In group living animals where natal dispersal is delayed, prospecting allows individuals to asses their future dispersal and breeding opportunities and, in males of some species, may minimize the costs of delaying dispersal by enabling extra-group breeding while still resident in the natal group. While evidence of prospecting is widespread, comparatively little is known about the development of this behaviour and few studies have investigated the factors that may affect investment in prospecting, as it is typically difficult to monitor such mobile individuals. Prospectors typically encounter neighbouring groups during extraterritorial forays and resident individuals in these groups respond aggressively to approaches by extra- group males, given the potential loss in direct and indirect fitness that prospectors may inflict. As with prospecting behaviour, few studies have investigated the causes of individual differences in investment in repelling prospectors and measured the costs of such territory defence. In this dissertation, I exploit our ability to closely monitor prospecting males in meerkats, to investigate the causes of individual variation in extraterritorial prospecting effort and aggressive responses to prospector intrusions. In Chapter 3, I show that, as adults, heavier males invest more in prospecting than lighter ones, and that males time their forays in order to maximize their chances of dispersal, while minimizing the associated costs by prospecting when neighbouring groups are in close proximity to their own. In Chapter 4, I demonstrate that males that are heavier in early life start prospecting at a younger age and contribute less to helping later in life, than lighter males. In Chapter 5, I show that the threats posed by prospectors towards residents are associated with high investment by resident males in repelling intruders, which has measurable costs in terms of weight gain and cooperative contributions to offspring care. Finally, in Chapter 6, the experimental presentation of scent cues reveals that meerkats discriminate between resident and extra-group male scent cues, and that resident dominant males exhibit stronger responses to indirect evidence of prospectors than other group members.