Friday, 2 March 2012

New World Primates 9: They came by night

The last of the New World primates we have at Bristol can be found in Twilight World. The Grey-legged Dourocouli, Aotus griseimembra, belongs to a widespread group of monkeys which have, uniquely for higher primates, become nocturnal. There are of course numerous nocturnal primates in the Old World, incuding the bush babies of Africa, lorises of Asia, and many lemurs in Madagascar, but they all belong to a group generally referred to as the Strepsirrhine primates. These are distinguished from monkeys and apes, the ‘higher’ or Haplorrhine primates, by features of their face and skeleton, most obviously the moist, dog-like noses and tendency to rely more on scent and less on vision.

Strepsirrhine primates became extinct in the Americas, and when the ancestors of the New World primates colonised, probably by rafting from Africa across the then much narrower South Atlantic, they had already evolved into strictly diurnal animals. The nocturnal dourocoulis have re-evolved the ancestral primate condition of a nocturnal omnivore with a fondness for fruit. They do not exhibit much diversity however compared to the range of different nocturnal Strepsirrhines, probably because there were plenty of small nocturnal mammals in existence when they arrived, especially the large variety of opossums and frugivorous bats, which leave little ecological space for small nocturnal primates. As part of that adaptation, their eyes have become enormously enlarged, as can be seen on this skull:

A.trivergatus skull
The home range of a dourocouli family is extremely small, with densities of up to 16 groups per km2 being recorded, and they tend to use familiar routes around their territory, which probably helps them to remember familiar routes to and from their feeding and sleeping sites. The diet is mainly fruit, especially small fruits, but they also feed on nectar, leaves, flowers, and insects. Insects are usually large orthopterans and moths, but some other insects may also be taken. They are also prey themselves, and among their predators would be owls, snakes (especially boas) and various small carnivores such as the Margay or the Tayra. The large eyes on the owl butterflies in our butterfly house probably mimic these mammalian predators even more than they do owls in fact, and as owl butterflies are active at dusk they are probably important prey for dourocoulis.

Caligo atreus - dourocouli prey!
Although the range of the various species extends from Central America south to Argentina, they reach high altitudes in some places and in the Chaco region of Argentina even sub-zero temperatures are recorded in their range.

As monkeys, douroucoulis are not especially large, with the largest usually around 1.3kg. They seem to live in small family groups of an adult pair plus a few offspring from previous years. Youngsters stay with the adults until they are 2 or 3 years old, and females usually breed in the wild when they are 4 or 5 years. Lifespan is probably around 12 in the wild, more in captivity.

A.griseimembra family
As is common with New World primates, the male plays a major role in infant care, and once the infant is past a week old it is carried almost constantly by the male, only returning to its mother to feed until it is weaned at around 5 months old. Sometimes older juveniles will also assist, and as a result infant survival is extremely high – 96% has been recorded in wild Azara’s dourocoulis. Although captive dourocoulis are usually kept in permanent monogamous groups, in the wild both males and females are regularly replaced by intruders after serious fights which can even be fatal. The extreme support given by males to infants means that females have as much interest in acquiring a successful male as males do in attracting a healthy female, so the social structure is more even-handed than in Old World monkeys where most of the burden of infant care is carried by the females.

Nocturnal animals tend to rely on scent or sound to distinguish species rather than vision, and this has resulted in considerable confusion in their taxonomy, as even distantly related forms can look very similar. Until recently it was thought there were only 2 or 3 species, but in fact studies of chromosome number, detailed anatomy, and DNA have shown there are at least 11 good species and probably many more. Most of these have fairly large ranges and seem reasonably secure, especially as they can make use of disturbed forest fairly well, but a few species have tiny ranges and are threatened by deforestation. An additional threat has been collection for medical experimentation, as they are one of few species that are resistant to the deadly malarial parasite Plasmodium falciparum.

In captivity comparatively few zoos house dourocoulis, with A.griseimembra being the only one held in any numbers. Even that has under 100 individuals worldwide. Part of the problem is the general one of displaying nocturnal animals, but another problem has been the taxonomic problems of identifying which species a zoo actually has. In the past several mis-matched species pairings have resulted in sterile hybrids rather than useful parents of a new generation of captive dourocoulis, even though their husbandry in itself is not especially problematic.

This draws to a close this series on the New World primates at Bristol. Looking over the variety of different species, several themes can be picked out by which the New World primates differ significantly from those of the Old World:

1) A strong tendency for a more egalitarian social structure with females playing a far more central role than in most Old World primates

2) Much greater male input to infant care, especially transport of small babies

3) A lack of any significant terrestrial forms – as far as I am aware even in fossil forms there was never a New World equivalent to a baboon or macacque.

4) Much greater diversity in ecology, with miniaturization, gum-feeding, nut-eating, and other specialisations compared to Old World monkeys, which basically are either frugivores/omnivores or folivores.

The reasons for this diversity are not entirely clear, but I believe some of the reasons may include:

1) The different colour-vision in New World primates, with trichromatic vision being restricted to a subset of females, with males and other females having different forms of dichromatic vision. This means that each monkey in a group will see with different eyes, and be better at finding different types of food as a result.

2) A different set of competitors, both living and extinct. Until very recently terrestrial New World monkeys would have been in competition with a large set of forest herbivores such as ground sloths, glyptodonts and others, which may have restricted any evolutionary move to terrestrial feeding. The nocturnal marsupials may also have been too much competition for small nocturnal primates to survive.

3) Different biochemistries. The need for greater Vitamin D levels by South American primates may have limited their ability to move away from the equatorial forests into other habitats.

Next week, a new bird species at Bristol.

(Images from wikipedia, marwell zoo)

For more on dourocoulis, see the studbook here:

1 comment:

  1. Grey-legged Dourocouli are stunning creatures, Ive seen them at folly farm, cant wait to see them again