Variation in great-call structure of hybrid gibbons in Central Borneo

Abstract

A study on the variation in great-call structure of hybrid gibbons (Hylobates agilis x H. muelleri) was conducted throughout the headwaters of the Barito River in Central Kalimantan, Indonesia. A total of 88 females from 22 sites were recorded between September– November, 2004 and May– August, 2005. A detailed spectrographic analysis was made on 469 great-calls, which revealed that six of the 45 variables measured could accurately differentiate between agile and Muller’s gibbons: (1) Number of great-call notes, (2) Great-call rate of emission, (3) Number of notes, (4) Climax rate of emission, (5) Mid-climax note duration, and (6) Inter-climax note interval. Of these, only the variable climax rate of could reliably distinguish between the calls of agile (n = 23 females), Muller’s (n = 14 females) and hybrid gibbons (n = 51 females). A coefficient of variation indicated that intra-individual variation in great-call structure was low for each of the six variables. Non-parametric tests revealed that intra-population variation in great-call structure varied significantly (P< 0.05). A discriminant function analysis found that great-calls could be correctly assigned to the individual that produced them at a rate higher than would be expected by chance (cross-validated classification rate: hybrid = 81.9%; agile = 54.4%; Muller’s = 52.4%). There was a significant (P < 0.05) level of inter-population variation in great-call structure. The cross-validated correct assignment rate was 94%, indicating that the agile, Muller’s, and hybrid populations could be reliably distinguished by the two functions of the discriminant function analysis. One of the main variables in each of the two functions was climax rate of emission. Within the hybrid population, females located on the western side of the hybrid zone had a slower rate of emission (mean = 2.07 notes/s) than those located on the eastern side (mean = 4.55 notes/s). This finding is likely due to a larger proportion of agile genes on the western side of the hybrid zone, and a larger proportion of Muller’s genes on the eastern side. It appears that the average rates of emission on both sides of the hybrid zone have increased since last surveyed by Mather (1992), indicating that the rate of emission of hybrid great-calls is accelerating. There was a significant level of variation between mother-daughter pairs, one generally having a faster rate of emission than the other. It appeared that the regular participation of daughters in song bouts was to practice their own song, rather than learn that of their mother. Female neighbours also varied significantly in their call structure. The wide range of inter-individual variation indicates that great-call structure is inherited and not learnt. The hybrid zone appears to have increased in size since it was last surveyed (Mather, 1992). Fifteen of the 29 females recorded between the western side of the Busang River and the northern side of the Joloi River were hybrids. These females were sparsely distributed from the village of Parahau down to the northern banks of the Joloi River. They had a faster rate of emission (range = 0.67-4.89 notes/s) than the agile gibbons in the same region (range = 0.37-0.65 notes/s). It is likely that limited hybridisation has long been present in this region, but that hybrids have become more abundant than when last surveyed.