Routes matter: the effect of seasonality on bamboo lemur navigational strategies

Seasonal environments challenge animals, increasing the complexity of locating resources. Navigation may rely on ‘habitual routes’ or ‘Euclidean’ maps: following established routes or computing novel paths, respectively. Folivores are presumed to rely on evenly distributed resources and thus have a reduced reliance on navigation. We tested whether the navigation strategy of southern bamboo lemurs, Hapalemur meridionalis, a primarily folivorous strepsirrhine that periodically consumes fruit, varies across seasons. We recorded behaviour and location of three social groups during full-day, focal follows during January–December 2013. We analysed the macronutrient components of food items. Bamboo lemurs travelled through habitual routes during periods of food scarcity but relied on nodes (i.e. intersections where directional decisions are made) to navigate throughout the year. Likely, habitual route navigation promoted an energy-saving strategy during lean periods by easing lemur locomotion across the forest. In contrast, lemurs relied on Euclidean navigation during food abundant periods. The continuous use of nodes suggest that nodes may act as cognitive anchors to support efficient directional decisions across seasons. Linearity increased during periods of resource abundance when feeding on energy-rich items and during periods of scarcity to reach latrines. By travelling increasingly linearly towards energy-rich items, lemurs maximized energy intake during periods of food abundance. Overall, bamboo lemurs' combination of both Euclidean and route-based navigational strategies demonstrates a cognitive adaptation for coping with seasonal resource variability in a way that challenges previous research postulating constrained spatial skills in folivorous strepsirrhines.