A brief clip of a Siamang (Symphalangus syndactylus) vocalizing.
Some mornings in the forests of Indonesia, a male and female gibbon will perform a musical duet. The pair will call out, staking their claim in the forest, often answered by neighboring pairs, their treetop display the result of much practice so that the pair can get things “just right.” In order to achieve an effective display the two gibbons need to form a pair bond, but the strength and permanence of the pair-bond between these apes has long been obscured by the sociological biases of the scientists studying them. Indeed, the ideas of what a “nuclear family” should be and what monogamy means in nature have been heavily influenced by the culture and moral sense Western scientists have brought to their work in the past, but the reality of gibbon monogamy is much more complex (and startlingly familiar) than has been understood previously.
In the Western world, the term “monogamy” is typically assumed to mean “having only one mate,” often being related to the concept of only having that one mate for life (which, in and of itself, is not a necessarily consequence of monogamy). If a pair of a particular species of organism, one male and one female, were observed to remain in close proximity to each other and mated only with their partner, the relationship was termed monogamous, no distinction being made between the social and reproductive aspects of the behavior. Gibbons* seemed to be a perfect example of monogamy in non-human primates, there being little sexual dimorphism between males and females, one adult member of each sex being found in a pair (lone gibbons are rarely seen) that defend a territory by duetting and chasing members of their own sex out of the territory. Such observations helped support the idea that the gibbons had a strong pair bond which singing together helps reinforce, forming a sort of “nuclear family” with offspring leaving their parents when they become subadults to pair up with another subadult of the opposite sex, in turn staking claim to their own territory. Indeed, the “faithful” behavior of the gibbons forms an appealing natural narrative, but we now know that such stories were far too good to be true.
*For the purposes of this essay we’ll primarily be looking at the Siamang (Symphalangus syndactylus and White-Handed (or Lar) Gibbon (Hylobates lar).
A female White-Handed Gibbon (“Solstice”) at the Philadelphia Zoo.
Habituation is a problem for zoologists, and many times a researcher will habituate themselves to one group of animals and study them in detail, but such an approach can have drawbacks. If there are other groups nearby that are not habituated to the researcher they may avoid contact with the study group when they would otherwise interact, and this seems to be the case with gibbons. In previous studies researchers would often habituate themselves to just one pair, other neighboring pairs typically avoid the scientists. The results? Mating outside of the pair that might normally occur was blocked as the non-habituated gibbon wouldn’t allow researchers to get close enough, running off through the canopy instead. Furthermore, many studies take place over the course of a few months or a year, these short studies mask what is actually going on in the Indonesian forests.
As I noted previously, it used to be thought that gibbon pairs formed when a subadult male and subadult female from two “families” met up and gained access to their own plot of forest, and this does in fact happen. What was unexpected, however, is that gibbons are much more opportunistic and do not stick with the same mate their entire lives. A longer 6 year study, undertaken by Ryne Palombit and others, showed that gibbons will often leave their partner if an opportunity arises nearby, i.e. a male might abandon his female partner if the male of another nearby pair dies or disappears. In turn, another male might move in when the 1st male leaves or the abandoned female might die, but it was a bit surprising to find that not all pairs were subadults that were in permanent, monogamous relationships. The realization that gibbons “cheat” and “divorce” leads us to a very important realization; there is a difference between social monogamy and reproductive monogamy, the presence of one not necessarily indicating the other. This illuminates the fact that reproductive monogamy is much rarer than previously thought, mating being a much more opportunistic affair than one dictated by social bonds. Indeed, there may very well be advantages to being socially monogamous that are distinct from reproductive monogamy, the pair bonding of gibbons perhaps having much more to do with defending a territory (and hence resources) than whatever sense of long-term affection was attributed to them previously.
Things can get even stranger, though, as researchers discovered when multiple groups were habituated and the free interchange of members between groups could be documented.
A diagram of the interactions between groups of White-Handed Gibbons W and G. Members of each group are marked by the letter W or G and the different life stages are as follows; (AM) = Adult Male, (AF) = Adult Female, (SM) = Subadult Male, (SF) = Subadult Female, (JM) = Juvenile Male, (JF) = Juvenile Female, (GI) = Infant born to “G” mother. Adapted from Palombit, R. (1994) “Dynamic pair bonds in hylobatids: implications regarding monogamous social systems.” Behavior, vol. 128 (1-2), pp. 65-101
What we first see in the above diagram is what might be expected in White-Handed Gibbons; a subadult male and female left their own families and were able to establish themselves in a patch of territory (which can be difficult to find). Almost immediately, though, things take an odd turn. Rather than staying with their original “nuclear” family, the juvenile brothers of the subadult male move with him and live with him and the subadult female. This was interesting enough alone, but then something even more unexpected occurred; the adult male of the original “G” group disappeared, probably succumbing to an injury sustained earlier, and the three dispersed W males (the subadult and his juvenile brothers) move into the territory of the original G family, leaving the subadult G female behind. The subadult W male moved between the subadult female and her mother, but ultimately the interactions between the mother [G(AF) in the diagram] and her offspring [G(SF)] became very hostile and the males stayed with G(AF). Later, G(AF) gave birth to a baby gibbon most likely fathered by W(SM), although it is not revealed what happened to the subadult female [G(SF)].
