Variation in an individual animal's behavioural traits has been observed in a wide range of species and is believed to have important fitness consequences. As part of a larger study, it was observed that squirrels occurred at different densities on a 315 ha island, and this subsequently led to variations in faecal cortisol metabolite levels. This study aimed to examine whether living at different densities would also lead to variations in activity and aggression levels and in their survival probability, breeding and body condition. In order to examine variations in activity and aggression, behavioural tests (open field test, mirror image stimulation, breath rates and struggle rate tests) were conducted on 32 individuals (15 female, 17 male), a total of 69 times. Activity in the wild was investigated through radio tracking. There was a significant correlation between the time a squirrel was active and in their aggressive behaviour during the behavioural tests, with more aggressive individuals also being more active. Squirrels in the low density area spent a greater proportion of time active and engaged in aggressive behaviour in the open field test but also moved further in the wild and had a lower breath rate and higher struggle rates. Squirrels in the high density area were the least aggressive, had a smaller home range and higher breath and lower struggle rates. However, this was found to have no effect on survival probability, breeding or body condition. These variations may be the result of sampling dispersing squirrels.