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.

The selective costs and benefits affecting the evolution of group living have long interested behavioural ecologists because knowledge of these selective forces can enhance our understanding not only of why organisms live in groups, but also why species exhibit particular patterns of social organisation. Tamarins form stable and permanent mixed-species troops providing an excellent model for examining the costs and benefits hypothesised for group living. However, testing hypotheses in the wild is difficult, not least because participating species are rarely found out of association. In contrast, in captivity it is possible to compare matched single- and mixed-species troops and also to study the same individuals in single and mixed-species troops to see what effect the presence of a congener has on behaviour. In this way, captive work can help us confirm, reject, or refine the hypotheses, and aids in the generation of new ones, for relating back to the wild. The utility of this approach is demonstrated in this thesis which explored some of the foraging benefit hypotheses and, in particular, the underlying notion that individuals in tamarind mixed-species troops can increase their foraging efficiency through social earning. Single and mixed-species troops of Saguinus fuscicollis and S. labiatus were studied at Belfast Zoological Gardens. It was found that social interaction with conspecifics and congeners facilitated learning by individuals of various types of food-related information (food palatability, location, and method of access). However, although social learning operated in mixed-species troops, it did so under the shadow of inter-specific dominance. The results were used, in conjunction with field observations in Bolivia, to make inferences about the adaptive function of social learning in the wild. These findings strengthen the hypotheses which suggest that increased opportunity for social learning, through an increase in troop size and as a result of species divergence in behaviour, is an adaptive advantage of mixed-species troop formation in tamarins.

The present study gathered both general and specific information about the behavior of a captive family group of Saguinus midas midas [red-handed tamarins] housed at the Washington Park Zoo, Portland, Oregon, USA. Saquinus midas midas is a rarely studied species, and detailed information about the behavior of this species is virtually nonexistent. For this reason, this study collected information about both activity budgets and social grouping within this family group. The primary focus of this study was, however, the socialization of captive juvenile tamarins with respect to several behaviors relevant to reproductive success among the Callitrichidae. The behaviors assessed were: scent marking [reproductive suppression]; mounting, thrusting, allogrooming, huddling [pair-bonding]; food transferring, and infant carrying [infant caretaking]. Socialization was defined as the process of social learning that guides young primates in the day to day life of a species and was assumed to occur via two potential processes: active participation and passive observation. It was supposed that juveniles may learn behaviors relevant to pair-bonding and infant caretaking by participating in interactions with other group members. It was also supposed that juveniles may further learn about these behaviors by observing a pair-bonded male and female. Recognizing the potential importance of both socialization processes, this study attempted to answer several questions with respect to the aforementioned behaviors: do juveniles and adults engage in these behaviors, and, if the behavior is a social interaction, with whom?