David J. Chivers

Long-Term Changes in Behaviour

One of the main aims in producing a book of this kind on the ecology and behaviour of a community of primates is to add the dimension of time, which is so often lacking in the published results of field studies, and yet which is so crucial to their interpretation. Having described the behaviour of each species in some detail — in relation to each other and to the environment in which they occur — we should not consider these behaviours, and the weather and trees, over the 10-year period of study and, where possible, try to explain the changes observed.

Selection of Fruit by Gibbons (Hylobates muelleri£ agilis) in the Rain Forests of Central Borneo

Gibbons (Hylobates spp.) are among the main frugivorous primates in Southeast Asia, yet little is known about the criteria by which they select fruit for consumption. We studied two gibbon groups for 14 mo in the lowland dipterocarp forests of Central Borneo to determine their selectivity for different fruit species and traits. Ideal gibbon fruit were yellow, large, with a juicy-soft pulp, thin skin and available in large crops. Gibbons ultimately sought seedless fruit, but when seeds were present they selected fruit with a single, well-protected seed. Given that few fruit exhibited all the desired traits, we also carried out a multiple regression using the selection ratios of the various fruit species and their associated fruit traits to determine which traits ultimately determined gibbon choice. The analysis was stratified to account for differences in fruit availability. Selection was strongest when fruit were abundant in the forest and was based on seed width (<21 mm), color (yellow-orange), and fruit weight (1–5 g). No selection is apparent when food abundance was intermediate, but when fruit were scarce they preferentially ate larger fruit (6–30 g).

Diet and Gut Morphology in Primates

There has been much discussion of the conspicuous differences in (1) body size and shape, (2) efficiency of the gastro-intestinal tract and (3) behavioural biology between those mammals subsisting mainly on animal matter and those that graze or browse on grass and (e.g. Moir, 1968; Vallenas et al., 1971; Jolly, 1972; Jarman, 1974; Wilson, 1975; Janis, 1976; Clutton-Brock, 1977). The former have been labelled as “insectivores” or “carnivores”, according to the kind of animal matter consumed, thereby following taxonomic, and thus evolutionary, relationships; the latter are usually called “herbivores”, irrespective of the kind of plant matter eaten, even though the distinction between ruminating, refecting and other herbivores has been stressed in the wide debate. More precise subdivisions are necessary, therefore, although these gross categories still have a use, especially in correlative studies; the two extremes are best labelled, however, “faunivory” and “folivory”, respectively.

Long-Term Observations of Siamang Behaviour

Long-term observations are presented on the behaviour of the siamang ape, Symphalangus syndactylus, in the lowland forest of central Malaya. The data were collected during two dry and three fruiting seasons between 1969 and 1973 inclusive on two groups with adjacent ranges; comparisons are made within and between sample periods, and between groups. The influence of weather on daily activities is considered. Food intake is analysed in terms of number of food trees, number of visits to these trees, and the cumulative time spent feeding on various food categories. Ranging behaviour is investigated in terms of distance travelled, area covered, and distribution of time and of food trees about the range. The occurrence of calling is described and compared with that of the white-handed gibbon in the same area. A discussion ensues on each of these aspects of behaviour in turn. Emphasis is laid on the similarity of behaviour of the two groups at any one time, and on the degree of their response to the fluctuations of environment variables. Finally, the application to siamang of ranging concepts currently used in animal behaviour is considered briefly.

A Phylogeny of Gibbons (Hylobates spp.) Based on Morphological and Behavioural Characters

A new estimate of the phylogenetic relationships among the nine gibbon taxa is presented, based on a compatibility analysis of 55 morphological and behavioural characters. The phylogeny presented here differs somewhat from those of other studies, but there appears to be a basic consensus that: the siamang and concolor gibbons represent the earliest forms to speciate; the agile, lar, moloch, Müllers, and pileated gibbons represent the lates forms to speciate; with the Kloss and hoolock gibbons representing intermediate forms. Although the hypotheses regarding the evolutionary direction of these characters remain subjective due to the lack of gibbons in the fossil record, the method used is totally objective in its approach and is repeatable when further data become available.

Measuring food intake in wild animals: primates

The last 30 years have seen the rapid accumulation of large quantities of data on the diets and feeding behaviour of free-ranging primates in all continents. Much is known about what, when and where different primates eat through the seasons, on their activity patterns and use of space, but in the wild it is difficult to quantify actual amounts of food and of primary and secondary nutrients ingested.

