On the self-regulation of sentinel activity among Arabian babbler groupmates

Abstract

In many cooperatively breeding species, one of the group members (called a sentinel) flies or climbs up occasionally to a high position, looks around and utters alarm calls if a predator approaches, while other group members are foraging. Bednekoff's (1997, American Naturalist, 150, 373–392; 2001, Annales Zoologici Fennici, 38, 5–14) game-theoretic model assumes that group members trade the need to forage against the advantages of earlier risk detection when sentinelling. The model predicts that in the presence of another sentinel which may provide timely alerts, a sentinel will terminate its sentinel bout earlier when its energy level drops below a threshold that is higher than the depletion threshold that would incentivize it to resume foraging if it were sentinelling alone. We propose a complementary hypothesis, that this effect of shortened sentinel duration in the presence of another sentinel will be more pronounced for subordinate group members, for whom the fierce competition they suffer while foraging gets attenuated when a groupmate sentinels. We tested these hypotheses in Arabian babbler, Argya squamiceps, groups by comparing sentinel activity under natural conditions vis-a-vis sentinel activity in a feeding experiment in which one or two individuals were fed. In natural conditions, dominant males not only acted as sentinels longer and more often, but they also initiated a larger share of their sentinel bouts when another group member was already on guard. Following experimental feeding ad libitum, in line with the game-theoretic predictions (1) the probability of starting a sentinel bout was not affected by the presence of another sentinel, nor by its sex or rank, and (2) sentinel bouts that terminated in the presence of another sentinel were significantly shorter than sentinel bouts ending alone. Our findings suggest that the seemingly systematic exchange of sentinels may be a consequence of differential needs and incentives rather than active coordination.