Manipulating foraging group size: spider monkey food calls at fruiting trees

The food calling behaviour of the Costa Rican spider monkey, Ateles geoffroyi, was studied to determine if food calls function as conditional signals designed to attract conspecifics and thereby manipulate the size of the feeding subgroups. First, individuals were shown to manipulate the number of conspecifics joining them by altering the frequency of calling, Then, based on the expected levels of feeding competition and male-female strategies, four predictions were made concerning the frequency with which individuals should call. As predicted, calls were given more often by subgroups containing dominant individuals than by subgroups with only subordinate individuals, in large trees more often than in small ones, and more frequently when food resources were abundant than when they were scarce. However, contrary to what was predicted, subgroups containing males did not call more than subgroups with only females. In general, the observations suggest that spider monkeys can conditionally broadcast information to manipulate their subgroup size and do so in ways that may decrease feeding competition. Several authors have argued that animal communication should not be viewed as the sharing of information, but as the attempt of the sender to manipulate the actions of the recipient (Maynard Smith 1974; Dawkins & Krebs 1978). Using such a perspective, we can see that the sender can provide information to the mutual benefit of both the sender and recipient, or it may manipulate and/or deceive the recipient to the benefit of only the sender. The calling that sometimes accompanies the discovery of a food source is a situation where animals may manipulate information. For instance, house sparrows, Passer domesticus (Elgar 1986), and toque macaques, Macaca sinica (Dittus 1984), are known to give calls when entering food patches; the calls appear to be conditional to food abundance. Similarly, observations in nature (Wrangham 1977) and experimental evidence (Hauser & Wrangham 1987) both suggest that chimpanzees, Pan troglodytes, withhold information from other group members to avoid food sharing. In this paper, we examine food calling at fruit trees by Costa Rican spider monkeys, Ateles geoffroyi. Spider monkeys have a flexible pattern of social organization in which subgroup size is extremely variable; the size and composition of a subgroup can change a number of times a day (Klein 1972; Symington 1987; Chapman 1988a). When a spider monkey enters a fruit tree, it often *Present Address: Department of Anthropology, Harvard University, Cambridge, MA 02138, U.S.A. utters a 'whinny' (Klein 1972). Eisenberg (1976) describes the whinny as a position indicator that accompanies feeding and suggests that the call may serve to indicate food availability. During a feeding bout, other monkeys may join the caller at the fruit tree and feed with it. We examine whether whinnies function as conditional signals to attract conspecifics to the feeding site. In addition, we look at the frequency of food calling to determine whether variation in calling frequency is associated with variation in the number of monkeys attracted, a pattern that suggests that calling spider monkeys manipulate the size of their foraging subgroup. The ecological literature contains a number of field and experimental studies that have addressed the question of why animals feed in groups (van Schaik 1983; Pulliam & Caraco 1984). These studies have examined the relationships between some assumed measure of individual fitness and the size of the foraging group. When group size is too low, animals may have a greater risk of being preyed upon and/or suffer lower feeding rates as more time needs to be spent scanning for predators (Hoogland 1979; van Schaik et al. 1983). When group size increases, competition for food may increase (Janson 1986; Chapman 1988b). Our first three predictions are based on the assumption that calling carries a potential risk of increasing food competition. Feeding competition is thought t 9 occur in spider monkey foraging groups: individuals compete aggressively for access to food trees (Klein 1972; Klein & Klein 1977) and subgroups 0003-3472/90/050891 + 06 $03.00/0 9 1990 The Association for the Study of Animal Behaviour