The Social Organization and Mating System of Khao Yai White-Handed Gibbons: 1992-2006

Appendices

Appendix 1: Multimale Single-Female Groups

Multimale groups developed when an adult male immigrated and joined an existing pair (n = 11), except once when a multimale unit formed by female-female replacement. In this case, a resident female was replaced by another female; a young adult male group member and his father subsequently both became the female’s new partners. Although multimale groups usually formed through male immigration, not all male immigration events resulted in multimale groups because male immigration sometimes led to a rapid male replacement when the former resident male was ousted by the newcomer. Male immigration was competitive and usually accompanied by intense aggression between males, except when a father immigrated into the home range of his son.

With the notable exception of cases where social fathers migrated onto their sons’ home ranges, successful immigrants all displaced the resident males from their close relationships with resident females. Male replacement had previously been noticed in this population (Treecuson and Raemaekers 1984; Brockelman et al. 1998), and the Khao Yai long-term monitoring suggests that it amounts to a common cause for group composition changes.

I have observed variable patterns of multimale group formation. Two males in this study, for example, delayed natal dispersal before immigrating into a neighboring group, and an additional observation that closely resembles these cases has been described in detail elsewhere (Brockelman et al. 1998). Another two immigrants were secondary dispersers (one secondary dispersal event occurred after a male was replaced by another immigrant and the other for unknown reasons). An unexpected new pattern observed in this study was males transferring into groups where their sons were the resident males (n = 2). In the first case, son Amadeus dispersed from group “A” in 1999 and founded group “T” with the previously solitary female Brenda (Fig. 1). The following year, a new male immigrated into Amadeus’ father Fearless’ group and replaced Fearless as the female’s primary mate (see below). Fearless’ group was multimale for about 6 months until Fearless briefly transferred into neighboring group “E” before moving again to join his son Amadeus on home range “T.” Before and until shortly after his father’s arrival, Amadeus had fiercely resisted immigration attempts by other males, whereas no aggression was observed against his social father. Initially, the female had duetted almost exclusively with Amadeus. Subsequently, however, during a time when Amadeus moved back and forth between his group and neighboring group “SD,” Fearless became the primary duet partner of the female, and remained so even after Amadeus’s permanent return. Social grooming between Fearless and Amadeus had been frequent when they were residents in group “A,” but no grooming was observed between them in group “T,” although both groomed with the female.

In the second case where a social father followed his son, mature son Christopher of group “C” dispersed into group “A” in 2000. About 4 years later, an unknown male immigrated into the group of his father, Cassius II. The immigrant male frequently provoked agonistic interactions with Cassius II. Within a week, Cassius II transferred to group “A” and was accepted by his son Christopher and female Andromeda without hostility. A nearly mature son of Cassius II and brother of Christopher, Chikyū, co-dispersed with Cassius II. Three months later another younger brother, Chuu, likewise transferred into group “A.” The duet pattern in the group remained unchanged after the arrival of the new males, and Christopher is still the regular duet partner of the female at the time of writing. By 2005, Chikyū had reached adulthood, which made the unit the only known multimale gibbon group at Khao Yai with three fully adult (although related) males living with one female. Social relations among the males appear relaxed, and no overt aggression has been noticed, perhaps due to their kinship. Christopher has been seen allogrooming with his younger brothers, who have likewise been groomed by Cassius II, but no allogrooming has been observed between Christopher and Cassius II. All males have been seen allogrooming with female Andromeda.

In multimale group “N” adult males Claude and Nithat were also presumed to be father and son, but unrelated to the immigrant female Hima. After Hima’s arrival in group “N,” she displayed continuous hostility towards resident female Natasha. Hima consistently interfered with Natasha’s foraging, threatened, and chased her. Even though Natasha began lagging behind the group soon after Hima’s appearance, and showed submissive behaviors toward Hima, the young immigrant female continued to dash back and chase Natasha out of fruiting trees. Multiple times Natasha escaped from Hima by descending to the forest floor, which is a very rare behavior in wild gibbons, where she remained cowering while Hima hovered above her. Twice, contact aggression was observed, but more fighting may have occurred, because a few weeks later Natasha disappeared with unknown fate.

