**4. Discussion**

Monogamy is common among birds [95], but established in only about 3–9% of all mammals and about 15–29% of all primate species [45, 48, 96]. Among hominoid apes, only gibbons typically live in social monogamy (in the sense of [46]).

Various hypotheses explaining the proximate and ultimate mechanisms, which led to the evolution of social monogamy among gibbons are under debate [48, 97–99]. In these discussions, monogamy among gibbons is usually treated as, and implicitly assumed to be, a comparable, uniform entity. Cowlishaw [100], for instance, assumes that the pair bond is created by the different resource interests of the partners. The female is interested in the territory and the food resources in it, whereas the male is interested in the female partner.

Although several reports suggested that gibbon taxa might exhibit subtle distinctions in their group coherence or group composition (see Introduction), quantitative data for representative numbers of pairs have been lacking. It is generally assumed that pair bonds in all gibbon taxa are built up and maintained in the same way, and that males are mainly responsible for maintaining the pair bonds [3, 52].

As will be discussed below, this study provides evidence to the contrary. We compared indicators of pair bond strength and sex-specific pair bond investment between 7 pairs of crested gibbons, 9 pairs of pileated gibbons, and 11–17 pairs of siamangs (depending on the variable in question).

## **4.1 Pair bond strength**

We determined three variables to compare pair bond strength between siamangs and crested gibbons (synchronization of behavioral variables, relative partner-distance, and allogrooming).


time in the largest distance class 4 (>3 m) than pileated gibbon (*H. pileatus*) pairs, suggesting that pair bond strength in siamangs may be more pronounced than in pileated gibbons. Similarly, Palombit [22] found that siamang pairs spent significantly more time in close proximity to one another than white-handed gibbons (*H. lar*). However, we found no significant differences in the other distance classes or in the mean relative distance between pair partners.

3.Allogrooming: The three gibbon taxa did not differ in the number of grooming sessions/hour (average male and female), the proportion of time spent grooming, and the average duration of grooming sessions. As a result, siamang pairs, crested gibbon pairs and pileated gibbon pairs are involved in similar numbers of grooming sessions and spend similar amounts of time grooming.

In summary, pileated gibbons appear to spend more time apart by the largest distance class than siamangs. Based on this variable alone, their pair bond may be weaker than that of siamangs. No consistent differences in pair bond strength were found between siamangs and crested gibbons or between crested gibbons and pileated gibbons.

### **4.2 Pair bond maintenance**

We examined which sex invests more in the pair bond by measuring the amount of grooming directed at the respective partner. For simplicity, we indicate the male proportion only; the female partner's proportion is its complement to 100%.

Our results show that in pileated and crested gibbon pairs partner-directed grooming is mostly provided by females, whereas males are the main groomers in siamang pairs. This result is further supported by additional data we collected from the literature. In most siamang pairs, males are the main groomers. Furthermore, male proportion in grooming session duration and time spent grooming are higher in siamangs than in pileated gibbons, whereas the male proportions in the numbers of grooming sessions per hour do not differ between siamangs and pileated gibbons. Siamang males groom their partners more often than crested gibbon males do, but time spent grooming and male proportion in duration of grooming do not differ between siamangs and crested gibbons. Our pairwise comparison revealed statistically significant differences for *Symphalangus*/*Nomascus*, but not for *Symphalangus*/*Hylobates* or *Hylobates*/*Nomascus*.

These results suggest that each genus differs in the mechanism of how pair bonds are created or maintained. Especially siamangs differ compared to pileated and crested gibbons: male-driven in the former, female-driven in the latter two. Obviously, the pair bond in gibbons does not appear to be a uniform entity. Date compiled in **Table 3** also suggest that field and zoo observations are consistent (except that only one wild pair exhibits a "Class 1" male grooming proportion of 0–33%).

Our results support vocal and molecular studies suggesting that gibbons are a much less homogenous group than generally assumed [39, 101, 102]. It is becoming more and more obvious that including one gibbon taxon into comparative studies in order to represent "the gibbon" is not useful practice anymore.

In our overall sample of dwarf gibbon pairs (*Hylobates*, N = 26) as well as in the subset of *H. pileatus*-pairs (N = 11), females provided more partner-grooming than males in most pairs. In *H. lar*-pairs (N = 11), on the other hand, the amount of grooming provided by males and females was very variable (**Table 3**) and the reason for this variability in this sample is not clear.

Kleiman [45] proposed that males should be the more active groomers in monogamous primates because their dominance situation is reversed as compared to primates with polygynous social organizations. Simple dominance relationships, however, do not seem the only variables influencing partner-directed allogrooming in gibbons.

If partner-directed allogrooming reflects the investment into a pair bond [62, 63], then our results document that the readiness to invest differs among pairs. In most (but not necessarily all) pairs, both partners appeared to be interested in maintaining the pair pond, and both partners provided at least some allogrooming. In addition to individual differences, the interest in a pair partner may vary with time. Probably, the benefit of a pair bond is related to the reproductive potential of a partner. Observations on wild *H. lar* and *H. moloch* suggest that the reproductive status of females may play an important role [13, 103, 104]. Males may have a higher interest to invest into the pair bond with females when they are receptive, in order to guard them more efficiently, copulate more frequently and improve the probability of their paternity. If partner-directed grooming is part of a mate-guarding strategy with fluctuating relevance to the groomer, it becomes clear why data of relatively large numbers of pairs need to be compared in order to discover species-specific differences.

How do our findings compare to the predictions of the three hypotheses for the evolution of pair bonds presented in the Introduction?


Only very limited information on the direction of partner-grooming is available for the fourth of the gibbon genera, the hoolocks (genus *Hoolock*). Ahsan [105], who studied three groups of the western hoolock gibbon (*H. hoolock*) at two sites in Bangladesh, reported that grooming was most frequent between adult pairs and that it was "mostly performed by the adult male". Unfortunately, the author did not publish the quantitative data in support of his statement. Sankaran [94] observed three

*Taxon-Specific Pair Bonding in Gibbons (Hylobatidae) DOI: http://dx.doi.org/10.5772/intechopen.95270*

groups of the same species in the Gibbon Wildlife Sanctuary in Assam. However, none of his males provided more than 50% of the partner-grooming (**Table 3**). Apparently, the results of the two studies differ, but the overall sample size is too small to assess the directionality of partner grooming in hoolock gibbons with any reliability.

It has also been reported that allogrooming between pair mates is virtually nonexistent in wild *Hylobates agilis* [21] and *H. klossii* [106], in contrast to the situation in wild *H. lar* and siamangs [10, 13, 19, 103]. This suggests that the range of variation in gibbon pair bonds may be larger than what we covered in our study. Several species of dwarf gibbons (*Hylobates*) are hardly represented or not represented at all in our data, including *H. agilis* and *H. klossii*.

Within crested gibbons (*Nomascus*), most of our data are from one species, *N. leucogenys* (N = 14 pairs), whereas few pairs of other light-cheeked species and only one male of a black-cheeked species (*N. concolor* in a mixed pair) are available.
