Vigilance and its complex synchrony in the red-necked pademelon, Thylogale thetis

Abstract : Several adaptive functions, including gaining information from other group members and detecting predators, are generally ascribed to vigilance in groups of animals subject to predation. Most studies of the effects of neighbors on vigilance have focused on individual vigilance. We investigated the effects of neighbors on vigilance in wild red-necked pademelons Thylogale thetis foraging at night in nonpersistent aggregations in a clearing in rain forest. Neither the total number of pademelons in the clearing nor the numbers at various distances around focal individuals affected the individual vigilance of focal animals. However, focal animals' individual vigilance did change with the distance to their nearest neighbor and also with distance to cover. Pairs of individuals closer than 10 m apart tended to synchronize their bouts of individual vigilance and foraging. The degree of synchrony within pairs increased with both distance to cover and the total number of pademelons foraging in the area and decreased with increasing distance to the pair's nearest neighbor but did not vary with the distance separating the members of the pair. Thus, despite their individual vigilance being unaffected by the number of other pademelons in the feeding aggregation, pademelons were nonetheless sensitive to the presence of conspecifics and adjusted their behavior in relation to their separation from neighbors. Thus, some vigilance benefits may be obtained from the presence of conspecifics even in species that aggregate only temporarily on food patches without forming more permanent social groups.