Patterns of Infant Carriage by Barbary Macaques Macaca sylvanus in Gibraltar

The Barbary macaque (Macaca sylvanus) shows the most extensive infant-carrying behaviour in the genus Macaca. Different hypotheses have been investigated and discussed in previous studies, but none was able to explain all types of observed behaviour. In this study, a group of 46 wild Barbary macaques in Gibraltar was observed during three months of early infant life. Data were collected on dyadic and triadic interactions of all categories of carriers (males, females, juveniles). There was a high level of competition for access to infants. Males preferred to carry infants of multiparous and high-ranking females. Females other than the mother were very rarely involved in infant handling. Juveniles carried their newborn relatives. Adult carriers and mothers used infants as tools to form coalitions to maintain or improve rank position. There is some evidence that position in the hierarchy correlates with reproductive success in Barbary macaques, it is suggested that infant handling is a mating strategy. As the treatment of infants by carriers is consistently very gentle, infants may benefit from integration into the social network. Introduction Newborn infants are of general interest in Old World monkey societies. Although the mother is the main caretaker, other members of the group of different age, sex and relatedness can be involved in interactions with infants (Whitten 1987; Maestripieri 1994). Because Old World monkeys have large body size, the infant birth weight/mother weight ratio is low. Therefore, the mother's cost of carrying its infant is low and helping behaviour from siblings, relatives or fathers is not expected (Wright 1990). Additionally, paternity is very uncertain in many species of Old World monkeys due to promiscuous mating (Van Schaik and Paul 1997). Various authors tested the kin selection hypothesis. The results were conflicting (Strum 1984; Taub 1984; Paul, Küster et al. 1992), so alternative hypotheses have been established and examined. In the tribe Papionini, the Barbary macaque (Macaca sylvanus) shows the most extensive relationships between infants and other troop members (Maestripieri 1998). Gentle interactions take place in two different ways (for a detailed survey see Deag 1980). First: Dyadic interaction involves a partner and an infant. The partner carries the infant, holds it on the lap, guards and plays with it. Dyadic interactions can last from a few seconds up to several minutes. Second: Triadic interactions involve two partners and an infant. The infant is held by one or both partners. Various characteristic behaviour patterns occur, such as teeth chattering, lip smacking and inspection of the infant's genitalia. The infant plays a passive role. Triadic interactions last only a few seconds. Male and female Barbary macaques seem to have different motivations to interact with newborns. Separate hypotheses have therefore been proposed (for females see Paul and Kuester 1996a; for males see Paul, Kuester et al. 1996). Males: (1) Males use infants for their mating strategy. (2) The "agonistic buffering" hypothesis suggests that triadic interactions reduce aggression among males. Females: (1) The "learning to mother" hypothesis assumes that nulliparous females are primarily involved in infant handling. They learn necessary skills for rearing an infant. (2) The "by-product" hypothesis suggests that infant handling is a nonadaptive behaviour. The affiliation of a female towards an infant may be a natural response to all newborns. (3) The "reciprocity" hypothesis proposes that a non-mother female helps to rear an unrelated offspring to ensure that this help will be reciprocated when the handler herself gives birth. (4) The "reproductive competition" hypothesis assumes that females harm infants to reduce their future survival and thus obtain better conditions for their own offspring. Two other hypotheses have been established for all handlers: (1) The "coalition formation" hypothesis suggests that animals interact with infants and with each other to establish profitable relationships. (2) The socialization hypothesis assumes that the partner integrates the infant into the social network of the troop (Small 1990; Timme 1995). The present paper is too brief to be able to test all of the above hypotheses. Instead, it attempts to examine this complex behaviour from another perspective by choosing the newborns as focal animals. In this study, data were collected on dyadic and triadic interactions of all categories of carriers (males, females, juveniles) to find out whether there are differences among infants, whether there is competition for access to infants, whether the animals use infants to form coalitions and whether the handled infants benefit themselves. Methods During the study period, approximately 230 monkeys were living on Gibraltar. They were divided into seven troops. All of these troops arose by fission from the two main groups (Middle Hill and Queen's Gate) over the last decade (Fa and Lind 1996). All troops except the Middle Hill troop have intense contact with tourists. This study was conducted with the Middle Hill troop because its natural behaviour is least biased. The troop's home range is situated in a military area without public access. Several studies have been conducted since 1994 by students of the Anthropological Institute of Zürich, providing demographic data for the last five years. During the study period, there were 46 animals (for age and sex composition, see Table 1). All macaques were fed with fruits and vegetables once a day by government employees. As the provisioned food could not satisfy the daily need, the animals spent a lot of time feeding on natural resources such as seeds, roots, fruits and flowers. Recording of the daily travel pattern yielded an estimation of the home range of approximately 50 hectares. Table 1: Age and sex composition of the Middle Hill troop in July 1999 following the categories of Merz (1984) Adult Subadult Juvenile 2 Juvenile 1 Infant Total