Interspecific associations of Diana monkeys, Cercopithecus diana, in Sierra Leone, West Africa: biological significance or chance?

Abstract

The advantages of interspecific associations are invoked frequently without first testing the null hypotheses: (1) observed associations occur by chance, and (2) when associations do occur, the organism's behaviour is unaffected. During a study of two groups of Diana monkeys on Tiwai Island, Sierra Leone, the duration and frequency of interspecific associations were recorded along with behavioural and ecological data. Interspecific associations occurred in 36·6% of the samples from one group and 89·5% of the samples from the other. Predicted values of association based on null hypothesis 1 were generated using both Waser's gas model and computer simulation based on species density and group spread. For interspecific associations of Diana monkeys with the six other species of forest monkeys resident on Tiwai Island, only associations with olive colobus, Procolobus verus, could not be accounted for by random events (i.e. exceeded null model predictions). For both study groups, associations were non-random with respect to their range use. The frequency of different behavioural categories of Diana monkeys was not independent of association status, but both varied with time of day. No significant relationship between interspecific association and behaviour was found for either study group, when controlling for time of day. The effects of random processes must be considered before biological significance is attributed to interspecific associations.