ESTIMATING THE COMPLEXITY OF ANIMAL BEHAVIOUR: HOW MOUNTAIN GORILLAS EAT THISTLES

Mountain gorillas use elaborate, multi-stage procedures for dealing with plant defences. This paper investigates the use of mathematically-inspired, informational measures to gauge the complexity of one of these tasks, eating thistle Carduus nyassanus, from field observations of 38 adults and juveniles. Behaviour was analysed at two levels, a detailed, movementbased description of the form of actions, and an organizational description of techniques that were composed of a series of many actions. Complexity, as measured by counting the sizes of behavioural repertoires, correlated at the two levels. Repertoires were shown to be incomplete, but the rates of cumulative increase in actions differed between tasks. Thistle eating was the most complex, and apparently involved many more actions than even chimpanzee tool-use. Techniques were highly selective arrangements of actions, so that their organization (sequence, bimanual coordination, hierarchical structure) reflected cognitive capacity. Although ideally it would preferable to estimate complexity of task organization, this may seldom be feasible, and was not in this case. Instead, the length of a regularly occurring sequence of actions may be the best practical estimate of an underlying complexity of mental process. Confidence in this measure will be increased if it broadly agrees with other, independent estimates of task complexity; in the case of gorilla plant processing...