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Current theory frames animal dispersal as an outcome of potentially complex, multi-factorial interactions and tradeoffs that may vary across individual, sex, rank, age, social group, species, habitat and time. Empirical data relevant to a broad range of the potential costs and benefits incurred by dispersal are, not surprisingly, limited for many mammals and other vertebrates. Here we present the first report on dispersal in a wild population of the Neotropical primate Saimiri sciureus (Primates: Cebidae). Long-term observations (1998-2001) of this squirrel monkey represent part of a broader study of the forest community at Raleighvallen in the Central Suriname Nature Reserve. These new dispersal records for S. sciureus are combined with comparable information from congeners, S. boliviensis in Peru and S. oerstedii in Costa Rica. The resulting three-way compilation includes the ecological, social and mating context for each congener. Further enhancing the inherent phylogenetic control of a within-genus comparison, these data were collected with the explicit intent of joint analyses, and the study sites for these small, arboreal social mammals are three of the least disturbed extant Neotropical forests in the historical record. Saimiri appears to merit description as the genus with the most diverse set of species dispersal patterns yet documented among mammals. (1) S. sciureus of both sexes undertake dispersal on several to many occasions during their lifetime. Females and immatures commonly transfer between troops. The large portion of male S. sciureus spend their adult years as solitary or peripheral males. Few males attain secure residence in a mixed-sex troop, a prerequisite for mating success. (2) On attainment of sexual maturity, male S. boliviensis emigrate with their same-age cohort, first joining all-male bands, and eventually entering mixed-sex troops with this same natal male birth cohort. Natal female S. boliviensis are philopatric and form cohesive matrilines. Within-troop competition determines each matriline's priority of access to fruit resources. (3) In contrast to both S. sciureus and S. boliviensis, S. oerstedii males are philopatric and maintain tight affiliation with same age-cohort males. Natal female S. oerstedii emigrate as juveniles prior to their first mating season, and may undertake secondary dispersals in subsequent years. Squirrel monkeys represent a genus with realistic prospects of discriminating the costs and benefits germane to species-typical dispersal strategies. To this end, we collate 30 different causal parameters commonly invoked as influencing mammalian dispersal patterns. Each of these factors is assessed separately for possible influence on the empirically determined sex and species differences. We predict the possible consequences of direct and inclusive fitness interactions on dispersal outcomes for future testing with genetic data. Components of Saimiri selective regimes particularly salient to female dispersal strategies include food competition, foraging benefits provided by kin and inbreeding avoidance. Dispersal patterns among male Saimiri are constrained by mate competition and the consequent reproductive skew, in addition to enhanced predation risk during dispersal forays. Little evidence, however, suggests that relative to familiar landscapes, exploitation of novel ranging areas substantially increases foraging costs or predation risk for dispersing squirrel monkeys of either sex. We then compare the species-specific dispersal regimes initially identified with the univariate array of proposed costs and benefits to the tradeoffs predicted by a selection of contemporary multivariate dispersal models. The multivariate models did not, however, improve substantially upon the collective insights on cost-benefit regimes achieved with the univariate hypotheses. Conclusions regarding the selective regimes structuring dispersal among squirrel monkeys are best considered provisional until genetic data become available allowing tests of our inferences concerning kin relationships and population structure of the study populations. In the end, this fine-grained analysis is consistent with contemporary understandings on the costs and benefits structuring dispersal strategies. Yet our findings provide an uncommonly clear demonstration that the outcome of the interaction of individual factors (food competition, mate competition, and inbreeding avoidance) depends in very subtle ways on the natural history of the organism. This text and two companion papers, II. Within-species and local variation and III. Cognition, comprise a monograph on the evolution and ecology of squirrel monkey dispersal patterns. Implications of the various results are relevant to resource and mate competition, inbreeding avoidance and kin-recognition, predation risk, coalition and alliance formation, cognitive capacities required for implementation of dispersal tactics, impacts of pre-Columbian anthropogenic disturbance on extant Neotropical biota, and conservation management, among others. [ABSTRACT FROM AUTHOR]

Cognitive skills essential to dispersal remain a thorny, seldom-broached topic, especially among the putatively 'clever' primates. This essay, the final installment of a three-part monograph, considers the cognitive mechanisms underlying expression of three extremely distinctive species-specific dispersal outcomes within squirrel monkeys (genus Saimiri, Primates: Cebidae). Findings from two companion reports, which assess the costs and benefits structuring between-species differences (I. Divergent costs and benefits, Boinski et al., 2005a) and variation within-species (II. Within-species and local variation, Boinski et al., 2005b), provide the groundwork for my often speculative discussion. (1) In Costa Rica, female S. oerstedii do not form kin-based alliances. All females disperse prior to their first mating season and may disperse on numerous occasions throughout adulthood. Male S. oerstedii are philopatric and exhibit close social bonds with other natal males, particularly those from the same birth cohort. (2) Male dispersal and female philopatry, the prevalent pattern in most mammals, including primates, characterizes Peruvian S. boliviensis. Both sexes form life-long alliances with same-sex kin. After natal dispersal, male birth cohorts join all-male groups, from which they attempt to immigrate into mixed-sex troops. Female kin in a S. boliviensis troop form matrilines critical in within-group food competition. (3) All male and most female S. sciureus disperse from several to many occasions during their lifetime. In contrast to the other two species, male S. sciureus never exhibit stable alliances with other males, including probable kin. Similarly, female coalitions are transient, detectable only during periods of relative food abundance. What are the implications of this marked between-species disparity in dispersal outcomes for squirrel monkey cognition, and, by extension, the cognition of other social mammals? Two timely issues are addressed. First, squirrel monkeys exemplify the provocative parallels in the assessments required of individuals embedded within three circumstances usually treated separately: dispersal; coordination of group travel; and fission-fusion adjustments of group composition. Are arguments that dispersal is more or less cognitively demanding than either coordinated travel or fission-fusion social structures justified? Fundamentally, all three processes are reducible to frequency-dependent decision-making by individuals based upon concurrent social and ecological assessments across multiple dimensions, such as time, space, and participant number. Second, a common approach to identify the covariation of selective regimes and apparent cognitive abilities are taxonomically inclusive, multivariate parametric statistical models, which incorporate information on ecology, behaviour, morphology and phylogeny. However, such correlative analyses add little to what is arguably the major challenge in contemporary field investigations of animal behaviour: How can we distinguish complex, multivariate decision-making algorithms from simple 'rule of thumb' solutions? Must field workers await the findings of laboratory-based neuroethological and neuroanatomical investigations to improve understanding of what innate versus learned behaviour contributes to complex social and ecological decisions in group-living mammals, such as those incarnate in dispersal? My suggestion is that more research emphasis be given to detailed, longitudinal field observations of recognized individuals from infancy onwards. The resulting empirical data, although in most instances onerous to collect, will enable construction of a rich, multivariate, quantitative and qualitative longitudinal picture of individual development and changing contexts of experience. In turn, these descriptive data will afford a strong basis for rejecting or accepting predictions distinguishing experiential, socially learned and innate components of dispersal behaviour. [ABSTRACT FROM AUTHOR]