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

Ecology, March, 1998 by Colin Hughes

The schedule of age-specific reproduction is partly determined by developmental changes in the rate at which members of both sexes employ alternative mating tactics. Young female Purple Martins mated to young males choose older males as EPFs partners, perhaps because age is an index for fitness (Morton et al. 1990, Wagner et al. 1996). In Bobolinks, too, young males suffered more losses to EPFs than old males, though young females had fewer EPFs offspring than old females (Bollinger and Gavin 1991). No age specific effects were found in Tree Swallows (Dunn et al. 1994). It will be interesting to see how age-related changes in EPFs affect lifetime reproductive success.

What predicts the frequency of extra-pair young?

Molecular techniques have made it clear that individuals may achieve a large fraction of their reproductive success through extra-pair mating. One obvious next step is to understand what factors determine general patterns in extra-pair mating. One factor that has been extensively investigated is males' guarding of mates during females' fertile periods (e.g., Moller and Birkhead 1993). If mate guarding has evolved because it increases paternity, then EPFs and mate guarding are predicted to be negatively correlated, and polygynously mated males are predicted to suffer more EPFs than monogamously mated males. Analyses have provided mixed support for these two predictions.

The frequency of mate guarding bears no simple relationship to paternity determined from molecular data. Contrary to expectation in Eastern Bluebirds and Great Reed Warblers, the frequency of mate guarding is positively correlated with frequency of EPFs (Gowaty and Bridges 1991a, Hasselquist et al. 1995). However, in Red-winged Blackbirds, there is strong experimental evidence that guarding increases males' paternity (Westneat 1994). Males that were supplemented with food spent less time foraging, guarded more, and had higher paternity (0.88 of chicks raised on their territories) than unsupplemented males (0.69). The food supplements also reduced female forays away from the territory, but the effect was much smaller than on male behavior, so it is reasonable to conclude that the increase in the primary mate's paternity was due largely to the effect of mate guarding.

Similarly, polygynously mated males do not necessarily suffer lower paternity in their own nests. In neither the Pied Flycatcher, Ficedula hypoleuca (Lifjeld et al. 1991), nor Great Reed Warblers (Hasselquist et al. 1995), did polygynous males suffer greater loss of paternity to EPFs than monogamous males. Within polygynous male bobolinks' territories, primary females had more EPF offspring than secondary females, and monogamously mated females had an intermediate number (Bollinger and Gavin 1991). A comparison of two congeners, in the same environment, showed that there were no EPFs in either polygynous Wood Warblers or monogamous Willow Warblers (Gyllensten et al. 1990).

These results provide weak support for the explanatory framework based on male tactics for increasing fertilization rate and reproductive success. A fruitful new line of research focuses on development of a parallel set of expectations based on the female perspective (e.g., Stutchbury et al. 1994, Stutchbury and Morton 1995). In birds, recent evidence suggests that the resolution of reproductive conflicts of interests between males and females favors female interests. Patterns of paternity suggest females control paternity in Red-winged Blackbirds (Gray 1996), Alpine Accentors and Dunnocks (Davies et al. 1996), and Purple Martins (Wagner et al. 1996). In Purple Martins it was additionally shown that the balance shifted toward males' interests in pairs where males were significantly larger than their mates.