Status, age, and sex effects on performance of discrimination tasks in group-tested rhesus monkeys (Macaca mulatta).

Abstract

To assess the relation between performance and social or demographic variables, this study group tested a captive monkey colony on visual and manual discrimination problems. Animals could choose between differently colored, sand-filled boxes, where hue signaled the initial probability of finding buried food items. Dominant animals and subadults were most successful in locating and retrieving incentives, but sex did not affect performance. Rank effects occurred without overt aggression, suggesting deference by subordinates as a mediating mechanism. Age effects may reflect changing attention patterns only evident in complex arenas where cue salience becomes diluted. Because these findings differ from studies of singly tested animals, they show that, in a social context, an individual's rank and age may define opportunities to gain or efficiently use information.

Low-status monkeys “play dumb” when learning in mixed social groups

“…Subordinate monkeys performed poorly on a simple discrimination task in the presence of higher-ranking animals, replicating previous findings for group-tested monkeys (12). Nevertheless, under segregated social conditions, subordinates were as capable of discrimination learning as were dominant animals.…”

“…The outcomes of dominance-submission encounters were used to assign rank to individuals. A hierarchy that produced the fewest reversals was then constructed, with the constraint that all members of a matriline be grouped together (12,13). Based on the resulting matrix, we defined two social classes, a ''dominant'' class (n ϭ 27), including the 3 highestranking matrilines and 1 nonnatal male, and a ''subordinate" class (n ϭ 28), including the 3 lowest-ranking matrilines and 2 nonnatal males.…”

“…Videotaped trials were scored for individual duration measures (time spent in zones and at boxes) and individual frequency measures (number of visits to zones and to boxes, manual searches, and peanut retrievals) (11,12). We present two representative measures of discrimination performance, physical proximity (time spent in zones) and foraging success (peanut retrievals), for the last 6 acquisition trials and the 6 test trials.…”

“…We modified the traditional task to allow group testing, and incorporated components of natural foraging behavior to enhance ecological relevance (11). As reported previously, in a social context only the dominant members of a 74-member colony showed evidence of discrimination learning (12). It remained unclear exactly why subordinate monkeys performed poorly.…”

Use of Social and Ecological Information in Tamarin Foraging Decisions

“…If there is a strong cost to following both strategies at the same time, an individual should adopt whichever strategy yields the higher expected fitness. If there is little or no tradeoff, however, the animal should adopt both strategies (Giraldeau and Caraco, 2000) The success of finders and joiners depends on patterns of resource distribution, patch size and food abundance, individual social rank, tolerance, and the number of other group members also using these strategies (Barnard and Sibly, 1981;Di Bitetti and Janson, 2001;Drea, 1998;Drea and Wallen, 1999;Mottley and Giraldeau, 2000;Ranta et al, 1996;Robinette and Ha, 2003;Vickery et al, 1991;Winterhalder, 1996). Several authors note, however, that given asymmetries in the search effort of finders and joiners, the stability of such a system is a function of the advantage finders obtain by having first access to a feeding site (finder's advantage) relative to the advantages joiners experience by consuming foods acquired with reduced searching effort and potentially lower predation risk (Barta and Giraldeau, 1998;Di Bitetti and Janson, 2001;Ranta et al, 1996;Rita et al, 1997).…”