Pigtail Macaque (<i>Macaca nemestrina</i>) Performance Differs Across Multiple Cognitive Domains in Comparison to Rhesus Macaques (<i>Macaca mulatta</i>)

Rhesus and pigtail macaques are closely related and have similar social structures, yet differences in their behavior, socio-ecology, and personality have been observed, although not systematically documented. Given these differences, it is important to assess pigtail macaque cognition independently, rather than relying on rhesus macaque findings as a proxy. To gain a better understanding of pigtail macaque cognition, we used a battery of three cognitive tasks. Rhesus macaques were tested on the same tasks to validate our methods and to allow for comparison. Across just three tasks, we found significant differences between the two closely related species. In the three cups task, which tests short-term memory, both pigtail and rhesus macaques performed significantly better when they had to recall the location of a hidden food reward after a 0 s delay compared to a 15 s delay. However, in the 15 s delay condition, only rhesus macaques performed above chance levels, whereas pigtail macaques did not. In the reversal learning task, which tested rule learning and cognitive flexibility, we found species differences in learning performance. For the quantity discrimination task, which tests numerosity, we found that both rhesus and pigtail macaques were more accurate at discriminating "easy" ratios of foods (e.g., 1 vs. 5 or 2 vs. 6) than the "hard" ratios (e.g., 2 vs. 3 or 4 vs. 5). However, pigtail macaques were more accurate than rhesus macaques in the hard ratio trials. These contribute to a novel understanding of cognition in pigtail macaques while also increasing research rigor in translational research.