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

Ecology, March, 1998 by Colin Hughes

Polyandry

Polyandry is the long-term association of a female with more than one male. Recent molecular work with a classic example of polyandry, the Tasmanian Native Hen, Tribonyx mortieri, suggests that the species is characterized by genetic monogamy, though polyandry did occur in one of six groups (Gibbs et al. 1994). In other studies, molecular methods have confirmed polyandrous mating; examples include the Dunnock (Burke et al. 1989), and the Galapagos Hawk, Buteo galapagoensis (Faaborg et al. 1995). Groups of male Galapagos Hawks have long-term stable membership; the data showed that group members were not close relatives and had approximately equal chances of siring young. In Brown Skuas, Catharacta lonnbergi, males within groups were also unrelated, but had highly variable probabilities of siring young (Millar et al. 1994). The authors emphasize, however, that parentage studies based on data from a single year should be interpreted with caution, as reproductive success may vary widely between years.

An alternative form of polyandry is sequential polyandry, in which a female lays clutches for two or more males in quick succession, leaving each male in turn to raise a brood. A study of sequentially polyandrous Spotted Sandpipers, Actitis macularia, showed that early-pairing males had higher than expected reproductive success because sperm storage by females allowed these males to father offspring in later broods (Oring et al. 1992).

In some species the mating system includes more than one of the patterns described above. In the Dunnock, a small European passerine, the mating system embraces monogamy, polygyny, polyandry, and polygynandry (Davies 1985). Burke et al. (1989) used DNA fingerprinting to resolve parentage, and showed that patterns of paternity provided reasonable explanations for behavioral observations of paternal care and intrasexual aggression. Males fed broods according to their access to females during the mating period, which was a good predictor of their paternity. Alpha males attempted to drive off beta males, apparently because they did not gain sufficient benefits from the cooperation of a second male to make up for the loss of paternity they suffered.

Which individuals are responsible for extra-pair young?

When molecular markers are sufficiently polymorphic and the population adequately characterized, parentage assignments can be made. Under these circumstances, factors influencing choice of extra-pair mates can be determined. In Red-winged Blackbirds, extra-pair fathers were generally (20 of 26 cases) neighboring males (Gibbs et al. 1990). Similarly, in Yellow Warblers, Dendroica petechia, 89% of extra-pair young were sired by a neighbor or next-to-neighbor (Yezerinac et al. 1995). And in Great Reed Warblers, Acrocephalus arundinaceus, nests closest to neighboring males had a greater chance of containing young sired by EPF (Hasselquist et al. 1995). Old males were disproportionately represented as EPF sires in Purple Martins (Morton et al. 1990, Wagner et al. 1996), while in Blue Tits (Kempenaers et al. 1992), and Red-winged Blackbirds (Weatherhead and Boag 1995) large males achieved more EPFs. In Tree Swallows, no physical or behavioral correlates of extra-pair fathers could be found, but only 21% of extra-pair young could be assigned to a known father, so the test was not strong (Dunn et al. 1994). Apparently, female Tree Swallows copulated with males nesting elsewhere or with unmated "floaters."