Diet and Activity Budget in Colobus angolensis ruwenzorii at Nabugabo, Uganda: Are They Energy Maximizers?

Abstract

Introduction: Colobine monkeys are specialized folivores that use foregut fermentation to digest leaves. The slow process of fermentation forces them to spend a lot of time resting and to minimize their energy expenditure to subsist on a lower-quality diet.

Methods: We recorded the diet and activity budget of Colobus angolensis ruwenzorii, which form a three-tiered multi-level society, at Lake Nabugabo, Uganda, over 12 months using scan sampling on adults and subadults, to determine whether they utilize the energy minimization strategy typical of colobines.

Results: We found that the annual diet was primarily comprised of high-quality food resources (young leaves 65% and fruit 31%), and fruits were the only plant part the monkeysselected when available. Both the fruits and young leaves of some species were preferred food items in some months, and mature leaf consumption correlated negatively with preferred food availability. Mature leaves appear to be a fallback food for this population but are rarely relied upon (3%). The C. a. ruwenzoriiat Nabugabo spent less time resting (40%) and more time moving (25%) than is typical for other species of black-and-white colobus.

Discussion/conclusion: The high-quality diet of this population appears to allow them to utilize an energy maximization strategy. Their reliance on food items that tend to be clumped in space and time likely explains the frequent fission-fusion behaviour that we observe between core units. Our findings demonstrate that the foraging strategies of colobines may be more flexible than was previously thought and illustrate how food availability and distribution can impact primate social organization.

Keywords: Colobines; Diet composition; Fallback foods; Food selection; Group cohesion.

Fig. 1

The location of Uganda in Africa and the location of Lake Nabugabo in Uganda indicated by the red star. The satellite images show the forest fragment where the study band of Colobus angolensis ruwenzorii live with their home range outlined in red. The photo is of an adult female C. a. ruwenzorii (photo by Julie Teichroeb). Satellite image (Google Earth, 2018).

Fig. 2

a Average daily rainfall (mm/day) for each month of the study period. b Food availability index scores for the different plant parts consumed. c The monthly diet of Colobus angolensis ruwenzorii near Lake Nabugabo, Uganda.

Fig. 3

Monthly activity budget for Colobus angolensis ruwenzorii near Lake Nabugabo, Uganda.