Group Fusion Among Wild Toque Macaques: an Extreme Case of Inter-Group Resource Competition

1. Two independent groups of toque macaques (Macaca sinica) fused into one larger cohesive group. The groups were part of a population of 18 to 22 groups that inhabit natural forest at Polonnaruwa, Sri Lanka, and that had been observed more or less continuously from 1968 to 1985. 2. The group fusion generally supports WRANGHAM'S (1980) model of the evolution of group living and female residence patterns in "female-bonded" primate groups. 3. The effect of the group fusion was that of a take-over by the dominant group (Group A) of the subordinate group's (Group SG) home range. It was the extension of a competitive relationship between the two groups over at least seven years before the fusion whereby they contested concentrated food resources, such as fruiting trees, more frequently with each other than with any one of their other respective neighboring groups. The relationship was highly asymmetrical as the dominant Group A always supplanted the subordinate one (Group SG). 4. The group fusion was preceded by a change in ranging behaviors whereby: (a) Group A itself avoided the encroachment of a dominant group in the northern regions of its home range by shifting its northern boundary southward; (b) Group A intensified its use of the southern parts of its range which overlapped with that of Group SG; and (c) Group SG used its area of overlap with Group A less intensely and shifted its range away from Group A and into previously unused areas. 5. The take-over occurred at a point in time in the history of these two groups when the defensive abilities of the subordinate group (SG) were at their weakest. This weakness was brought about by the death of two of the three matriarchs in the group shortly before the take-over. Other factors contributing to the merger were: (a) the emigration of all of the subordinate Group SG's males to the neighboring dominant Group A; (b) an ineffective defense by the new males of Group SG against the incursions of a new coalition of males from Group A that involved the previous ousted alpha male of Group SG; (c) the emigration of a juvenile female of Group SG to follow her ousted father to Group A; and (d) the friendly and protective behaviors of the alpha female of Group A towards the cowed females of Group SG during and after the take-over. 6. Aggression was most frequent at the onset of the take-over and was directed at the Group SG females primarily by the younger and subordinate females of Group A whose dominance ranks and future reproductive successes were threatened the most by the addition of new females to the group. Normally such aggression would serve as a barrier to group fusion. 7. In the merged group behaviors were differentiated most strongly according to familiarity or group of origin (kinship). This was evident from measures of (a) spatial affiliation, (b) affiliative behaviors such as grooming, and (c) agonistic support as was reflected by the proximity of ranks in the dominance hierarchy. Also, the probable father of the subjugated females defended them against the aggression of the dominant group's females. 8. The alpha female of the dominant Group A initiated more grooming, hugging and defence of the Group SG females than any other natal member of Group A. Consequently, the subjugated females spatially associated with her more than with any other natal animal of Group A. Shortly after the take-over much of her affiliative behavior had been directed towards the subjugated group's adult female, who had been carrying an infant. During these encounters the alpha female often fondled this infant and restrained it from nursing; but, her contribution to its death one month after the take-over is unknown. 9. The effect of the affiliative and protective behaviors of the alpha female of the dominant Group A towards the subjugated females of Group SG was to reduce the level of aggression between the females of these two groups. In this way her behavior facilitated the group take-over and promoted a peaceful coexistence in what was essentially a competitive relationship. 10. The integration of the subjugated females into the merged group within three months after the take-over was marked by: (a) a decrease in aggression from the females of the dominant Group A to a level not significantly different from that among the subjugated females themselves; and (b) an increase in spatial affiliation and grooming between the females of the merged groups. Nevertheless, even three years after the merger, the subjugated females often split away from the rest of the group and foraged independently in an apparent attempt to avoid resource competition with the dominant natal animals of Group A. 11. Both groups benefited from an increase in home range size as a result of the merger, and this increase was greater for the subordinate group than for the dominant one. But, whereas the dominant group gained priority of access to all resources in the enlarged home range, the subjugated group did not. Instead, the females of the subjugated group occupied the lowest ranks in the dominance hierarchy of the merged group and thereby lost priority of access to resources in the formerly exclusive areas of their range. The overall cost/benefit of the take-over therefore was greater for the subordinate group whose home range was taken over. 12. As a result of this socio-economic asymmetry the survivorship and reproductive success among the natal members of the dominant group were greater than among the members of the subordinate group all of whom (except one offspring that was born after the merger) died within eight years after the merger. The dominant group grew at the expense of the subordinate one (DITTUS, 1986). 13. This degree of behavioral and demographic asymmetry among the subjugated females was not observed among their male relatives who had immigrated into the dominant group before the take-over. 14. The take-over did not alter the relative sizes of the merged group with those neighboring groups that had dominated either Groups A or SG before the merger. Therefore, the dominance relations among these groups also remained unchanged. 15. The chance of any one group taking over another's home range and merging with it in any one year is very low (P < .004). It would most likely involve a small group with limited defenses against a larger one and under conditions where intergroup competition for shared resources is accentuated.