Clearly, the notion that White-Handed Gibbons are pair bonded for life to the sweethearts of their youth cannot be maintained, who pairs up with whom seeming to have a lot to do with opportunity and territory. There are a lot of unexplained factors here, though, like why the juvenile males followed the subadult W male everywhere he went, but it does serve to prove that long life-history studies are needed before we can say whether a species is monogamous or not. If researchers came into the study at a different point or only studied one group of animals for a shorter amount of time they probably would not have seen this sort of interaction, and in a forest that is (to borrow my professor’s phrase) “wall-to-wall gibbons” such switches probably are fairly commonplace events, especially when a member of a pair bond goes missing or disappears. As we saw from the above example, though, individuals can’t have it both ways; the territoriality of gibbons means that there are going to be aggressive interactions between the two “lovers” in neighboring territories, the gibbon that previously went between the two ultimately having to make a choice and pair bond to just one (even if only temporarily). Such a “commitment” can be critical, as it takes quite a bit of practice and investment of time for a male and female in a pair to learn to sing with each other like they do every few mornings, making it a poor strategy for males to try and move between females too often. All of this is not to say that pair bonding isn’t real or isn’t significant, it most certainly is, but it appears that it is not as powerful as once assumed and most species previously assumed to be strictly monogamous (be they mammals, birds, or another sort of creature) are nothing of the sort. Even strict serial monogamy (having more than 1 mate during a lifetime but only mating with the other member of the pair) seems to be out for gibbons, repeated extra-pair copulations being observed when mates had their backs turned.
If nothing else, observations such as these show that there is an important difference between reproductive monogamy and social monogamy. While reproductive monogamy requires social monogamy to at least some extent, it is fully possible to be socially monogamous (pair-bonded and have the majority of mating with one individual and act to defend a territory) but be, if I may use the term here, “unfaithful.” Part of the reason we’re recognizing this now is because of narrow-sighted research design, but also the desire that we may have for nature to vindicate our own social opinions and values, especially when it comes to primates. There are major debates over whether humans are more like the aggressive and violent Chimpanzee (Pan troglodytes) or the bohemian and liberated Bonobo (Pan paniscus)*, but why must we be like either? Do we really require our closest living relatives, molded by at least 5 million years of evolution, to edify or condemn our current behaviors? The African apes (and other non-human primates) can tell us much about ourselves, but even though we understand that we did not evolve “from” chimpanzees or bonobos many debates seem deeply rooted in the notion that this is what has occurred, the behavior predilections of our ancestral stock biologically determining whether we are brutes or sexually-liberated peacemakers. Trying to find our own social conscience in nature may be poetic but it does not result to good science when taken into the lab or field, especially since we often forget the parts of ourselves and our society that we’d rather not talk about. Looking out our living relatives and the rest of nature as it is and not as we hope that it should be is one of the most difficult things to do, but it reveals a natural world far more interesting and wonderful than one that we might intellectually try to create.
*I use these terms to exemplify the common public image of these animals. Chimpanzees were once thought to be peaceful, but longer-term studies have shown them to show a greater range of behavior, especially violent ones (strangely making them “more human”). Bonobos, while still poorly understood, are often championed as the evolutionary “alternative” where sex is a cure-all for social tension and violence is kept to a minimum. Such considerations could make for a post in and of itself, so I will tread no further in this politically and socially charged subject for now.
Fuentes, A. (1998) “Re-Evaluating Primate Monogamy.” American Anthropologist, New Series, Vol. 100, No. 4., pp. 890-907.
Geismann, T. (1999) “Duet Songs of the Siamang, Hylobates syndactylus: II. Testing the Pair-Bonding Hypothesis During a Partner Exchange.” Behavior, vol. 138, no. 8, pp. 1005-1039
Geismann, T. (2002) “Duet-splitting and the evolution of gibbon songs.” Biological Reviews, Vol. 77, no. 1, pp.57-76
Kleiman, D.G. (1977) “Monogamy in Mammals.” The Quarterly Review of Biology, Vol. 52, No. 1., pp. 39-69.
Mitani, J.C. (1984) “The behavioral regulation of monogamy in gibbons (Hylobates muelleri).” Behavioral Ecology and Sociobiology, Vol. 15, no. 3, pp. 225-229
Palombit, R. (1993) “Lethal territorial aggression in a white-handed gibbon.” American Journal of Primatology, Volume 31, Issue 4 , Pages 311 – 318
Palombit, R. (1994) “Dynamic pair bonds in hylobatids: implications regarding monogamous social systems.” Behavior, vol. 128 (1-2), pp. 65-101
Palombit, R. (1994) “Extra-pair copulations in a monogamous ape.” Animal Behaviour, Vol. 47, pp. 721-723.
Palombit, R. (1996) “Pair bonds in monogamous apes : A comparison of the siamang Hylobates syndactylus and the white-handed gibbon Hylobates lar.” Behavior, vol. 133 (5-6), pp. 321-356
Rasmussen, D.R. (1981) “Pair-Bond Strength and Stability and Reproductive Success.” Psychological Review, Vol. 88, No. 3, pp. 274-290
Reichard, U. (1995) “Extra-pair copulations in a monogamous gibbon (Hylobates lar).” Ethology, vol. 100, no2, pp. 99-112
Wittenberger, J.F.; Tilson, R.L. (1980) “The Evolution of Monogamy: Hypotheses and Evidence.” Annual Review of Ecology and Systematics, Vol. 11., pp. 197-232.