Do female tree swallows guard their mates by copulating frequently? A comment on Whittingham et al.

Abstract

In recent years, it has become increasingly clear that both intraand inter-sexual conflicts are important processes leading to variability in mating systems and mating behaviour (Davies 1989, 1992; Walter & Trillmich 1994). For example, paired females use different strategies to prevent their mates engaging in polygyny or extra-pair copulations: these include aggressive behaviour (Slagsvold & Lifjeld 1994) and mate guarding (Petrie 1992). The female mate-guarding hypothesis suggests that a female may copulate repeatedly with her mate to reduce the likelihood that he mates with other females (Petrie 1992). It is only one of many hypotheses that can explain why females copulate repeatedly with the same male (Hunter et al. 1993). The mate-guarding hypothesis predicts that within-pair copulation frequency will be correlated negatively with the male’s investment in extra-pair copulations or polygyny (Hunter et al. 1993). Whittingham et al. (1994) provided experimental evidence contradicting this prediction and concluded that ‘a female [tree swallow, Tachycineta bicolor] could not deter her mate’s extra-pair copulations by copulating more frequently with him’. Here, we argue that Whittingham et al.’s results do not allow rejection of the mate-guarding hypothesis. We argue that, for several reasons, the experimental procedure may be inadequate to test the hypothesis. Furthermore, we suggest that Whittingham et al.’s results may be taken as support for the female mateguarding hypothesis and are consistent with observations on other facultatively polygynous species. To investigate whether female tree swallows use frequent copulations to guard their mates, Whittingham et al. (1994) temporarily removed resident males, increasing in this way the opportunity for extra-pair copulation and polygyny for other (neighbouring) males. They found that in six cases, one of the neighbouring males copulated frequently with the temporarily widowed female. The simultaneous within-pair and extra-pair copulation behaviour of these males was observed throughout the period of experimental male removal. Of crucial importance in this experiment were the observations of the within-pair copulation behaviour of the females (N=6) whose mates were gaining extra-pair copulations in comparison with the within-pair copulation behaviour of neighbouring control females (N=6) in a paired design. These control females were females whose mates did not gain extra-pair copulations. Whittingham et al. did not make a distinction between males trying to become polygynous or performing extra-pair copulations. From their description (see page 995, last paragraph), it seems that after a male had been removed, one of the neighbouring males quickly took over the female and copulated repeatedly with her. This leads to several problems with the experimental set-up. First, because the replacement males quickly took over the widowed females, the other males (i.e. control males) probably did not try or did not have the opportunity to obtain extra-pair copulations or to become polygynous. Unfortunately, Whittingham et al. did not mention whether several males courted the experimentally widowed females, and in particular whether the control males tried to engage in extra-pair copulations or were approached by the widowed female. They did mention, however (page 995, first sentence), that some females had mates who were gaining both extra-pair copulations and secondary mates, *Present address: Department of Biology, Queen’s University, Kingston, Ontario K7L 3N6, Canada.