White-handed Gibbons: Tough Monogamous Love?

Native to Southeast Asia, white-handed gibbons (Hylobates lar) can be found in the rainforests of Indonesia, Thailand, Myanmar, and Laos. These gibbons are not easy to find because they spend 99% of their time off the ground. But if you do spot them, they are busy launching themselves up to 15 metres from branch to branch, at speeds up to 30 kilometres per hour.

During my visit to Monkeyland, I heard a loving story about two resident white-handed gibbons, Atlas and Siam. Atlas is a 21-year-old male who was rescued within South Africa, while Siam is an 11-year-old female who was rescued from a zoo in France. Our guide, Prince, shared that white-handed gibbons are known for their monogamous and mate-for-life relationships. They do not spend more than twenty minutes without each other. When time is up, these gibbons sing their melodious duets so that they can reunite for a grooming session.

As we walked along the guided trails, there sat Atlas casually on the ground, blocking our path. In a matter of minutes, we heard rustling of leaves on the branches above us. Here came Siam, swinging and free-falling in this natural playground of hers. It was time for her to reunite with her partner, Atlas. What was intriguing about their relationship was despite Atlas’ sterility, Siam was willing to partner with him. This prompted me to research more on pair bonding and monogamy in this species.

Tough Love

In 1996, Ryne A. Palombit reported findings from his comparative study between the siamang (H. Syndactylus) and the white-handed gibbon regarding their heterosexual pair bonds and approaches they employed to maintain these bonds. Palombit evaluated that white-handed gibbons have limited physical interactions beyond grooming. He went as far as to comment that grooming “are virtually the only contexts in which adult gibbons touch one another” (p. 343). Moreover, in Thad Q. Bartlett’s Intragroup and Intergroup Social Interactions in White-Handed Gibbons (2003), he cited Ellefson, who reported “grooming accounted for 3% of the activity period of adults” (p. 252). As such, white-handed gibbons’ primary social activity constitutes a minute fraction of their time awake. In other words, white-handed gibbons barely socialize amongst themselves.

In another twist in this heart-breaking backdrop to a supposed love story, Palombit (1996) further reported an asymmetry in white-handed gibbon relationships. “Female gibbons rarely initiate grooming sessions and provide far less grooming to males than they receive from them” (p. 344). And when they do initiate, the female gibbons engage in presenting, rather than offering the act of grooming. ‘Presenting’ refers to “elevating of the head, limb, or torso towards or directly in front of another individual” for the purpose of inviting for grooming (p. 326). As such, female gibbons demand their partners to be disproportionately more invested in the relationship while reciprocating minimally. Talk about tough love!

This tough love becomes a greater challenge during times of food scarcity. During seasons when fruit have yet to ripe, there were low rates of social interaction between gibbons at Khao Yai National Park, Thailand. A plausible explanation is gibbons prioritise energy conservation when there is a lack of high-caloric resources, thus forgoing non-subsistence activity (Bartlett, 2003, p. 253).

Questioning Monogamy

Atlas and Siam are engaged in a monogamous relationship, perhaps because there are currently only two white-handed gibbons in Monkeyland. However, studies have shown that gibbons can and have exhibited multi-male and polyandrous relations for various reasons.

Scientists Tommaso Savini, Christophe Boesch, and Ulrich H. Reichard had set out to test the relationship between resource distribution and incidence of polyandry in Khao Yai white-handed gibbons. They reported in 2009 that there was a negative relationship between home range quality and home range size (p. 509). ‘Home range’ is the ecological space where an animal primarily lives in and gathers its resources from. Thus, the lower the quality of the home range i.e. sparsely distributed resources, the larger the home range size.

Subsequently, their study revealed a positive relationship between home range size and incidence of polyandry (p. 509). The larger the home range size, the higher the probability of polyandry. A way to understand this is the secondary male gibbon can aid in the protection of the territory and female (p. 510). As such, with a larger home range size, multiple males may cooperate to defend the integrity of the territory. Therefore, we can observe how the environment shapes the social structure of gibbons.

More recently, a 2013 report by Claudia Barelli et al confirmed extra-pair paternity (EPP) amongst white-handed gibbons after investigating the genetic relationships between males, females, and offspring of the wild population in Khao Yai. But such EPP has to firstly be preceded by extra-pair copulations (EPCs). The investigation revealed a case of two successive offspring with extra-pair partners. A hypothesis could be the primary male was sterile, and as compensation, the female turned to EPCs. Other possible hypotheses include material benefits or paternal care the female may enjoy from EPP (p. 1193). The latter is less probable because previous studies have shown that the males do not provide direct paternal care to their offspring (p. 1186). Therefore, there is a flexible option for promiscuous reproduction.

My mind drifts back to Atlas again. If the hypothesis was true, I wonder what would happen if a new male white-handed gibbon is introduced to Monkeyland. It seems that Atlas might have to share Siam with another male. Nevertheless, Prince informed me that Atlas seems to believe he is a capuchin because he spends most of his time with them. I would not be surprised if Atlas is nonchalant about entering an extra-pair partnership. Till then, I give my best wishes to both Atlas and Siam… perhaps more wishes to Atlas though!

References:

Barelli, C., Matsudaira, K., Wolf, T., Roos, C., Heistermann, M., Hodges, K., . . . Reichard, U. H. (2013). Extra‐pair paternity confirmed in wild white‐handed gibbons. American Journal of Primatology, 75(12), 1185-1195. doi:10.1002/ajp.22180

Bartlett, T. Q. (2003). Intragroup and intergroup social interactions in white-handed gibbons. International Journal of Primatology24(2), 239-259.

Palombit, R. A. (1996). Pair bonds in monogamous apes: A comparison of the siamang hylobates syndactylus and the white-handed gibbon hylobates lar. Behaviour, 133(5-6), 321-356. doi:10.1163/156853996X00486

Savini, T., Boesch, C., & Reichard, U. H. (2009). Varying ecological quality influences the probability of polyandry in white-handed gibbons (hylobates lar) in thailand. Biotropica, 41(4), 503-513. doi:10.1111/j.1744-7429.2009.00507.x

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