**3. Results**

#### **3.1 Strength of pair bonds**

#### *3.1.1 Synchronization of behavioral variables*

The average degree of behavioral synchronization across 11 behavioral variables is shown in **Figure 2**. Values range from 15.5% to 63.9% in siamangs, from 8.0% to 38.7% in crested gibbons, and from 19.2% and 42.7%. As shown in **Table 1**, however, the overall degree of behavioral synchronization does not differ significantly between the genera (Kruskal-Wallis test*,* P *=* 0.186).

#### *3.1.2 Relative partner-distance*

Average relative partner distances and time proportions spent in four distance classes for each study group are listed in **Table 2**. Considerable differences were found *Taxon-Specific Pair Bonding in Gibbons (Hylobatidae) DOI: http://dx.doi.org/10.5772/intechopen.95270*

#### **Figure 2.**

*Comparison of the average degree of behavioral synchronization between siamangs (Symphalangus, N = 13 pairs), crested gibbons (Nomascus, N = 7 pairs), and pileated gibbons (Hylobates pileatus, N = 9 pairs). Box plots show mean values, standard deviations and minimum and maximum values. The difference between the genera is not statistically significant (Kruskal-Wallis test, P > 0.05, see text).*


#### **Table 1.**

*Average degree of synchronization [% ± standerd deviation] across 11 behavioral variables for siamangs (Symphalangus syndactylus), crested gibbons (Nomascus spp.), and pileated gibbons (Hylobates pileatus). Abbreviation: p = error probability.*

among pairs. Time spent in distance class 1, for instance, varies from 0.3% to 49.7% in siamangs, from 5.6% to 32.3% in crested gibbons, and from 0.0% to 20.5% in pileated gibbons. Similarly, time spent in distance class 4 varies from 1.3% to 61.2% in siamangs, from 14.1% to 47.4% in crested gibbons, and from 31.6–84% in pileated gibbons. The time gibbon pairs spent in each of the four partner distance classes are shown in **Figure 3**. The three taxa do not differ significantly among each other in the time groups spent in any of the four partner distance classes (Kruskal-Wallis tests, P *>* 0.05), except for time spent in distance class 4 (P = 0.014). Dunn *post-hoc* tests revealed that pileated gibbon pairs spent more time in distance class 4 than siamangs (P < 0.02). Moreover, the difference in distance class 2 is close to significance (P *=* 0.051).


#### **Table 2.**

*Average relative partner distances and time proportions spent in four distance classes: (a) siamangs (Symphalangus syndactylus, N = 17 groups), (b) crested gibbons (Nomascus spp., N = 7 groups), (c) pileated gibbons (Hylobates pileatus, N = 9 groups).*

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

#### **Figure 3.**

*Time proportion spent in 4 distance classes (left) and of the mean relative partner distances (right) in siamangs (Symphalangus, N = 17 pairs), crested gibbons (Nomascus, N = 7 pairs), and pileated gibbons (Hylobates pileatus, N = 9 pairs). Box plots show mean values, standard deviations and minimum and maximum values. In a comparison between the genera (Kruskal-Wallis tests), only one of the five variables (distance class 4) are statistically significant (P < 0.05, see text).*

The relative distance between pair partners is also shown in **Figure 3**. The three taxa do not differ in this variable (Kruskal-Wallis test, P *>* 0.05).

#### *3.1.3 Allogrooming*

The number of grooming sessions/hour (average of male and female) varies from 0.0 to 3.9 in siamangs (*Symphalangus*, N = 12 pairs), from 0.5 to 2.0 in crested gibbons (*Nomascus*, N = 7 pairs), and from 0.0 to 2.1 in pileated gibbons. The difference is not statistically significant (Kruskal-Wallis test, P *>* 0.05). The average duration of grooming sessions varies from 0 s to 76.0 s in siamangs, from 50.5 s to 132.1 s in crested gibbons, and from 0 s to 101.0 s in pileated gibbons. This difference is not statistically significant (Kruskal-Wallis test, P *>* 0.05). The proportion of time spent grooming varies from 0% to 66.9% in siamang pairs, from 9.3% to 28.7% in crested gibbon pairs, and from 0% to 57.7% in pileated gibbons. The difference is not statistically significant (Kruskal-Wallis test, P *>* 0.05). As a result, siamang pairs, crested gibbon pairs, and pileated gibbon pairs spend similar amounts of time grooming (**Figure 4**).

#### **3.2 Mechanism of pair bonds**

In order to study which sex invested more in maintaining the pair bond, we determined the %-proportion of partner-directed grooming for each adult. Because male and female proportions in a pair complement each other to 100%, the grooming proportion of one sex will suffice to provide the full information. The results are summarized in **Figure 5**.

