Negative or positive density-dependence in movements depends on climatic seasons: The case of a Neotropical marsupial

One of the major challenges in animal ecology is to understand the factors and processes driving movement behaviour. Although density may influence movement patterns, the occurrence and nature of density-dependence in animal movements are still unclear, particularly whether it may vary among populations of a species, or across time within a population. Here, we evaluate the occurrence and nature of density-dependence in the movements of a Neotropical marsupial, the Grey four-eyed opossum Philander frenatus (Didelphidae, Didelphimorphia). We quantified fine-scale path tortuosity of individuals inhabiting continuous forest areas and forest fragments, in different climatic seasons (humid vs. super-humid). We also determined the relative importance of population size compared to sex and body mass on movements, using a model-selection approach. In forest fragments, path tortuosity increased with population size in the super-humid season, but decreased in the humid season. In the continuous forest, path tortuosity was affected only by sex and body mass, being slightly higher in males and negatively related to body mass. The occurrence of density-dependence on movements only in forest fragments is likely to reflect the higher overall density of P.frenatus in small forest fragments. The variation in the nature of density-dependence between climatic seasons is likely to reflect a trade off between foraging over large areas (humid season, low resource availability) versus avoiding agonistic encounters (super-humid season, high resource availability). Our results show that (i) density-dependence in movements may be context-dependent occurring only in areas of relatively high overall population density; and (ii) density may affect movements in different ways at different climatic seasons.