Sexually selected sentinels? Evidence of a role for intrasexual competition in sentinel behavior

Abstract

Although the evolutionary mechanisms that favor investment in cooperative behaviors have long been a focus of research, comparatively few studies have considered the role that sexual selection may play. For example, evolutionary explanations for sentinel behavior (where 1 individual assumes an elevated position and scans the surroundings while other group members forage nearby) have traditionally focused on the inclusive fitness benefits arising from its effects on predation risk, while its potential role in defense against intrasexual competitors remains largely unexplored. Here, we provide experimental evidence of a role for sentinel behavior in intrasexual competition, in a cooperatively breeding songbird, the white-browed sparrow weaver (Plocepasser mahali). First, dominant males sentinel substantially more than other group members (even when controlling for variation in age and body condition), consistent with a role for sentineling in intrasexual competition for mates and/or territory. Second, experimental playback of an unfamiliar male's solo song elicited a marked increase in sentineling by the dominant male, and the vocal response to the playback also positively predicted his sentinel effort following the simulated intrusion. A second experiment also suggests that sentineling may facilitate mounting rapid anti-intruder responses, as responses to intruder-playback occurred significantly earlier when the dominant male was sentineling rather than foraging at playback onset. Together, our findings provide rare support for the hypothesis that sentinel behavior plays a role in intrasexual competition, and so highlight the potential for sexually selected direct benefits to shape its expression in this and other social vertebrates.

Keywords: Plocepasser mahali; intrasexual competition; sentinels; sexual selection; vigilance..

Figure 1

The contributions to sentineling of each dominance and sex class. Bars present mean ± standard error. The data were derived from 53 observation sessions on 25 groups, yielding a sample of 25 dominant males, 25 dominant females, 28 subordinate males, and 11 subordinate females.

Figure 2

The sentinel activity of dominant males pre- versus post-playback of either the control or male solo song playback treatments (n = 10 groups). The bars present mean ± standard error. Unfilled bar, pre-playback; filled bar, post-playback.

Figure 3

The total duet output of the resident pair in response to the playback of male solo song (n = 10 groups) positively predicted the subsequent sentinel effort of the resident male once the pair had returned to the foraging context.

Figure 4

Latency to (a) first duet of resident pair and (b) first entry of dominant male to playback tree for dominant males exposed to male solo song in 2 contexts (sentineling vs. foraging; n = 8 groups).