Territorial defence behaviour in red squirrels is influenced by local density

While many animals defend territories to secure resources such as food and mates, little is known about why territory owners of the same species vary in their territorial defence behaviour. We tested whether potential intruder pressure, defence of offspring, resource-holding potential or aggressiveness of the individual territory owner best explained intraspecific differences in territorial defence in a wild population of North American red squirrels (Tamiasciurus hudsonicus). We assessed territorial defence behaviour of individual red squirrels by recording whether or not they produced territorial vocalizations, known as rattles, both in response to a territorial playback and during natural observation sessions without an experimental stimulus. We compared the relative fit of four a priori models to explain territorial defence intensity in red squirrels and found that rattling behaviour in red squirrels under natural conditions was best explained by the intruder pressure hypothesis. Red squirrels were more likely to vocalize if they were surrounded by a higher density of conspecifics on neighbouring territories, indicating that they adjust territorial defence in response to potential intruder pressure. However, vocalization responses of red squirrels to the playback were not affected by local density, which was reflected in similar support for the four a priori models. The differing effects of local density on red squirrel vocalization rate during natural observations and following playbacks indicates that the effects of local density on the territorial behaviour of red squirrels depends on the particular context in which this behaviour is expressed.