Surviving in a degraded forest environment: foraging strategies and space use of northern pigtailed macaques (Macaca leonina) in Sakaerat Biosphere reserve, Northeastern Thailand

Space-use patterns and foraging strategy in degraded habitat are crucial to understand the ecology, adaptation, and conservation of primates. However, detailed ranging and behavioral data are scarce for many species, especially from Southeast Asia.
We aimed to determine the northern pigtailed macaques (Macaca leonina) dietary, ranging, and habitat use patterns in the Sakaerat Biosphere reserve in Northeastern Thailand, a degraded habitat with ancient plantations and sub-optimal resources availability. We studied these patterns for eleven months in regard of fruit availability in a wild troop of these macaques. We used Characteristic Hull Polygons (CHP) combined with spatial statistics to estimate home ranges and core areas. We ran a monthly phenology survey to measure fruit availability over the study period. We predicted that macaques would increase their ranging during low fruit abundance to gather enough food (i.e. energy-maximizing strategy) and would use more intensively plantation and edge areas. We found a total home range of 535.5 ha and an average core area of 219.3 ha, with an average daily path length of 2,226 m. During high fruit abundance, macaques spent more time around fruit-tree species in the dry evergreen forest. During low fruit abundance, they extended their home range to plantations forest and edge areas. Fruit consumption was positively correlated to fruit abundance and there was a clear shift from flesh and pulp to dry fruit during low fruit abundance season. While the portion of home range used decreased in period of high fruit availability, daily path length and core area size did not change. Overall, as predicted, our preliminary data shows that the study troop used an energy-maximizing strategy and was able to expand the range of resources consumed in fruit scarcity period.
Conversely, a study done using the same method on the same species in the nearby pristine Khao Yai National park found an energy-minimizing strategy, that is, decreased ranging area in periods of food scarcity. In conclusion, as in previous studies, our results show indeed that northern pigtailed macaques adapt their diet, monthly range and habitat use according to food abundance. However, they indicate they adapt it in an opposite way. In a degraded forest environment, the northern pigtailed macaques seem to change their survival strategy by increasing their range in periods of food scarcity to seek additional resources. These findings pose the question of substantial modification of ecological strategies by species constrained by human alteration of their habitat.