Do seasonal changes in metabolic rate facilitate changes in diving behaviour?

Macaroni penguins were implanted with data loggers to record heart rate (f(H)), abdominal temperature (T-ab) and diving depth during their pre-moult trip (summer) and winter migration. The penguins showed substantial differences in diving behaviour between the seasons. During winter, mean and maximum dive duration and dive depth were significantly greater than during summer, but the proportion of dives within the calculated aerobic dive limit (cADL) did not change. Rates of oxygen consumption were estimated from f(H). As winter progressed, the rate of oxygen consumption during dive cycles (s(V)over dot(O2DC)) declined significantly and mirrored the pattern of increase in maximum duration and depth. The decline in s(V)over dot(O2DC) was matched by a decline in minimum rate of oxygen consumption (s(V)over dot(O2min)). When s(V)over dot(O2min) was subtracted from S(V)over dot(O2DC), the net cost of diving was unchanged between summer and winter. We suggest that the increased diving capacity demonstrated during the winter was facilitated by the decrease in s(V)over dot(O2min). Abdominal temperature declined during winter but this was not sufficient to explain the decline in s(V)over dot(O2min). A simple model of the interactions between s(V)over dot(O2min), thermal conductance and water temperature shows how a change in the distribution of fat stores and therefore a change in insulation and/or a difference in foraging location during winter could account for the observed reduction in s(V)over dot(O2min) and hence s(V)over dot(O2DC).