The behaviour of mixed-species tamarin groups (Saguinus labiatus & Saguinus fuscicollis)
Saguinus labiatus lahiatus and S. juscicollis weddelli form stable mixed-species groups in the wild. This thesis investigates the costs and benefits of such an association, with particular emphasis on determining species differences, the "behavioural differences hypothesis". A nine month field study was conducted in northern Bolivia, which showed that the species differed in mean height used in the forest, method of locomotion, and preferred insect foraging strategies. Subsequent investigations were conducted on captive single species and mixed-species groups at Belfast Zoo. Preliminary observations of mixed-species groups in standard enclosures and while free-ranging, demonstrated that they behaved in a similar way to their wild counterparts, where the S. labiatus had priority of access to food and utilised a higher mean height in the cage. Therefore, it is reasonable to relate the results of captive investigations to the wild situation. The "behavioural differences hypothesis" was investigated through the presentation of novel objects in various parts of the environment. Both species' reactions to objects varied according to predictions based on their vertical partitioning. S. labiatus were found to use a more visually orientated approach than S. juscicollis, and this can be related to insect foraging strategies in the wild. An experiment was conducted with novel food and non-food objects, in order to test the "social facilitation hypothesis", which predicts that the behaviour of one species can orientate the other towards the presence of food. Results suggest that S. labiatus have priority of access to objects and are first to consume food, but S.juscicollis are able to use this behaviour and always gained some food. The results suggest that overall both species benefit from increased foraging efficiency. The hypothesis that individuals in mixed-species groups benefit from decreased predation through increased vigilance was investigated, and supported, as members of both species were shown to benefit from a decrease in individual vigilance effort, but an overall increase in total vigilance per unit time. This was shown in both general vigilance and in the active monitoring of a threatening stimuli. Findings are discussed in terms of costs and benefits towards the participants in mixed-species groups, and are compared to the main theoretical viewpoints in the literature.