Cascading competition: the seasonal strength of scramble influences between-group contest in a folivorous primate

Abstract

Habitat-wide depletion of food leads to scramble feeding competition and animals can mitigate costs with behavioral adjustments in ranging and activity budgets. In seasonal environments, scramble feeding competition may be more intense in lean seasons leading to shifts in the ranges of neighboring groups and cascading changes for other types of competition. Here, we specifically investigated the interplay between within-group scramble (WGS) and between-group contest competition (BGC) using four groups of ursine colobus monkeys (Colobus vellerosus) at Boabeng-Fiema, Ghana. These groups were known to be impacted by WGS and our first goal was to assess whether this competition was occurring in both the abundant and lean seasons. Within-season, between-group comparisons showed WGS in both seasons but greater behavioral compensation in the lean season, especially for larger groups. We then focused on differences in ranging behavior within groups, between seasons to examine whether greater WGS in the lean season influenced the intensity of BGC (rates and “wins” of between-group encounters). The largest and the smallest groups entered more trees and ranged slightly further per day in the lean season suggesting stronger WGS. These two groups also faced more between-group encounters with the small uni-male group winning more often in the lean season. Both middle-sized multi-male groups shifted their ranging towards the edge of the fragment in the lean season. They also engaged in fewer between-group encounters, which they usually lost. This study demonstrates the interplay between different types of competition. It also confirms that male membership disproportionately influence group competitiveness in C. vellerosus. Animals face different forms of competition over limited resources every day but the ways that these competition types influence one another is rarely studied. We focused on within-group scramble competition (WGS) for food in four groups of ursine colobus monkeys and examined how seasonal changes in this type of competition influenced between-group contest competition (BGC). We found that the lean season for food availability showed greater WGS, and that animals compensated for this by using more of their home range. Some highly competitive groups were able to move into shared areas with other groups in the lean season, winning encounters with other groups, while groups with poor competitive ability were forced to range into lower quality areas and avoided other groups. Thus, WGS had cascading effects leading to changes in BGC, showing the importance of studying different types of competition collectively.