Are sentinels rewarded by their groupmates? A study in a cooperative breeder

Abstract: The seemingly altruistic sentinel shifts performed alternately by individuals in group living animals have been extensively studied. However, the reactions of group members to the sentinel following its guarding shift have been almost ignored. To address this issue, we examined the reactions of groupmates to the sentinel in 20 groups of the cooperatively breeding Arabian babbler. We first induced sentineling of dominant and subordinate individuals of both sexes by feeding them ad libitum. Immediately after a sentinel bout, sentinels were not granted more copulations or more allopreening, neither did they suffer less aggression as compared to a control period not following sentinelling, even not after warning their groupmates from an approaching predator. Even more surprisingly, subordinates, who normally copulate less than dominants, copulated even less than usual after a sentinel bout. These findings support further the hypothesis that in this species, sentinelling serves first and foremost the sentinel itself, and accordingly, groupmates do not reward the sentinel. Significance statement: Sentinel behaviour in cooperatively breeding animals gave rise to numerous debates regarding this seemingly altruistic behaviour. All the group members benefit when sentinels spot predators and pronounce loud alarm calls. Do groupmates pay back sentinels for defending them? To address this question, we examined the reactions of groupmates to the sentinel in 20 groups of the cooperatively breeding Arabian babbler. Immediately after a sentinel bout in the breeding season, sentinels did not (1) copulate more often, (2) get more allopreening, and (3) suffer less aggression, as compared to a control period, even not after warning their groupmates from an approaching predator. These findings support the hypothesis that at least in this species, sentinelling is carried out for the sake of its performer and is not an altruistic behaviour.