State-dependent sentinels: an experimental study in the Arabian babbler

Natural patterns of cooperative sentinel behaviour in Arabian babblers. Turdoides squamiceps have proven consistent with state-dependent models of individually selfish anti-predator strategies. Here we demonstrate experimentally that sentinel effort within groups is determined simply hy individual state. The two highest-ranking males in eight groups were separately fed a supplement of mealworms each for one day at a time. Control days before and after each treatment confirmed that Ilo carry-over effects occurred, and that most normal sentinel activity was carried our by alpha males, then beta males and then by. the rest of the group. When supplemented. both alpha and beta males exhibited similar marked increases in sentinel activity relative to control days. Unsupplemented males and the rest of the group incompletely compensated for these increases with reductions in sentinel effort. Differences in individual bod! mass within groups reflected natural and experimental variation in sentinel effort. Alpha and beta males weighed more than other group members, and gained mass only when supplementally fed. There was no evidence either for competitive sentinel behaviour, nor for an); increased interference between malts during the supplementation treatments. These results therefore provide strong evidence in support of the state-dependent approach to cooperative sentinel behaviour.