Time as a constraint on the distribution of feral goats at high latitudes

We use data on feral goats on the Isle of Rum to explore the hypothesis that time is a major constraint on the latitudinal and altitudinal distribution of this species. Daylength and temperature were the principal factors influencing the main time budget elements (feeding, moving and resting). Because the goats do not feed at night due to low ambient temperatures, they faced particular problems during winter when daylengths were at their shortest. A mean monthly windchill-adjusted temperature of 5 degrees C appeared to be a critical threshold in terms of the animals' ability to survive on Rum. We used these data to model the animals' time budgets at different latitudes and altitudes. We show that winter daylength would be shorter than the required feeding time north of about 60 degrees latitude, with a narrowing altitudinal cone approaching sea level at this latitude. Biogeographic evidence confirms that feral goat populations are not self-sustaining north of this latitude.