Extra-pair paternity in birds: explaining variation between species and populations

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Abstract

Molecular techniques used to assign paternity have revealed previously unknown incidences of extra-pair paternity in socially monogamous bird species. DNA fingerprinting has now been used sufficiently often for mating-system biologists to appreciate the natural variation in the frequency of broods showing extra-pair young. The variation between species and between populations of the same species is surprisingly marked. Explaining this variation may help us to understand the factors promoting sexual selection. Recent comparative studies and detailed behavioural studies suggest that factors such as breeding density, genetic variation in the population and the intensity of sexual conflicts determine the costs and benefits to males and females of engaging in extra-pair copulations, and therefore contribute to the variation among populations.

Section snippets

Variation in benefits

The type of benefit to females from seeking extra-pair copulations could vary among species or populations or females of all species could gain the same benefit but its magnitude may vary. What are the benefits to females of seeking extra-pair copulations and under what circumstances will these be greater for a higher proportion of the population? In recent years, several possible benefits have been suggested[10]. Depending on the type of benefit, different predictions about the proportion of

Variation in costs

The benefits of seeking extra-pair copulations may be similar for a similar proportion of the population, but the differences in the proportion of extra-pair young may be caused by variation in the costs of seeking extra-pair copulations. Where costs are low it will pay more females to seek extra-pair copulations. What are the main costs to females of seeking extra-pair copulations and under what circumstances will these be low for a higher proportion of the population?

Constraints on female choice

It is unlikely that females are free to choose any male in the population as a mate (copulation partner). Females are not only restricted because of costs associated with searching and assessing males as discussed above, but the behaviour of her own partner and that of other females might also play an important role[31]. The importance of conflicts of interest among different individuals involved in extra-pair copulations and the consequences of the resulting arms races are discussed in more

Costs and benefits to males

So far we have adopted the working assumption that females benefit from seeking extra-pair copulations. However, there is an alternative view which emphasizes that it will always pay males to gain extra-pair matings[37]. Females might not resist these mating attempts, either because it costs them little to mate with an extra-pair male or because it costs them more to resist an extra-pair mating. Given that males benefit substantially from gaining additional offspring through extra-pair

Prospects

There are two main ways in which these ideas can be tested—firstly, by use of the comparative method and secondly by experiment. Comparisons between species have already been used to test some of the possible ideas[44], although many remain to be tested. However, even where species estimates of extra-pair paternity levels are repeatable[17], interspecific comparisons can be open to alternative explanations. Comparisons between populations of the same species or between sister species have fewer

Acknowledgements

We thank Claudie Doums, Tim Birkhead, Jan Lifjeld, Anders Møller, Ben Sheldon, David Westneat and an anonymous referee for their comments. We are most grateful to Gro Bjørnstad, Simon Griffith, Jan Lifjeld and Pat Weatherhead for kindly allowing us to use their unpublished information, and to Kelvin Conrad, Sven Jacobsson, Lukas Keller, Christin Krokene, Bruce Robertson and Nanette Verboven for helpful information.

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