestioned even when modern empirical data do not conform to this model. This article reviews the origins and history of these ideas and uses modern data to highlight the current and growing controversy surrounding the validity and general applicability of this paradigm.

Spatial patterns of extra-pair paternity in a waterbird colony: separating the effects of nesting density and nest site location

Centres of avian colonies are usually associated with reduced predation risk and, thus, attract individuals of high quality, while poor-quality individuals are relegated to peripheral zones. Assuming that the incidence of extra-pair paternity (EPP) is dependent on individual quality, we could expect lower incidence of extra-pair offspring in the central parts of colonies. On the other hand, central pairs often nest in higher densities, which might increase EPP rate. To test these hypotheses, we sampled 124 great cormorant Phalacarocorax carbo sinensis chicks from 30 broods from different zones of a colony and genotyped them at seven highly polymorphic microsatellite loci. Sibship reconstruction confirmed the presence of at least one extra-pair chick in 30.0 % of broods. We found that EPP varied significantly between the zones of the colony, with higher incidence of extra-pair broods in the peripheral zone (53.3 vs. 6.7 % of broods). Centre-edge difference in EPP was consistent with the expected distribution of pair quality and suggested that poor-quality peripheral females were more likely to solicit extra-pair interactions, possibly to gain ‘good genes’ for their offspring. By contrast, we found no evidence for density dependence in EPP rate, indicating that likelihood of raising extra-pair offspring was not constrained by limited availability of local males. The results indicate that spatial randomization of sampling within avian colonies is critical to obtain robust estimations of EPP for non-solitary species. To our knowledge, this study provides the first evidence for the centre-edge difference in EPP within a breeding colony of birds.

Social and genetic mating system of Ridgway's hawk (Buteo ridgwayi), an endemic raptor on Hispaniola

Patterns of social organization and mating systems have been shown to be functions of ecological factors such as resource allocation and breeding density. In some species, particularly birds, social organization and genetic mating systems differ with molecular studies providing evidence of extra-pair young frequently occurring within broods of socially monogamous species. Here we examine the social and genetic mating system of an ecologically little-known forest raptor endemic to the island of Hispaniola in the Caribbean. From 2005–2009, our field observations of over 60 breeding pairs verified a social mating system of monogamy for the species. During the same time period, we collected blood samples (n= 146 birds, 48 nests) and used microsatellite profiles from 10 loci to estimate genetic relatedness among nestlings in a brood and assign putative fathers. We found no evidence of extra-pair paternity in 41 broods. We had one instance where a social male was not assigned as the putative father, however, the confidence level of this assignment was not significant since the genotypes of the social and assigned males were very similar. Our results support our hypothesis that genetic monogamy would be exhibited by Ridgway's hawk, an island-endemic tropical raptor.

Experimentally simulating paternity uncertainty: immediate and long-term responses of male and female reed warblers Acrocephalus scirpaceus

In many socially monogamous species, both sexes seek copulation outside the pair bond in order to increase their reproductive success. In response, males adopt counter-strategies to combat the risk of losing paternity. However, no study so far has tried to experimentally prove the function of behaviour for paternity assurance. Introducing a potential extra-pair partner during the female fertile period provides a standardised method to examine how pair members respond immediately (e.g. increase mate guarding or copulation frequency) or long term (e.g. later parental investment and paternity uncertainty). In this study on a socially monogamous passerine species, we experimentally confronted pairs of reed warblers with a conspecific male (caged male simulating an intruder) during egg-laying. Our results revealed that occurrence of an intruder during that period triggered aggression against the intruder, depending on the presence of the female. The male territory owner also attacked the female partner to drive her away from the intruder. Thus territory defence in reed warblers also serves to protect paternity. The increase in paternity uncertainty did not affect later paternal investment. Paternal investment was also independent of the actual paternity losses. In females, the experiment elicited both, immediate and long-term responses. E.g. female copulation solicitations during the intruder experiment were only observed for females which later turned out to have extra-pair chicks in their nest. In relation to long term response females faced with an intruder invested later less in offspring feeding, and had less extra-pair chicks in their nests. Extra-pair paternity also seems to be affected by female quality (body size). In conclusion female reed warblers seem to seek extra-pair fertilizations but we could demonstrate that males adopt paternity assurance tactics which seems to efficiently help them to reduce paternity uncertainty.

Mating system and reproductive skew in the black rhinoceros

Only approximately 2600 black rhinoceros survive today, mainly in small, isolated populations of < 100 animals. The management of remaining black rhinoceros populations aims at preserving natural levels of genetic relatedness and optimizing breeding success, which requires an accurate knowledge of the mating system, reproductive skew and effective population size. DNA was extracted from faecal samples from a community of 35 wild black rhinoceros, and microsatellites were used to characterize patterns of paternity of 19 offspring born from eight females in this community. Paternity could be ascribed unequivocally for each offspring. Although our conclusions must be considered tentative, we present the first genetic evidence that black rhinoceros males are polygynous, with a high variance in reproductive success. We also describe a noninvasive management tool that can be used for the genetic management of this critically endangered species, both in the wild and in captivity.