Monday, 17 May 2010

Strangely musical - some unexpected discoveries about gibbons

Two of my favourite animals at Bristol are Samuel and Douana, our pair of Agile gibbons Hylobates agilis. I always feel that singling out a gibbon as “Agile” is a bit unfair, as all the gibbons are famous for their acrobatics. They are the most species-rich group of all the apes, with four living genera (Hylobates, Symphalangus, Nomascus and Hoolock), and numerous identifiable species and subspecies.

Unfortunately for both taxonomists and zoos, gibbons will hybridize freely both in the wild and in animal collections, and a lot of work now has to be done to identify the status of the various gibbons in captivity. Our own pair are both too closely related and probably of hybrid origin, so they are not being allowed to breed.

Gibbons are well known for their musical calls (when the wind is in the right direction ours can be heard all over Clifton), and love to great the dawn with territorial calls. This has resulted in ours being shut in at night – when we first got them they had access day and night to their outdoor gymnasium (which is made of telegraph poles in the centre of our lake) and we had complaints from the neighbours when they started singing at 4 am in the summer months.

Gibbons are famed for their monogamy and rainforest habitat, and like a lot of things animals are famous for this is not entirely true. Crested gibbons of the genus Nomascus often reach high altitudes, and in the summer will enter deciduous forest to feed on birch leaves. The monogamy of gibbons has also been called into question – Siamangs (Symphalangus) have been observed in multi-male groups and studies of several gibbon species have shown that a females’ infant is not infrequently fathered by a neighbouring male. It appears that the prime concern is not the partner but the shared resources – like politicians, male and female stay together for the sake of the territory.

The modern range of gibbons extends from eastern India (the Hoolock gibbon is India’s only living ape), across South East Asia to Indonesia. In almost all their range, only a single species of gibbon is found in one location. The exception is the Malay peninsula and Sumatra, where Siamangs (which at 20kg are about twice the size of other gibbons) share their range with Agile gibbons and Lar gibbons. As they also share the forest with Sumatran orangutans, this makes Sumatra the only place left on earth where 4 species of ape (including humans) can be found living in the same forest.

One of the remarkable features of gibbons is that they have very strange chromosomes. Given the well known damage that changes in chromosome number can have on humans (such as the duplication of chromosome 21 that causes Down’s syndrome), one might wonder how it is that different species have different chromosome counts, but part of the reason is that it is sometimes possible for an animal with “incorrect” chromosome numbers to breed successfully.

All the same, this is rare, and most primates have very similar numbers of chromosomes, averaging around the 40-odd mark. Since the human lineage split, there has been a reduction of one pair from the 48 chromosome count of gorillas and chimpanzees, probably caused by a fusion of two of the ape chromosomes.

When we come to gibbons, the count goes haywire. The different genera are distinguished by different diploid chromosome counts; Nomascus (2n=52); Symphalangus (2n=50); Hylobates (2n=44); and Hoolock (2n-38). For an outwardly pretty uniform group of animals, this is rather surprising, and gibbons are much studied by researchers investigating the evolution of karyotypes.

As might be expected of forest animals, the gibbons of Asia are often threatened or highly endangered. This is especially true of the northerly gibbons of the genus Nomascus, which reach into southern China. Gibbons once extended as far north as the Yellow River in historic times, but these are long gone, leaving only some fragmentary remains and some paintings. It is not clear what species they were – on biogeographical grounds they were most likely a Nomascus species, but the paintings resemble Hoolock gibbons.

Several species of gibbons in the various genera can be found in zoos. Lar gibbons are the commonest, with 488 on ISIS, but Siamangs (357) and White-Cheeked gibbons (172), plus a few other species can also be seen. At present, there are only 69 Agile gibbons on ISIS, with only 17 in the whole of Europe, so you would have to search hard to find them. Twycross Zoo has a famous primate collection, with 6 species of gibbon along with many other monkeys and apes, including 3 pairs of Agile gibbons.

Next time, a change to some plants which Bristol is famous for, and some unexpected speciation.

(Image from Arkive)

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