Hima began duetting with Nithat, the adult son of the group, when she arrived in the group. During the first days after Hima’s immigration, duets were also still heard from the resident pair Natasha and Claude. However, Hima’s ongoing threats presumably forced Natasha to stop singing shortly thereafter. Hima and Nithat continued to duet, and Claude began to also add replies to Hima’s great calls (for a description of gibbon duet calls see Raemaekers et al. 1984). The males did not overlap with their singing; Nithat would reply first, followed by a less-vigorous response from Claude.

Appendix 2: Multimale Single-Female Group “A”

Male Amadeus of group “T” (described above) tried unsuccessfully to establish polyterritorial polygyny with two females. He was Brenda’s pair mate from the summer of 1999. However, in the beginning of 2001, after young female Cyrana immigrated into group “E,” joining pileated female Emanuelle and male Bard (see Appendix 3), Amadeus also began traveling with members of group “E.” Male Bard was rarely seen with Cyrana and Emanuelle, and Amadeus copulated and sang duets with Cyrana, but also regularly traveled and interacted with Brenda. Repeatedly, Amadeus led Brenda toward the overlapping area between the home ranges of groups “T” and “E.” Amadeus appeared to increase group “T’s” share of the overlap zone as he foraged ever deeper into group “E’s” range. He would then slowly depart from the overlap area and travel even further into group “E’s” home range. He emitted contact calls and appeared to wait for Brenda to follow. Brenda rarely crossed deep into “E’s” home range and would eventually stay behind when Amadeus proceeded further. Their calling and activity in the overlap area regularly resulted in contact with group “E,” and Amadeus was then seen copulating and duetting with Cyrana. Although no quantitative data are available, Amadeus’ frequent movement back and forth and simultaneous interactions with the two females gave the impression that he was trying to persuade one of the females to join him and follow onto the other female’s home range. He also appeared to try to lead Cyrana toward Brenda and her home range. Cyrana followed him deeper into the group “T” home range than Brenda had followed onto the group “E” home range. However, Cyrana remained cautious and Brenda’s constant hostility apparently prevented spatial proximity between the two females and the development of a multifemale group. Brenda frequently threatened Cyrana, and long chases were witnessed during which Brenda pursued Cyrana back onto the “E” home range. Intergroup encounters during this period often exceeded two hours. Eventually, however, Brenda would leave the encounter area and forage away toward the opposite side of her home range. Amadeus often remained, traveled, and spent the night with Cyrana, before he would return to Brenda the following morning or during the day, usually when he heard Brenda singing solo female great calls. One morning, Brenda began calling close to the overlap between her range and that of group “ST.” After only a few minutes, a dark male rapidly approached her from the south. The pair copulated and started to duet when suddenly Amadeus brachiated at high speed down the slope and vigorously chased the intruder away. After about two months of changing location and trying to maintain simultaneous socio-sexual relationships with the two spatially separated females, Amadeus ceased traveling with Cyrana and returned to exclusively reside with Brenda. During all this time, Emanuelle had been with Cyrana and Amadeus on most days, but the group’s resident male, Bard, was rarely seen with them. There was low-intensity hostility between Emanuelle and Cyrana and soon after Amadeus ceased traveling with members of group “E,” Cyrana emigrated and Bard was seen back with Emanuelle again.

Appendix 3: Multifemale Single-Male Group “J”

Group “J” was identified as multifemale when it was first contacted in November 1998, because two females each carried approximately 2-month-old infants. The group composition remained stable for 26 months until January 2001, when one female and her now-juvenile offspring disappeared with unknown fate. Between November ‘98 and January ’01, the group was contacted on 25 days. Qualitative observations were available for ∼25 contact hours despite the females’ fear of humans because the adult male of the group, Frodo, was a known, habituated individual born in study group “A” (cf. Brockelman et al. 1998). Frodo dispersed from group “A” in 1990. Between 1991 and 1992 he was encountered a few times with a female of unknown origin (designated as group “K”). During the 1993 census, Frodo was in the company of a new female in the same area as before, but by the end of 1994 both individuals had disappeared. Frodo was rediscovered four years later in group “J.”

