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Male and female eastern whipbirds responded to simulated intruders within their territories. However, they showed little evidence of coordinating their response, as might be expected if they were cooperatively defending a shared resource or preventing their partner from being usurped. We found no evidence to support any of the 3 key predictions of the joint defense hypothesis in Table 1. First, pairs did not tend to respond to intrusion as a coordinated unit. In almost one third of trials, only one member of the pair approached the speaker, and when both members approached, they did not approach at the same time. Second, neither sex showed a more aggressive vocal or approach response to duetting intruders compared with lone intruders, suggesting that duets were not perceived as more threatening territorial signals than solo songs. Finally, both sexes reacted more aggressively to same-sex playback, whereas the joint defense hypothesis predicts an equally aggressive response to intruders of either sex. Indeed, paired females did not generally approach the speaker or increase their vocal rate in response to playback of a lone male, suggesting that females did not aggressively defend against lone male intruders. During interactions between neighboring pairs, we never observed opposite-sex birds in physical conflict or chases, and aggression was invariably directed at same-sex individuals (personal observation, Rogers et al. 2006). Our finding that females did not respond aggressively to male intruders and formed duets with their partners least often during playback of a lone male suggests that it is unlikely that female whipbirds use duets to prevent their partner from being usurped by male rivals (Table 1). Likewise, the results did not support predictions of the identity hypothesis. First, duets were always initiated by males, whereas the identity hypothesis predicts that both sexes should initiate duets. Second, during solo playback, initiation rates were higher for same-sex playback, rather than opposite-sex playback as predicted by the identity hypothesis. Lastly, females that failed to answer their mate were never subjected to aggression from their mate. Indeed, we never observed intrapair aggression in eastern whipbirds.
Our results provide the most support for the hypothesis that females use duets to defend their own position within the partnership (Table 1). Intrasexual aggression is indicative of individuals independently defending their position within a partnership (Seddon et al. 2002). In addition, paired females answered their partner more often in the presence of a lone female intruder than a lone male intruder and tended to respond to their partner more often in the presence of an unpaired female than a paired female rival. Consequently, the proportion of vocalizations that were duets was highest when the focal female was most likely to be usurped from the partnership. The fact that females duet with their partner in the apparent absence of a rival female (during playback of a lone male) could be because females are trying to ascertain whether another female is present. Eastern whipbirds inhabit areas of dense vegetation where visual cues are limiting, and male songs that receive an answer from a female do not appear to differ from those that do not (Watson 1969). Thus, on hearing a male intruder, a female may not immediately know whether he is alone. The costs of not responding to her partner in this situation may exceed any benefits of remaining silent.
Two mechanisms could explain how responding precisely to a partner might enable a female to maintain her exclusive position in the partnership (Table 1). These are mutually nonexclusive, and it is possible that both play a role in duet function in the eastern whipbird. Firstly, duets could be used to signal paired status to a same-sex rival. By responding precisely to their partners, females prevent mates singing solo songs that may otherwise attract female rivals (“acoustic mate defense”: Brown and Lemon 1979; Sonnenschein and Reyer 1983; Levin 1996a,b; Seddon et al. 2002; Gill et al. 2005). The following field observations suggest that unpaired females may be attracted to solo male song if a female fails to reply to her partner. While paired females did not tend to approach the speaker during playback of lone male song, opportunistic playback of lone male song to 4 unpaired females resulted in close approaches to within less than 5 m and frequent antiphonal duets with the speaker. It also seems possible that paired male eastern whipbirds actively seek out females. We never observed males attacking female intruders and our findings that males reacted strongly to playback of a lone female could have been indicative of attraction rather than territorial aggression. It is likely that in this climate of conflict, duets may have arisen to allow paired females to protect their position within the partnership through an acoustic display of pair status that deters rival females.
A second possible mechanism of partnership defense involves the use of duets as an intrapair signal. By signaling their commitment and willingness to invest in the partnership, females may attempt to elicit reciprocal investment from their partners (“maintaining the pair bond”: Wickler 1980, Smith 1994, Hall 2004). Wickler (1980) suggested that if duet coordination required effort to achieve, for example, if it required a period of learning, duets could be used as an effective signal of commitment. However, most studies since then have shown that antiphonal duets do not require a lengthy learning period (reviewed in Hall 2004). Further, in eastern whipbirds, females respond to the songs of male strangers as precisely as they respond to their own partner (Rogers, forthcoming), suggesting that signaling intrapair commitment is unlikely to be the only role of duets in the eastern whipbird.
Why should female eastern whipbirds need to defend their position in the partnership? Two features of eastern whipbird life history may explain the need for acoustic mate defense in this species. First, it is unlikely that females could successfully reproduce without access to a territory and paternal care. Male eastern whipbirds play an important role in caring for offspring (Rogers and Mulder 2004), and the high level of paternal care displayed suggests that polygyny and division of male care between 2 nests would involve significant reproductive costs to his original partner (Slagsvold and Lifjeld 1994). By defending their position in the partnership, females simultaneously maintain their position on a breeding territory and their exclusive access to male parental care. Using an acoustic method to defend their position may be particularly effective in the densely vegetated habitat preferred by eastern whipbirds. It also allows for the possibility of long-distance defense of a partner from variable distances. Second, female defense of her exclusive position within the partnership may be particularly necessary as competition for partners and territories appears intense. During our 3-year study, eastern whipbirds displayed extremely high rates of adult survival, limited dispersal, and low rates of territory turnaround, despite the presence of numerous unpaired adult floaters (Rogers and Mulder 2004). The population sex ratio was female biased both among nestlings and unpaired adults (Rogers and Mulder 2004) and several lines of evidence suggest that this may have resulted in high levels of intraspecific competition between females. Elsewhere, we have shown that female eastern whipbirds type match the songs of female intruders during solo singing bouts (Rogers et al. 2006) and may also use a novel vocal strategy (forming extrapair duets with male neighbors) in order to type match and temporally overlap the song of female rivals in escalating song duels (Rogers, forthcoming). The development of a distinctive female solo song type II which does not feature in duets is also indicative of a high incidence of rivalry between female eastern whipbirds (Rogers 2005). A high level of female–female competition has been identified as a principal factor promoting the development of female solo song (Langmore 1998b, 2000). In this study, females only produced a song type II in response to playback involving female intruders and never in response to solo male playback. A male-biased sex ratio has been used to explain why males may use duets to defend their female partners from rival males in a species forming female-initiated duets (subdesert mesites (Monias benschi): Seddon et al. 2002). However, to the best of our knowledge, this is the first time it has been suggested that a female-biased sex ratio, and high female–female competition, might explain the development of male-initiated duets as a method for females to defend their position in the partnership. Our results show a striking correspondence to those of a recent study on duetting in a suboscine passerine, the warbling antbird Hypocnemis cantator (Seddon and Tobias 2006). Like eastern whipbirds, warbling antbirds produced male-initiated duets, females responded to same-sex solo songs more strongly than to duets, and females answered their partner's songs more often in response to female solos than male solos or duets. It would be interesting to know whether acoustic mate guarding in antbirds is likewise underpinned by high levels of female competition and a female-biased sex ratio.
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