Group size and stability: why do gibbons and spider monkey differ? Primates 32(3

ABSTRACT. Gibbons and spider monkeys have similar diets, body size, and locomotor patterns. They are therefore expected to be subject to similar socioecological ru es. However their grouping pat-terns differ. Gibbons live in small stable groups, whereas pider monkey form unstable sub-groups that vary from small to large during different seasons. If similar principles apply to the two species, food abundance should vary more for spider monkeys than for gibbons; food density should be simi-lar for the two species when spider monkey sub-groups are the same size as gibbon groups; and the highest level of food abundance should be higher for spider monkeys than for gibbons. These predic-tions are upheld for a comparison of particular populations of Hylobates muelleri and Ateles geoffroyi