How do primates decide where to feed? Insights from wild western gorillas

Animal foraging movements are shaped by sensory-motor abilities, availability and distribution of resources and cognitive skills (memory of where, and possibly what and when, food is available). Inferring a movement process (i.e. the decision rules controlling where to go at any time) from a given movement pattern (i.e. the distributions of some key variables such as the step length or the turning angle) is usually tricky because the same process can generate different patterns depending on the context while similar patterns can be generated by different processes. However, animal movement studies combining statistical (pattern-based) and mechanistic (process-based) approaches can provide valuable insights into the knowledge an animal has of its environment. This knowledge can range between the two extreme cases of a fully naïve animal finding food only by chance and an omniscient animal knowing where, when, what and how much food is available at any time. Based on 2 years of 20 min scan sampling of ranging and feeding behaviour, we investigated the foraging movements of two habituated groups of wild western gorillas, Gorilla gorilla, a seasonally frugivorous primate species inhabiting Central African lowland forests. We showed that gorillas may choose the next feeding site by following a movement heuristic favouring the nearest-neighbour feeding site of the highest long-term interest (inferred a posteriori as the total time spent within it over a whole season) likely to yield food at this time. Thus, gorillas seemed to rely on an accurate spatial memory, enabling them to know where the places liable to yield food are located but have limited knowledge of how much food they can find at a given place at a given time. Our study shows how integrative statistical analysis and mechanistic modelling may help improve our understanding of movements and cognition in numerous species.