Group Encounters in Wild Gibbons (Hylobates Lar): Agonism, Affiliation, and the Concept of Infanticide
Authors: Reichard Ulrich , Sommer Volker 
Published: April 2008

1. Gibbons are the least studied apes and traditionally thought to live in nuclear families of 2-6 individuals including a pair of breeding adults who maintain lifelong, sexually monogamous relationships and vigorously defend territories against neighbours. The present paper challenges 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 habituated groups were nine times more frequent (0.7/day) and lasted significantly longer (median 70 min) than encounters with non-habituated neighbours (median 38 min). It was also found that gibbon group home ranges (mean 24 ha) overlap extensively (64%) with neighbours', all of whom were met. However, most previous studies centered on single groups surrounded by unhabituated neighbours. This produced underreporting of the importance of inter-group interactions, particularly the affiliative 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 all 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 between the rate of encounters and (crude) measures of resource availability could not be detected. 4. The study indicates that gibbon groups are structured by female resource-defense and male mate-defense because adult females led 76% of all travels into and out of fruit trees, whereas males moved to the front as soon as neighbours were encountered. Male-male interactions constituted 90% of all inter-group chases. This 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 for males. These explanations are more parsimonious than the traditional concepts of 'pairbonding' and 'territoriality'. 5. Contrary to earlier assumptions, encounters were nevertheless not always dominated by aggression. 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, 45% triadic and quartetic play). Same-aged play partners are not present in a gibbon's natal group. However, gibbon youngsters clearly preferred age-mates during inter-group play which may indicate that play is ontogenetically important. 6. The adult female of one group copulated not only with her pair-mate, but also with two neighbouring males. The overall proportion of extra-pair-copulations (EPC) was 12% and they occurred during a period when the female conceived. Encounters thus provide opportunities for alternative mating strategies. However, philandering males are faced with the dilemma to lower their mate guard which creates a risk of cuckoldry for themselves. This could explain why subadults are often tolerated in natal groups beyond sexual maturity, because they assisted the resident 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 without giving away that his female is unguarded) and may at the same time attract neighbouring males. 7. For the first time, close spatial proximity and body contact between intruding adult males and infants of neighbouring groups are reported, including play (during 6% of all inter-group play sessions). Immatures were at times relaxed but at other times frightened in the presence of neighbouring males. A near-zero mortality of infants at Khao Yai shows that infanticide is absent or at least not a regular occurrence. EPCs and a long period of pre-conception copulations could be strategies of females to confuse paternity and forestall infanticide. In any case, the non-monogamous mating pattern makes it likely that kin-relationships extend well into neighbouring groups. A reduced level of inter-group competition and aggression is therefore not surprising.