GROUP ENCOUNTERS IN WILD GIBBONS (HYLOBATES LAR) - AGONISM, AFFILIATION, AND THE CONCEPT OF INFANTICIDE

1. Gibbons are the least studied apes and traditionally thought to liv e in nuclear families of 2-6 individuals including a pair of breeding adults who maintain lifelong, sexually monogamous relationships and vi gorously defend territories against neighbours. The present paper chal lenges this view. 2. During a long-term study on white-handed gibbons (Hylobates lar) in Thailand's Khao Yai rainforest, 162 encounters were recorded between 3 habituated and 8 non-habituated groups. Encounters characterized 9% of the activity day (mean 8.31 h). Those between hab ituated groups were nine times more frequent (0.7/day) and lasted sign ificantly longer (median 70 min) than encounters with non-habituated n eighbours (median 38 min). It was also found that gibbon group home ra nges (mean 24 ha) overlap extensively (64%) with neighbours', all of w hom were met. However, most previous studies centered on single groups surrounded by unhabituated neighbours. This produced underreporting o f the importance of inter-group interactions, particularly the affilia tive aspects observed presently. 3. Encounters included many elements which seem to have a 'defensive' function. Chases occurred during 61%, contact aggression during 8-9%; each adult and subadult male chased a ll others and was chased by all others. Moreover, encounters occurred often in or near food trees and rates peaked during the early morning when ripe fruit were most abundant. However, a seasonal correlation be tween the rate of encounters and (crude) measures of resource availabi lity could not be detected. 4. The study indicates that gibbon groups are structured by female resource-defense and male mate-defense becaus e adult females led 76% of all travels into and out of fruit trees, wh ereas males moved to the front as soon as neighbours were encountered. Male-male interactions constituted 90% of all inter-group chases. Thi s pattern is compatible with the idea that conflicts over food sources (which can be shared) will rarely provoke contact aggression. Control of mate access, on the other hand, has a much higher relative value f or males. These explanations are more parsimonious than the traditiona l concepts of 'pairbonding' and 'territoriality'. 5. Contrary to earli er assumptions, encounters were nevertheless not always dominated by a ggression. Groups fed, traveled or rested together for prolonged times (35% of encounters). Inter-group play between immatures was likewise common (21% of encounters; 55% dyadic, 35% triadic and quartetic play) . Same-aged play partners are not present in a gibbon's natal group. H owever, gibbon youngsters clearly preferred age-mates during inter-gro up play which may indicate that play is ontogenetically important. 6. The adult female of one group copulated not only with her pair-mate, b ut also with two neighbouring males. The overall proportion of extra-p air-copulations (EPC) was 12% and they occurred during a period when t he female conceived. Encounters thus provide opportunities for alterna tive mating strategies. However, philandering males are faced with the dilemma to lower their mate guard which creates a risk of cuckoldry f or themselves. This could explain why subadults are often tolerated in natal groups beyond sexual maturity, because they assisted the reside nt adult male during agonistic encounters. Moreover, females gave solo great calls during a quarter of all encounters. These calls increase the costs of philandering for the paired male (who cannot answer witho ut giving away that his female is unguarded) and may at the same time attract neighbouring males. 7. For the first time, close spatial proxi mity and body contact between intruding adult males and infants of nei ghbouring groups are reported, including play (during 6% of all inter- group play sessions). Immatures were at times relaxed but at other tim es frightened in the presence of neighbouring males. A near-zero morta lity of infants at Khao Yai shows that infanticide is absent or-at lea st not a regular occurrence. EPCs and a long period of pre-conception copulations could be strategies of females to confuse paternity and fo restall infanticide. In any case, the non-monogamous mating pattern ma kes it likely that kin-relationships extend well into neighbouring gro ups. A reduced level of inter-group competition and aggression is ther efore not surprising.