**1. Introduction**

Whereas the genera of great apes are known to differ strongly among each other in their social structure, the small apes or gibbons clearly are a more uniform group [1–3]. Distributed in Asian rain forests, its members typically live in socially monogamous, unimale unifemale, territorial groups [4, 5], although some flexibility in group composition and sexual behavior occurs [6–8]. Non-monogamous (extra-pair) matings and groups with multiple adult males and multiple adult females occasionally occur in gibbons [7, 9–15].

It has repeatedly been suggested, however, that gibbon taxa may differ in subtle details of their social organization [16]:

Wild family groups of Malayan siamangs (*Symphalangus syndactylus*) appear to be more tightly knit than those of white-handed gibbons (*Hylobates lar*), with siamang intra-group distances being shorter and intra-group communicatory signals being fewer or less conspicuous to observers, and paternal infant-carrying only occurring in siamangs [17–21]. A greater heterosexual cohesion in pair bonds of siamangs, as compared to white-handed gibbons, was also found in a field study in Sumatra [22], but differences in paternal investment appear to be less clear-cut. Paternal infantcarrying appears to be absent in some wild and many captive siamang groups and varies dramatically among males of the same population [10, 23–25], while it may occasionally occur in other gibbon taxa, at least in captive groups [23, 26].

Several reports suggest that the black-cheeked species of the crested gibbons (genus *Nomascus*) differ from other hylobatids in their social organization by more often forming bi-female groups [27–33]. This may not appear to apply to lightcheeked crested gibbon species [34, 35], but see [36].

Recent studies on gibbon calls documented that the various taxa strongly differ in how they present their long and loud morning song bouts [37–40]. In some taxa, mated pairs produce duet song bouts but usually no solo songs (genera *Hoolock*, *Nomascus* and *Symphalangus*), others produce sex-specific solo songs in addition to duets (*Hylobates agilis*, *H. lar*, *H. muelleri*, *H. pileatus*), and others yet produce sex-specific solo song bouts only (*H. klossii*, *H. moloch*). In all members of the genus *Nomascus* and most species of the genus *Hylobates*, for instance, most of the singing is produced by males, whereas in *H. moloch*, males sing rarely and most songs are produced by females. This pronounced diversity of sex-specific investment in resource defense provides indirect evidence for taxon-specific differences in social organization, and, possibly, in previously unrecognized factors of ecological adaptation or inter-specific competition.

Moreover, duets strongly differ in their complexity among taxa, with the most complex ones being uttered by siamangs (*S. syndactylus*) [41, 42]. These differences in song organization also strongly suggest differences in social organization. Because duet song bouts are believed to serve, among other things, to strengthen or advertise pair bonds, duetting and non-duetting gibbon species should differ either in their pair bond strength or in how the pair bond strength is achieved [43, 44].

Although the reports cited above suggest that some gibbon taxa may differ in social organization, very little quantitative evidence for such species-specific differences in the social structure are currently available. Previous comparisons have been limited to sample sizes of 2–3 pairs per genus [17, 19, 22, 23], thus precluding statistical testing. Palombit [3] correctly identified a great need for detailed data on more hylobatid pair bonds, so that we may identify consistent social patterns in light of intra-specific variation.

Early reviews on monogamy found shared behavioral traits in monogamous primates and suggested that the males generally initiate grooming and groom females more often than the reverse situation occurs [45]. In socially monogamous pairs, pair partners usually maintain close spatial association and often perform spectacular, well-coordinated, pair-specific display behavior. This does not necessarily imply, however, that the sexes share mutual socioreproductive interests [46]. Shared interests may be not be required for the evolution of social monogamy, and pair formation does not require an absence of sexual conflict, or symmetric costs and benefits for males and females.

Several of the hypotheses explaining the evolution and maintenance of social monogamy in mammals make predictions regarding female and male contributions to the pair bond [47].


The goal of our study was to answer the following two questions: (1) Do gibbon taxa differ in the strength of their pair bonds? (2) Do gibbon taxa differ in the way pair bonds are created and maintained?

We present the first comparative and quantitative study on differences of the pair bond among multiple gibbon taxa. We have collected observational data on several captive groups of siamangs (genus *Symphalangus*), three species of the crested gibbons (genus *Nomascus*), and the pileated gibbon (*Hylobates pileatus*) as a representative of the dwarf gibbons (genus *Hylobates*). Our study will focus, therefore, on a comparison of these three genera. Photographs of three of the species we studied are shown in **Figure 1**. In addition, a compilation of previously-published data also permits us to make a limited comparison with other gibbons of the genus *Hylobates*.

In order to estimate pair bond strength, we quantified the following three generallyaccepted indicators of pair bond strength (behavioral synchronization, relative distance between mates, and amount of partner directed grooming) following [44, 53].

In order to gain insight in the pair bonding mechanism, we examined which sex invests more in the pair bond by measuring the amount of grooming directed at the respective partner.

Although allogrooming *per se* may serve hygienic, social, communicatory, stress relief or thermal functions [54–59], these functions may be influenced by a species's social organization. For social systems with stable pair structures, allogrooming has been proposed to serve a pair-bonding function [45, 60, 61] and to reflect the investment into a pair bond [62, 63] and, therefore, the ultimate costs and benefits which partners can expect from a relationship [64]. Thus, sex-specific differences in partner-directed allogrooming indicate – on a proximate level – ultimate sex-specific strategies.

#### **Figure 1.**

*Three of the gibbon species that were observed during this study. (a) Siamang (Symphalangus syndactylus), family group, showing from left to right: juvenile, adult male, and adult female carrying an infant. Siamangs are of mostly blackish fur coloration. Notice the half-inflated throat sacs, which play a role in siamang territorial vocalizations and can be inflated to about the size of the animals' head. (b) Northern White-cheeked Crested Gibbon (Nomascus leucogenys) pair, showing from left to right: adult female, and adult male. Adult male and female differ markedly in their fur coloration. Females are mostly yellowish, and males are blackish. (c) Pileated Gibbon (Hylobates pileatus) family group, showing from left to right: adult female carrying a neonate infant, adult male, subadult male, and juvenile male. Adult male and female of this species also differ markedly in their fur coloration. Females are pale grey or fawn-buff with black on crown, cheeks and chest, while males are blackish with white facial border, corona, digits, and genital tuft. Photographs by Thomas Geissmann.*
