Spider monkeys can be thought of as the ecological equivalents of the apes (especially the gibbons) of the Old World – large bodied fruit eaters with complicated societies. They spend almost all their time high in the canopy on a continuous search for fruit of numerous species, and are very important seed dispersers as a result. In hard times they may also eat leaves, some insects, or even decaying wood, but without an extensive variety of fruiting trees they cannot survive for long.
The society of spider monkeys is much more complex than one might think. In a fashion somewhat similar to chimpanzees, they have a ‘fission-fusion’ society, with a group comprising 20-30 individuals at any one time. This group however does not move around in a body, instead much smaller bands of 2-4 individuals, usually with a single adult male, move around and meet peaceably with other subgroups as they encounter each other. Different groups show more aggression, but this is usually vocal rather than physical, and it is not unknown for females to transfer from one major group to another and then return to their home band.
As part of this social structure, spider monkeys do not show the disparity in size that is common in primates where a dominant male monopolises reproduction. Males are slightly larger, but the difference is hard to observe from the ground. The fission-fusion structure is probably an adaptation to maximise the use of widely scattered fruit sources, which can only sustain a few animals the size of a spider monkey at one time. Also, there is no obvious automatic male dominance – both females and males may take precedence in a group, depending on the individuals involved. This lack of male dominance is a feature of many South American monkeys, which are far more variable in their social structure than Old World monkeys are.
Part of the reason for this difference may be the polymorphism in vision in New World monkeys, which means that some females may have superior fruit-finding skills than other females and males. Colour vision in primates is the result of opsin pigments in retinal cone cells, which can be sensitive to Short (S), Medium (M) or Long (L) wavelengths. The S pigment is common to all primates and other mammals, and is held on an autosome. The M and L pigments are produced on the other hand by genes on the X chromosome. Old world primates have a duplication of the pigment genes, and normally carry both an M and an L pigment gene on each X chromosome. New World primates have only space for a single gene, which may be either an M or an L pigment. As male mammals are XY, and only have a single X chromosome, whereas females are XX, this makes for the situation as follows:
Male spider monkeys: SM or SL opsins
Female spider monkeys: SML, SLL, or SMM opsins
Females with the SML configuration are the best at colour discrimination, important when searching out coloured fruit of the best quality. Other configurations may each have their own advantages, but it is not clear what they may be. Any advantages of one type over another may only be temporary, perhaps at particular seasons. All the South American primates seem to exhibit this variation, so it has obviously worked for at least 17 million years, when it is believed the ancestral form began to diverge into the variety of species we see today.
Unfortunately, the need of spider monkeys for primary rainforest is a major cause of their current status as endangered animals. An effective breeding population needs a large area of untouched forest, as they cannot make use of secondary forest in the way that smaller species such as marmosets can. They also have a very low reproductive rate, with an age at maturity of at least 5 years and an interval between births of 2-3 years. As a result, any loss of a population cannot be replaced easily, and many species are quite heavily hunted.
The two young males we currently have at Bristol are part of the globally managed breeding programme for this species. It is hoped that females may join the group as they become available, but the captive population of this species is small (only around 85 animals worldwide) and reproduction is slow as mentioned above. With a long lifespan – potentially over 40 years- there is however time to find a suitable mate. With the ongoing deforestation and hunting in Columbia however, the need for protection in the wild is dire. The well known political problems do seem to be being gradually resolved, but by the time the Columbia people, or at least their politicians, take effective action it may be too late. As far as Bristol Zoos’ influence is concerned, we are gradually getting more involved with conservation projects in the country, and in particular with husbandry and veterinary help in rescue centres and Columbian zoos, as I described last week.
Nect week, a much smaller monkey – the Squirrel Monkey, which also has a more complicated society than you might expect.
(images from wikipedia, Arkive)