No hostility was noticed between the females in group “J”; instead, the females were repeatedly observed calmly feeding within 5 m of each other in the same tree crown. The females likewise both tolerated close spatial proximity with each other’s infants during feeding and travel as the infants became more independent from their mothers. The females traveled together and in the company of the male on a daily basis and coordinated their movements through contact vocalizations. Their travel pattern resembled those observed in units with other social organizations, e.g., pair-living gibbon groups. Interestingly, both females were heard to sing duets with the male. Neither interfered with the song of the other nor did they sing “in parallel” as typical for maturing daughters with their mothers (cf. Brockelman and Schilling 1984; Raemaekers et al. 1984). Instead, on some days the male first sang a duet with one female and later with the other; on other days, only one of the females duetted with the male during the contact time. Such duet pattern was unique to this group.

Observers did not witness the emigration or death of one of the females and her offspring and the females’ social histories were unknown. It is possible that the females were mother and daughter (or sisters) and that Frodo immigrated and displaced the resident male at around the time when the presumably nulliparous daughter reached sexual maturity. Both females may subsequently have copulated and conceived with Frodo and the group remained stable until the onset of a new reproductive cycle when the females’ offspring were independent. Female sexual competition is one possible explanation for the disappearance of one of the females. Such a theory of the origin of a multifemale group would parallel the familial polygyny described by Srikosamatara and Brockelman (1987), although in this case the multifemale structure lasted much longer.

Alternatively, two unrelated females may have formed a multifemale group with the male. The presence of two infants, an absence of overt feeding competition, and the females’ unusual alternating duet singing with the male, which has never been observed in another gibbon group at Khao Yai or elsewhere, support such interpretation. Delayed female dispersal has in fact been rare in Khao Yai. Still, it remains unclear why one of the females then left after more than a year. Predation or another sudden death appears unlikely, because the female disappeared with her independent juvenile offspring, leaving voluntary emigration the most plausible explanation. Perhaps the benefits of multifemale grouping were outweighed by increasing costs of resource competition and increasing group size, or a better opportunity arose elsewhere?

A pressing question remains: why or how a female could join another female on her territory in the first place? So far, it has forcefully been argued that ecological constraints would not allow gibbons to jump over the poylgyny-threshold and form multifemale groups (Brockelman and Gittins 1984). However, in a recent study Savini et al. (2008) show that female reproduction is less food-limited then previously assumed, which suggests that under the right ecological conditions gibbons may be able to form multifemale groups. More research is needed on other multifemale gibbon groups to illuminate which specific ecological and social conditions allow for the development of socio-sexual multifemale grouping.

Appendix 4: Mixed-Species Multimale Multifemale Group “E”

The mixed-species multimale multifemale group “E” formed after a pair-living female disappeared and the adult male and two immature offspring were first joined by a pileated gibbon female (H. pileatus) and then within a few days by a white-handed gibbon female (H. lar). Khao Yai National Park marks the eastern distribution border of the subspecies Hylobates lar entelloides. The western part of Khao Yai National Park is inhabited by white-handed gibbons, whereas pileated gibbons live in the eastern part. A small hybrid zone exists ∼30–40 km east of the Mo Singto – Klong E-Tau research site (Brockelman and Gittins 1984; Suwanvecho 2003). Central Mo Singto has traditionally been believed to be inhabited exclusively by lar gibbons (Brockelman 1975; Raemaekers et al. 1984), but over the years pileated gibbons have occasionally migrated into the Mo Singto – Klong E-Tau area where their species-specific calls are sometimes heard.

The pileated gibbon female was first observed in group “E” in spring 1997 and remains there at the time of writing. Both females of group “E” were adult at the time of their immigration into “E’s” home range, but both appeared to be young, presumably nulliparous, judging from their small, nonpendulous nipples. By the end of 1997, the mixed-species trio was joined by a second adult male from the neighborhood, who emigrated after he was replaced by another male. The relationship between the males appeared relaxed and tolerant as no overt aggression – but also no allogrooming – was witnessed between the two. In contrast, relations between the females seemed tense. The pileated gibbon female repeatedly threatened and briefly chased the white-handed gibbon female. The males were not observed to intervene in situations of female hostility, and both females duetted with the males. The white-handed gibbon female emigrated in 1999 and was joined by a recently matured neighboring male. Both animals disappeared from the study site the following year. The pileated female remained with the two white-handed gibbon males until one of them likewise emigrated, changing the structure to pair-living. During the 2005 census, however, the latter pair was again found in the company of a white-handed gibbon female of unknown origin, now as a mixed-species multifemale group.