Influence of gibbon ranging patterns on seed dispersal distance and deposition site in a Bornean forest

Frugivores display daily and seasonal behavioural variation, yet the influence of this variability on subsequent seed shadows is rarely considered. We investigated the extent to which three aspects of gibbon ( Hylobates muelleri × agilis ) foraging and ranging behaviour (revisitation of favoured fruit sources, daily and monthly ranging patterns) influenced seed dispersal distances and deposition sites for two groups in dipterocarp forests at the Barito Ulu research site, Kalimantan, Indonesia. Dispersal distances and sites were estimated using gut retention times and ranging patterns collected over 12 mo. Gibbons dispersed few seeds (0.7%) under parent trees and most seeds (> 90%) were dispersed more than 100 m. Mean dispersal distances differed significantly between groups (339 m and 431 m) and across different months. Deposition site was only influenced by time of day, with all seeds swallowed in the first hour of activity being deposited under sleeping trees used that night. Both groups visited all 0.25-ha quadrats within their home range over the study period, indicating that gibbons potentially disperse seeds throughout their home range. Given the general uniformity of the gibbon seed shadow, the intensity of home range use and large seed dispersal distances, gibbons appear to be consistently effective seed dispersers and are probably one of the most important frugivores in Asian rain forests.

The significance of call duration in howler monkeys

The “loud” calls of forest primates consist of repeated sounds that elicit a response from other members of the species. Recent studies suggest that these calls are often displays by the males that permit them to assess the strength of their opponents. Previous research on red howler monkeys supported the hypothesis that howling functions in assessment of competing individuals, as an alternative to energetically expensive chases and fights. As the second step in the attempt to understand the evolution of howling in genus Alouatta,one aspect—call duration — was compared in two species,the mantled howler (Alouatta palliata)and the red howler (A. seniculus).In A. palliatathe median howl duration was 3.5 sec and the interhowl interval was 20.0 sec, while in A. seniculusthe median howl duration was 19.0 sec and the interhowl interval was 3.0 sec. During the dawn chorus, the total duration of calling in A. seniculusmay be 10 or more times greater than that in A. palliata.The latter species appears to be limited in the duration of howls it can produce, so it increases the amount of calling by reducing the interhowl interval. At least four factors may be important in the evolution of the observed differences in call structure: constraints of the acoustic environment, alternate forms of display used by A. palliata,the presence or absence of competing males within the troop, and the effect of female calls on male howling. The observations suggest that the use of the male loud call may reflect differences in the nature of male-male competition and female support more than it reflects the constraints of the acoustic environment.

Influence of Forest Seasonality on Gibbon Food Choice in the Rain Forests of Barito Ulu, Central Kalimantan

We describe the diet of two hybrid gibbon groups (Hylobates mulleri x H. agilis) in relation to forest seasonality. We collected data over 12 mo in lowland dipterocarp forest in the Barito Ulu research area, Central Kalimantan, Indonesia. Although non-fig fruit was the main dietary item (52–64% of diet), gibbon diet was most strongly influenced by the availability of flowers. During periods when flowers were most abundant and the gibbons increased consumption of them, they also ate figs or young leaves more often. We suggest that although flowers are nutritionally rich sources of food, providing relatively high levels of protein compared to fruit, they are unlikely to satiate gibbon hunger and they seek dietary bulk from figs or young leaves, because they are easily obtained. Rainfall also influenced food choice, and non-fig fruit availability had a weak influence on fruit selection for one group. The group concentrated feeding on the fruit of a few species when fruit was most abundant and ate a greater diversity of species when fruit was scarce. Gibbon diet appeared not to be influenced by changes in availability of figs, young leaves and diversity of fruiting species.

Orangutan Energetics and the Influence of Fruit Availability in the Nonmasting Peat-swamp Forest of Sabangau, Indonesian Borneo

Data on energy intake and the effects of fluctuations in fruit availability on energy intake for African apes, and orangutans in mast-fruiting habitats, indicate that orangutans may face greater energetic challenges than do their African counterparts. Comparable data on orangutans in nonmasting forests, which experience lower fluctuations in fruit availability, have been lacking, however, complicating interpretations. We conducted a 46-mo study of orangutan energetics in the nonmasting Sabangau peat-swamp forest, Indonesian Borneo. Sabangau orangutans experienced periods of negative energy balance apparently even longer than in mast-fruiting habitats, as indicated by comparisons of observed energy intake with theoretical requirements and analysis of urinary ketones. Daily energy intake was positively related to fruit availability in flanged males, but not in adult females or unflanged males. This may represent different foraging strategies between age-sex classes and suggests that fruit availability is not always an accurate indicator of ape energy intake/balance. Urinary ketone levels were not generally related to fruit availability, daily energy intake, day range, or party size. This is probably due to low energy intake, and consequently high ketone production, throughout much of the study period. Comparisons with published results on African apes support the hypothesis that orangutans are unique among hominoids in regularly experiencing prolonged periods of negative energy balance. This has important effects on orangutan behavior and socioecology, and has likely been a key factor driving the evolutionary divergence of orangutans and African apes.