Dispersal patterns among three species of squirrel monkeys (Saimiri oerstedii, S. boliviensis and S. sciureus): III. Cognition
Authors: Boinski Sue 
Published: July 2005

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.