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



#### **Table 3.**

*Male contributions (%) to intra-pair grooming in gibbons. Classes of male grooming proportion are defined as (1) 0–33%, (2) >33–66% and (3) >66%. Abbreviations: Hoolock: Hho = H. hoolock; Hylobates: Hag = H. agilis, Hla = H. lar, Hmo = H. moloch, Hpi = H. pileatus. Nomascus: Nco = N. concolor, Nga = N. gabriellae, Nle = N. leucogenys, Nsi = N. siki. Captive/wild: c = captive, w = wild. Data type: f = frequency, t = time. Grooming: – = no partner-directed grooming observed. Source: ts = this study.*

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

Especially in siamangs and crested gibbons, the unilateral distribution of male grooming proportion is surprisingly consistent. We wondered whether there was something about the pairs which do not exhibit consistent results. Of the gibbons we observed, only the siamang sample was large enough to test several potential influences statistically. In the 16 siamang pairs that showed grooming, "Having infants" had no influence on the proportion of male grooming (Mann–Whitney *U* test, 16 pairs, U = 15.5, P > 0.05). However, "Having a family group" did: Pairs without a family showed a smaller proportion of male grooming than pairs with offspring in the family (Mann–Whitney *U* test, 16 pairs, U = 11.0, P = 0.03). We also wondered whether there were any differences between pairs kept in smaller cages and pairs kept in bigger enclosures. In order to study the effect of cage size on the male proportion of pair-grooming in siamang pairs, we used the maximal possible distance in the pair's given environment as an indicator of cage size. We compared male grooming proportion between siamangs kept in small enclosures (N = 9 groups) to siamangs kept in large enclosures (N = 7 groups). The difference was not statistically significant (Mann–Whitney *U* test, P > 0.05). The correlation between cage size and male grooming proportion was also not significant (Spearman rank correlation, Rho = −0.165, P > 0.05).

Results for the dwarf gibbons are less consistent than those for siamangs or crested gibbons (**Table 3**). Could the differences within the first two genera be influenced by wild *vs.* captive gibbons? In siamangs, captive pairs did not differ from wild ones (Mann–Whitney U-test, 23 captive pairs vs. 5 wild pairs, U = 51.0, P > 0.05). In dwarf gibbons, on the other hand, captive pairs differ significantly from wild ones

#### **Figure 6.**

*Male contributions to intra-pair grooming in gibbons. (a) Siamangs (Symphalangus, N = 28 pairs); (b) crested gibbons (Nomascus, N = 18 pairs); (c) dwarf gibbons (Hylobates, N = 23 pairs); (d) hoolock gibbons (Hoolock, N = 3 pairs). Abbreviations in (c) identify the following species: a – H. agilis, l – H. lar, m – H. moloch, and p – H. pileatus.*

(Mann–Whitney *U* test, 20 captive pairs vs. 6 wild pairs, U = 24.0, P = 0.027). It should be noted, however, that all available data for wild dwarf gibbons stem from only one species (*H. lar*), whereas several other species are represented in the captive sample of the same genus. If the comparison is restricted to *Hylobates lar*, the difference is not statistically significant (Mann–Whitney *U* test, 5 captive pairs vs. 6 wild pairs, U = 8.0, P > 0.05). Therefore, the variability of male grooming proportion among dwarf gibbons may be influenced by, and differ among, the species.

The frequency distribution of male grooming proportion is shown in **Figure 6**. These data differ significantly among the genera (Kruskal-Wallis test, df = 2, P *<* 0.0001). As revealed by the Dunn *post-hoc* tests, the male proportion in partner grooming is significantly higher in *Symphalangus* than in both *Nomascus* (P *<* 0.001) and *Hylobates* (P *<* 0.005), whereas no differences were found between *Hylobates* and *Nomascus* (P *>* 0.05).
