Age differences in spatial positioning of males in a chimpanzee unit-group at the mahale mountains National Park, Tanzania

Abstract

Proximity partner choice by male chimpanzees of various age classes was analyzed in relation to their spatial positioning. Field work was carried out twice at the Mahale Mountains National Park, Tanzania. Proximity data were recorded at 3 and 10m from the focal animal. The data for the proximity between the focal male and other individuals allowed the males to be classified into two categories according to both criteria: early adolescence to young adult, and prime to old age. Between the males, the 3m proximity data permitted a classification into two categories as above, but those for 10m did not. These two spatial distances thus probably have different meanings for the males. The numbers of male proximity partners and proximity with the alpha male also allowed the males to be classified into two categories: early and late adolescence, and young adult to old age. Together, the above results support the classification of males into three age-graded categories: (1) early and late adolescence, (2) young adult, and (3) prime to old age. This does not arise because the males of each category form an age group. Prime or older males are most frequently in proximity, while their juniors consistently attempt to approach them. However, even prime or older males are not equally in proximity with one another. Their proximity partners change as time passes. Probably recognizing such changes, they form coalitions or are in rivalry. The sexual interest of adolescent males is probably a factor stimulating them to separate from their mothers, and to approach older males. Young adult males, even though physically mature, do not have equal proximity relations with older males. They are not yet sufficiently qualified to join the coalitions formed by their seniors.