| Abstract: | Howler monkeys, Alouatta caraya, were collected in family groups from islands in the Rio Parana in northern Argentina. The body weights, lens weights, dental age and reproductive status of 261 individuals illustrated sexually dimorphic body weights after a period of accommodation as young adults, a large proportion of older females (even though there are more young adult males than females) and a higher percentage of pregnancies among these older females. Seventeen families, representing the entire social system of one island, showed that about three quarters were adult (equally divided between the sexes but with fewer older males than females) and the remaining quarter were immature (with an equal sex ratio). Half of the adult females were either pregnant or suckling young. The mean family size is seven which includes peripheral males. Group size and composition as well as adult to immature proportions compared closely with data on A. caraya in the Chaco of Argentina and the 1951 census of A. palliata on Barro Colorado Island. It can be said that when these two species of Alouatta occur in small size families, they have in common: a high proportion of adults to immature, a low proportion of infants, and can have either an equal sex ratio (A. caraya) or preponderance of females (A. palliata). |
