A field study of the activities of howler monkeys

Abstract

An observational study was made of group organization and activities of howler monkeys on Barro Colorado Island in the Panama Canal Zone during the dry season of 1962. Data were collected to provide information on diurnal patterns and relative frequencies with an attempt to quantify observations. The quantitative data presented in general corroborate and elaborate on the qualitative observations made during various census-taking studies. Results reveal several diurnal patterns in howler behaviour. Typical howling was most frequent early in the morning, but was also heard at other hours during the day. Other vocalizations were also maximal at daybreak and also decreased markedly thereafter. The group travelled most frequently at midday with a tendency for the entire group to rest late in the morning and again early in the afternoon. Travel was noted at other hours too with possible peaks shortly after daybreak and mid-morning. Even during periods of general travel, however, some animals could be found resting for short intervals. Shifting about with no general group direction was characteristic during most of the daylight hours. Feeding was noted throughout the day with possible peaks in the early morning and late afternoon. Direct social interactions were relatively infrequent with the exception of play wrestling which at times involved adult animals of both sexes in addition to juveniles. Sexual behaviour was seen relatively rarely, but observations reinforce the impression of general tolerance of others in the group. Other low frequency activities were also described. Data were also presented for the activities of a solitary male and cebus and spider monkeys in the immediate vicinity of a howler troop. Relations between howler troops appeared to be restricted to vocal exchanges which possibly indicate territorial defence. Groups remain in definite home ranges with favourite sleeping sites, travel routes and feeding areas which might be considered as core areas. Notes were also presented on the behaviour of spider and cebus monkeys living in the same areas as the howler monkeys under primary observation, and the feasibility of field work with these animals was discussed. Both the cebus and spider monkeys are considerably more active than the howlers, travelling more rapidly and often over greater ranges.