In these analyses, one pair of siamangs (Kr2) and one pair of pileated gibbons (PT3) had to be excluded because pair partners were not observed to groom each

#### **Figure 4.**

*Average intra-pair grooming frequency per hour, mean duration of grooming sessions, and proportion of time spent grooming in siamangs (Symphalangus, N = 11 pairs), crested gibbons (Nomascus, N = 7 pairs), and pileated gibbons (Hylobates pileatus, N = 9 pairs). Box plots show mean values, standard deviations and minimum and maximum values. In a comparison between the genera (Kruskal-Wallis tests), none of the three variables are statistically significant (P > 0.05, see text).*

#### **Figure 5.**

*Average male-female proportions of intra-pair grooming frequency per hour, mean duration of grooming sessions, and time spent grooming in siamangs (Symphalangus, N = 10 pairs), crested gibbons (Nomascus, N = 7 pairs), and pileated gibbons (Hylobates pileatus, N = 8 pairs). Box plots show mean values, standard deviations and minimum and maximum values. In a comparison between the genera (Kruskal-Wallis tests), all three variables are statistically significant (P < 0.05, see text). Abbreviations: M = males, F = females.*

other at all and male–female proportions of grooming variables could, therefore, not be calculated. Neither Kr2 nor PT3 were newly formed pairs, and the reason why no grooming occurred among pair partners is unknown.

Male proportions in the number of grooming sessions per hour varied from 8.5% to 78.3% in siamangs, from 2.9% to 62.5% in crested gibbons, and from 0.0% to 85.4% in pileated gibbons. The difference between the genera is statistically significant (Kruskal-Wallis test, P *=* 0.032). The Dunn *post-hoc* test revealed no significant pair-wise differences, but as a trend, male proportions were higher in siamangs than in pileated gibbons (P < 0.1). Male proportions in grooming session duration varied from 26.7% to 74.6% in siamangs, from 16.6% to 68.2% in crested gibbons, and, and from 0.0% to 48.0% in pileated gibbons. The difference between the genera is statistically significant (Kruskal-Wallis test, P *=* 0.043), and the Dunn *post-hoc* test revealed that male proportions were higher in siamangs than in pileated gibbons (P < 0.05). Male proportions in the time spent grooming varied from 3.3% to 90.4% in siamangs, from 0.9% to 69.1% in crested gibbons, and from 0.0% to 84.3% in pileated gibbons. The difference between the genera is statistically significant (Kruskal-Wallis test, P *=* 0.035), and the Dunn *post-hoc* test revealed that male proportions were higher in siamangs than in pileated gibbons (P < 0.05). As a result, siamang males groom partners in longer sessions and spend more time grooming them than pileated gibbon males. Only as a trend, siamang males also tend to groom their partners during more grooming sessions than pileated gibbons.

In addition to the grooming data collected by focal animal sampling, we also collected data on male–female grooming proportions for three additional siamang groups (An, Be, Zu) during the scan sampling observations. Male grooming proportions in these groups amounted to 95.4%, 85.7% and 100%, respectively.

Finally, we compiled data from the pertinent literature on other gibbon groups. If several reports were available on the same group, we used the study with the larger data base. These data are summarized in **Table 3** and also includes members of the dwarf gibbons (*Hylobates*) and hoolock gibbons (*Hoolock*) other that the species observed by us. The sample size for the hoolocks (**Table 3d**), however, comprises only three groups and is too small for statistical analysis. Pairs that did not exhibit partnerdirected grooming are also excluded from the analysis. Our resulting sample comprises 76 pairs. For summary statistics, we split male grooming contribution evenly into three classes: (1) 0–33%, (2) >33–66%, (3) >66%. Pairs should be evenly distributed across these classes if male and female contributions were balanced. As shown in **Table 3**, this is not the case in siamangs (N = 28). Most pairs fall into class 3, suggesting that siamang males, as a rule, provide most of the intra-pair grooming. In crested gibbons (N = 22) and dwarf gibbons (N = 26), the situation is exactly reversed. Most pairs fall into class 1, indicating that females provide most of the intra-pair grooming in *Nomascus* and *Hylobates*. The difference from the expected value of 50% is statistically significant for the genera *Nomascus* and *Symphalangus* (One-sample sign test, P *=* 0.002, and P *=* 0.013), but not for *Hylobates* (One-sample sign test, P *>* 0.05). As indicated by the species labels in **Figure 6c**, the distribution appears to differ among species of the genus *Hylobates*. Whereas partner-directed grooming is mainly provided by females in *H. pileatus* (N = 11), the distribution appears to be more randomly distributed in *H. lar* (N = 11). Although the difference between the two species is statistically significant (Mann–Whitney *U* test, P *=* 0.032), the samples are relatively small and the result should be regarded with caution. If only *H. pileatus* is considered, the difference from the expected value of 50% is still not significant (One-sample sign test, P *>* 0.05), but the sample is very small in this case (N = 11).

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