Year-round recordings of behavioural and physiological parameters reveal the survival strategy of a poorly insulated diving endotherm during the Arctic winter.

Warm-blooded diving animals wintering in polar regions are expected to show a high degree of morphological adaptation allowing efficient thermal insulation. In stark contrast to other marine mammals and seabirds living at high latitudes, Arctic great cormorants Phalacrocorax carbo have very limited thermal insulation because of their partly permeable plumage. They nonetheless winter in Greenland, where they are exposed to very low air and water temperatures. To understand how poorly insulated diving endotherms survive the Arctic winter, we performed year-round recordings of heart rate, dive depth and abdominal temperature in male great cormorants using miniature data loggers. We also examined the body composition of individuals in the spring. Abdominal temperatures and heart rates of birds resting on land and diving showed substantial variability. However, neither hypothermia nor significantly lower heart rate levels were recorded during the winter months. Thus our data show no indication of general metabolic depression in great cormorants wintering in Greenland. Furthermore, great cormorants did not reduce their daily swimming time during the coldest months of the year to save energy; they continued to forage in sub-zero waters for over an hour every day. As birds spent extended periods in cold water and showed no signs of metabolic depression during the Arctic winter, their theoretical energy requirements were substantial. Using our field data and a published algorithm we estimated the daily food requirement of great cormorants wintering in Greenland to be 1170+/-110 g day(-1). This is twice the estimated food requirement of great cormorants wintering in Europe. Great cormorants survive the Arctic winter but we also show that they come close to starvation during the spring, with body reserves sufficient to fast for less than 3 days. Lack of body fuels was associated with drastically reduced body temperatures and heart rates in April and May. Concurrent, intense feeding activity probably allowed birds to restore body reserves. Our study is the first to record ecophysiological parameters in a polar animal on a year-round basis. It challenges the paradigm that efficient thermal insulation is a prerequisite to the colonization of polar habitats by endotherms.