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

Abstract

<b><i>Introduction:</i></b> 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. <b><i>Methods:</i></b> We recorded the diet and activity budget of <i>Colobus angolensis ruwenzorii</i>, 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. <b><i>Results:</i></b> 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 monkeys<i></i>selected 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 <i>C. a. ruwenzorii</i>at Nabugabo spent less time resting (40%) and more time moving (25%) than is typical for other species of black-and-white colobus. <b><i>Discussion/Conclusion:</i></b> 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.

Temporal patterns in the social network of core units in Rwenzori Angolan colobus monkeys: Effects of food availability and interunit dispersal

“…Association patterns fluctuated at both the node and network level, with the largest changes correlating to seasonal shifts in fruit availability. As predicted, core units were more likely to associate, and did so with a larger number of other core units, when fruits, a preferred resource for this population (2021), were abundant. This increase in association was correlated with the timing of male dispersals between core units in the band.…”

“…Rainfall data (mm per month) was obtained from https://www.worldweatheronline.com/masaka-weather-history/masaka/ug.aspx for the nearby town of Masaka (12.5 km away). We considered the availability of young leaves and fruits as these food items comprise the majority of the C. a. ruwenzorii diet at this field site (96%, 2021). Food availability indices were calculated for each of these plant parts, for each month of the study period.…”

“…We calculated the food availability index for both young leaves and fruits separately by multiplying the mean monthly phenology score for each plant part in each of the 44 species by the total basal area of that species, and summing these values for all the tree species consumed (Dasilva, 1994; Fashing, 2001; Saj & Sicotte, 2007). These methods have previously been shown to capture fruiting and leafing peaks in line with the colobus dietary choices (2021).…”

“…Greater overall food availability should allow larger aggregations to form because food competition is alleviated (e.g., Foster et al., 2012; Schreier & Swedell, 2012b; Wittemyer et al., 2005). Young leaves make up the majority of the annual diet (65%) of C. a. ruwenzorii at Nabugabo but fruits are the only food type positively selected for and these are a high‐quality resource (2021). We therefore predicted that: (a) core unit clustering would be influenced most strongly by the availability of fruits, and that core units would associate more during times of high fruit availability.…”

The Influence of Seasonal Availability of Young Leaves on Dietary Niche Separation in Two Ecologically Similar Folivorous Lemurs

“…Contrary to A. meridionalis, the highly folivorous guerezas Colobus guereza showed a resource-maximising strategy with an increase in daily distances travelled, time spent feeding, number of feeding patches visited, and dietary breadth during the period of scarcity of the preferred food items [Harris et al, 2009]. A similar resource-maximising strategy was found in C. angolensis, mainly linked to high-quality specialist diet and a clumped distribution of their main food items [Arseneau-Robar et al, 2021]. This difference might be explained by the fact that C. angolensis and C. guereza are highly specialised (they mainly rely on a few food items when available, thus needing to travel longer distances when these food items are scarce), while A. meridionalis at Ampasy is more of a generalist folivore since it forages on a wide range of plant species.…”