**2. Methods**

Our data collection methods have previously been described [44, 53]. Siamang (*Symphalangus syndactylus*) data were collected in a consistent form by one of us

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

(M.O.) between April 1985 and March 1993. A total of 17 siamang groups were observed at the following zoos: Antwerp (An), Belgium, Branféré (Br1, Br2, Br3), France, Budapest (Bu), Hungary, Berlin Zoo (Be), Dortmund (Do), Dresden (Dr1, Dr2), Duisburg (Du), Frankfurt (Fr), Krefeld (Kr1, Kr2), Munich (Mn), Germany, Studen (St), Zurich (Zh), Switzerland and Washington (Wa), U.S.A., with group size ranging from two to six animals.

Crested gibbon data were collected in the same way by S.R.-W. between August and October 2001. A total of seven crested gibbon groups (*Nomascus*) were observed at the following zoos: Duisburg (Du), Eberswalde (Eb), Osnabrück (Os1, Os2), Germany, and Mulhouse (Mu1, Mu2, Mu3), France, with group sizes ranging from two to five animals. Three crested gibbon species are represented in our sample, including the Northern White-cheeked Crested Gibbon (*N. leucogenys*): Du, Mu1 and Os2; the Southern White-cheeked Crested Gibbon (*N. siki*): Mu2; and the Southern Yellow-cheeked Crested Gibbon (*N. gabriellae*): Eb, Mu3 and Os1. The gibbon classification used here follows [65].

Data for Pileated Gibbons (*Hylobates pileatus*) were collected in the same way by J.v.d.L. and K.N between February and May 2007. A total of nine groups were observed at the following zoos: Phnom Tamao, Cambodia (PT1–7), and Zurich, Switzerland (Zu1, Zu2), with group size ranging from two to five animals.

In order to assure comparability of data collected by the observers M.O., S.R., J.v.d.L. and K.N., dual observations were carried out on 31 July 2001 and on 13 February 2007, respectively, until consistent values of inter-observer concordance were obtained [66].

At each zoo, observation time for each sampling method was distributed evenly across the animals' activity period between 0700 and 1800 h (until 1700 h during the winter months, and between 0800 and 1800 h for crested gibbon groups Du, Eb, Mu, Os2).

We used focal animal sampling with the continuous recording rule [66–69] to collect information on the frequency and duration of grooming behavior between mates. Focal animals were changed every 20 min. Each of 11 siamang pairs was observed for 80 h, except for pairs Mu (50 h) and Du (90 h), and each crested and pileated gibbon pair for 35 h. Grooming occurred in discrete sessions that could be counted. We allowed an interval of up to 10 seconds between bouts of grooming before we counted them as two sessions, rather than one.

We used scan sampling to record behavioral synchronization of activities between mates. We defined 11 behavioral categories: socio-positive behavior (including allogrooming, embracing) and infant care, play, agonistic, territorial, sexual, comfort-related, feeding and food-related behavior, observe, rest and sleep, excretion, and locomotion. Scans were made every 1 min (or every 2 min in siamang groups Dr1, Kr1, Kr2, St). Siamang pairs were scanned for synchronization of behavioral categories during blocks of 5 or 10 min, separated by intervals of 20 min. Crested and pileated gibbon pairs were scanned for synchronization in parallel to the focal animal observations of grooming behavior. Each of 13 siamangs pairs was observed for 20 h, except pairs Zu (15 h), Be and Fr (30 h), and Du (40 h). Each crested and pileated gibbon pair was observed for 35 h. The occurrence of synchronized behavior between pair-mates is expressed in % of the total number of scans for a given pair.

We also used scan sampling to record the distance between mates. Distances were recorded to an accuracy of 0.5 m. If the individuals were closer to each other than 0.5 m, we recorded distance according to the following definitions: 0.3 m: shortest distance without body contact, 0.2 m: body contact through extremities, 0 m: body contact through trunk. Siamang pairs were scanned during blocks of 10 min,

separated by intervals of at least 10 min. During each scan sampling block, distance was recorded every 10 s. Crested and pileated gibbon pairs were scanned for the distance between mates every 1 min, and scans were carried out in parallel to the focal animal observations of grooming behavior. Each of 17 siamangs pairs was observed for 10 h, except pairs BrA, Bu, DrA (20 h), Be, Du, KrA (30 h), and Fr (210 h). Each crested and pileated gibbon pair was observed for 35 h.

The size of the enclosure varied between zoos (some gibbon groups were held in cages, others on islands). In small cages, the cage walls set outer limits to the inter-individual distances. Because small cages may have forced our pairs into closer proximity than bigger enclosures, we did not directly use absolute inter-individual distances in our comparisons. Instead, we calculated the relative distance (%) between mates, i.e. the inter-individual distance relative to the maximal possible distance in the pair's given environment (cage or island). This method was described by [44]. In order to test whether cage size had an influence on pair bonding behavior, we used the maximal possible distance in the pair's given environment as an indicator of cage size. In siamangs, our largest sample, this value ranged from 5.7 m in the smallest cage to 43.5 m on the largest island. We arbitrarily defined cages with values of less than 10 m as "small enclosures", the others as "large enclosures".

In addition to determining relative distance, we used scan sampling to estimate the time pair partners spent in each of the following distance classes: 1: body contact or distance of less than 0.3 m, 2: 0.3–1 m, 3: >1 m–3 m, 4: >3 m.

For comparison of our data on partner-directed behavior with literature data, we used male and female proportions of these behavioral variables, where male and female proportions complement each other to 100%. Proportions should be independent of the observation method and permit comparison of data from different observers.

One-sample sign test tests were used to compare classes of sex-specific grooming proportions within genera. For comparison of data among three genera, we used Kruskal-Wallis tests with Dunn's *post hoc* tests [70]. In order to compare data between *H. pileatus* and *H. lar* (i.e. after inclusion of data compiled from the literature), we used the Mann–Whitney *U* tests [71]. All tests were two-tailed, and the null hypothesis was rejected at P *=* 0.05. Statistical were calculated using the software StatView 5.0.1 and SPSS 17.0 on a Macintosh G4 computer.
