Biased hand use in captive emperor tamarins (Saguinus imperator)

Comparing forelimb use across primates can improve the understanding of the mechanisms and functions of brain asymmetry. We broadened the comparative framework by investigating hand use biases across spontaneous behaviors in 11 captive emperor tamarins of 2 family groups. We found a right preference across 58% of pooled unimanual bouts and 60% of all tasks. The maximum biases for a single task were 88% right-hand use and 78% left-hand use. On an individual basis, for most tasks, 6 tamarins preferred their right hand, 2 tamarins the left, and 2 animals were ambidextrous. Only a single tamarin did not switch between left and right hand bias according to task, but displayed a consistent right-hand bias. Our accompanying review of the 2 dozen previous callitrichid hand use studies confirms this picture. Across the board, these report hand preferences for single tasks. Task specialization is found in less than half of the studies, hand specialization in less than a third, whereas population level handedness is almost never reported. Importantly, right-hand use is preponderant, but very rarely consistent across tasks or subjects. This might be attributable to the fact that often-employed simple (food-reaching) tasks may not be coupled to a specialized hemisphere and therefore not constrain forelimb use. A better understanding of what causes directional forelimb use in callitrichids and other animals is currently hampered by a lack of standardized methods including measurement criteria, task difficulty, and social setting as well as a dearth of taxonomic diversity and field studies.