Integrating molecular techniques with field methods in studies of social behavior: a revolution results

Ecology, March, 1998 by Colin Hughes

Success in extra-pair matings, as well as susceptibility to their effects, may be related to age. In Purple Martins, Progne subis, and Bobolinks, Dolichonyx oryzivorus, the frequency at which males lose paternity in their own nests has been shown to decline with age (Morton et al. 1990, Bollinger and Gavin 1991). There are no demonstrations of age-specific effects of EPFs on female fitness, but it is intriguing that older female Bobolinks chose extra-pair males more often than did younger females (Bollinger and Gavin 1991). A thorough understanding of how mating strategies influence lifetime reproductive success must consider these age-related changes.

The frequency of extra-pair fertilization may also affect the pattern of parental investment, as males should not invest in offspring other than their own if they have a means of detecting unrelated young. On the other hand, if a male gave less care to a brood in which he had lower confidence of paternity, he may jeopardize the survival of any offspring in the brood of which he actually is the father. In such circumstances, females should weigh the benefits of mating with extra-pair males against lost assistance in raising offspring. The most convincing demonstration of this phenomenon comes from a study of Reed Buntings, in which male assistance changed according to the change in paternity from one brood to the next, in the same year, with the same mate (Dixon et al. 1994). Males fed a second brood more than they fed their first brood when they had higher paternity in the second brood. Conversely, they fed second broods less when they had lower paternity. This male response to loss of paternity may be widespread; there is evidence for it in Dunnocks (Burke et al. 1989) and Barn Swallows, Hirundo rustica (Moller 1988, 1991). However, in studies of other species there was no relationship between male parentage and the parental care they provide, such as in Indigo Buntings (Westneat 1988), Yellow Warblers (Yezerinac et al. 1996), and Galapagos Hawks (DeLay et al. 1996).

Females may avoid inbreeding or outbreeding depression by seeking EPFs. In the Splendid Fairy Wren, Malurus splendens, described behaviorally as having monogamous pairs living in groups with helpers, an extraordinary proportion of offspring are sired by extra-pair, apparently extra-group males. Allozyme data showed that [greater than]65% (and perhaps 100%) of offspring resulted from EPFs (Brooker et al. 1990, Rowley and Russel 1990). In this case, females may be seeking unrelated males because the sedentary nature of the birds makes it likely that within-group mates are close relatives. In contrast, in Pied Flycatchers, genetically similar pairs had fewer extra-pair young than less closely related mates (Ratti et al. 1995). This result is consistent with females seeking an optimal degree of outbreeding. Where other species fall along this continuum will be readily tested by analysis of existing data sets.

Polygyny

Classic polygyny refers to the prolonged association of one male with more than one female. The "polygyny threshold model" hypothesizes that this mating system arises when males control access to resources in such a way that some females benefit more by breeding with an already mated male than by breeding with an unmated male (Emlen and Oring 1977). However, this reasoning must be modified in light of the molecular evidence on paternity. Parentage assignment in Red-winged Blackbirds, Agelaius phoeniceus, showed that 28% of chicks are not offspring of their social father (Gibbs et al. 1990). An average of 21% of male reproductive success derived from fertilization of females other than those nesting within their territories. Between 0 and 67% of individual males' success was realized with out-of-territory females. Furthermore, residents that were more successful at fertilizing females nesting within their territories were also more successful in achieving EPFs. These results suggest that a female's choice of mate is partly independent of her choice of where to nest, which may explain the lack of support for models of the evolution of polygyny outlined above (Lightbody and Weatherhead 1988, Davies 1989). In light of such findings, new models for the evolution of polygyny should hypothesize how females choose the optimal available combination of high-quality territory and high-quality mate.