Abstract
Observations on 12 groups comprised of two adult males and one adult female (some included one or two fledglings), tame, individually marked, Arabian babblers ( Turdoides squamiceps) in the rift valley in Israel revealed that the babblers compete to guard. The pattern of guarding and the way by which one sentinel replaces another reflect the dominance relationships within the group. The dominant (alpha) male guarded more than any other individual. It interfered with and replaced the guarding by the adult beta male more than it did with the yearlings. About one-third of the replacements occurred less than one minute after the sentinel had assumed guarding. Whereas the dominant often replaced its subordinates directly; subordinates hardly ever replaced their dominants directly. The alpha male often allofed the beta male during the replacement. Replacements and allofeeding of the beta males by the alpha males increased significantly during courtship, when competition over breeding was maximal, and dropped back to their previous level at the start of incubation, highlighting the competitive basis underlying the act of guarding. Competition over altruistic acts, as shown here for guarding, is not compatible with explanations based on the assumption that altruistic acts reduce the fitness (reproductive success) of the altruist. We suggest, in contrast, that by investing in guarding and by intervening in the guarding of its competitors, a babbler demonstrates and signals its quality and its control over its competitors, thereby increasing its prestige and consequently its direct fitness.