Skills used in food processing by vervet monkeys, Cecropithecus aethiops
Abstract
The principle objective of this study was to describe and record all the gathering
and processing skills of vervet monkeys for a variety of different foods. The study
was conducted on two troops of vervet monkeys living in the Palmiet Valley,
Natal, South Africa. There was sufficient data to analyze the processing of four
foods of differing complexity; termites, leaf shoots, sugarcane and fruit.
Milton (1988) proposed that the intellectual difficulties of finding and processing
food led to the evolution of intelligence. In the only study of food processing skill,
Byrne and Byrne (1993) showed that gorillas use a hierarchical organization
perhaps reflecting imitation at the program-level. The question to be asked in this
study was, would vervets also organize their processing into a few techniques for
specific foods and would it then be possible to identify learning mechanisms used
by the monkeys? The current literature suggests that monkeys use simple solutions
to their foraging problems, there is no evidence for imitation of feeding skills in
monkeys.
At the most detailed level of analysis, grip types used in the processing of foods
were described. Existing definitions in the literature were not adequate to explain
the monkeys' hand use, and new definitions were added. High individual
idiosyncrasy was a feature of grip usage across all four foods, although firm
conclusions are not possible because of the known effects of sample size. Cluster
analysis was considered the most appropriate method to look at individual
variation in grip usage. There was an age effect for leaf shoots and sugarcane,
with juveniles restricting their usage to the necessary core grips.
The hand preferences for individuals across tasks gave no support for the theory of
the evolution of laterality presented by MacNeilage et al. (1987). There was a low
degree of individual preference for five out of six tasks, with only termite feeding
showing a hand preference. There was some evidence for a right hand reaching,
left hand manipulation preference, opposite to MacNeilage's prediction. There was
an age effect in direction and strength for two tasks, adults having a stronger left
hand preference in contrast to a weaker right hand preference in juveniles for leaf
shoots and large fruit.
Matrices of the transitional probabilities between two elements, were used to
construct the common pathways of processing skill for each individual. Flow
diagrams were then created to represent the minimal decision processes used by
the monkeys. The diagrams were used to compare individuals' choice of pathways.
Cluster analysis was used to analyze pathway choice in detail; none of a variety of
independent variables could explain the high individual variation. Whether whole
foods or just parts of foods were eaten did explain some of the variance for
sugarcane and fruit. The most parsimonious explanation is that social enhancement
resulting in trial and error learning best described individuals' acquisition of
processing skill, although a number of other factors may explain the observed
results.
Type
Thesis, PhD Doctor of Philosophy
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