Abstract

The way interactions between two free-ranging animals can be investigated is often limited because the two individuals are not directly observed but only remotely localized. Furthermore, the successive relocations of the two animals are not necessarily acquired synchronously, and the delay between two successive locations may be too long to make it possible to determine the behaviour at the small scale at which interactions may occur. I will focus here on the case, which is usual for most species of vertebrates, of individuals that restrict themselves their movements within an area called home range. This is a nice way to share the whole space, allowing each individual to be quite familiar with its environment. Home ranges may correspond to more or less exclusively used areas, either because they are defended against conspecifics (they are then called territories) or as a by-product of optimisation of foraging, or may largely overlap, depending on species, sex, and context. With possibly non-synchronous (and even non time-stamped) relocations acquired at a low rate, one can (i) estimate  ‘static interaction’, i.e. the overlap of space use distribution, which is a measure of the potential for possible interactions, (ii) map where this potential is maximum (which may correspond to territory boundaries when space is defended), and (iii) and highlight whether one individual spatially dominates the other (when space is not defended). Synchronized locations make it possible to go further, in particular to determine whether two individuals that share some space meet each other more or less than expected if they moved fully independently of each other. When such relocations are acquired with a rate to warrant that they are are highly serially correlated, it becomes possible to investigate ‘dynamic interactions’, i.e. whether two individuals tend to move together or to avoid each other, by comparing the distribution of observed inter-individual distances with the distribution of distances expected for two individuals that would move independently of each other. Additional possibilities come from estimating the probability that the two individuals meet (i) at some place at a given time (ii) at some place at any time; (iii) anywhere at a given time and (iv) anywhere at any time. Altogether, these various possibilities open interesting perspectives towards a better understanding movement ecology of home-ranging animals.