It is assumed that, like fur seals and sea otters, penguins depend on their pelt for insulation. Penguin feathers are narrow and short, the central axis is solid, and distribution is dense (11-12/cm 2). Based on temperature gradients between skin and core, when the bird is in air the feathers account for 80% of the insulation and the rest is due to blubber. Penguins frequently come out of the water to preen, an act that reconditions and replenishes air trapped between feathers that serves as insulation. Unlike blubber, feathers during swimming lose their insulative properties as air is swept out. Also, as the penguin descends the air is compressed and the insulative layer is reduced in thickness (Stonehouse 1975).
What happens to penguin body temperatures during diving?
It has been found that abdominal temperatures show a progressive decline during most dive bouts. Similar decreases in body temperatures have been observed for king penguins and emperor penguins. Many scientists have put forth the suggestion that this decline in abdominal temperatures may be due to the ingestion of cold food while underwater or conduction to cold seawater from exposed surfaces on the feet and flippers (Green et al. 2003). However, data from king penguins shows that lowered abdominal temperatures are somehow facilitated. Abdominal temperatures of king penguins may fall to as low as 11 C during sustained deep diving. These temperatures are 10 to 20 C below stomach temperature, suggesting that the low abdominal temperatures are not the result of ingesting cold food (Handrich et al. 1997). It is proposed that these temperature reductions lead to lowered metabolic rates in diving birds through the effects of cold temperatures on metabolically active tissues and reduced thermoregulatory costs. In diving birds, a lowering of abdominal temperatures and metabolic rate is suggested to be sufficient to bring most natural dives observed in the field within the cADL (see Physiological Constraints: Asphyxia) (Green et al. 2003). Slower metabolism of cooler tissues resulting from physiological adjustments associated with diving may help to explain why penguins can dive for such long durations, and the ADL (aerobic dive limit) of penguins may be prolonged by this temperature-induced metabolic suppression that is independent of stomach-cooling (Handrich et al. 1997).
Figure 4. Relationship between the drop in abdominal temperature and duration of diving bouts recorded from 13 breeding female macaroni penguins. y=0.04x+0.91. Figure from Green et al. (2003).
Macaroni penguins showed a progressive decrease in abdominal temperatures during periods of diving interspersed with surfacing; this is probably the result of many smaller decreases associated with individual dives that simply accumulate. The abdomen may not have sufficient time to return to its initial temperature during the surface interval between dives, and the overall decrease in temperature may be the result of accumulation of these cycles. This pattern was also found in emperor penguins, where abdominal temperature started to decrease as soon as a dive began and continued to decrease until the animal surfaced. Upon surfacing, abdominal temperature immediately increased until the dive commenced. However, the increase at the surface was not sufficient to match the decrease while diving, so there was a net effect of progressive decline in abdominal temperature during diving bouts (Green et al. 2003).
The results of Handrich et al. (1997) show that during deep dives, temperatures in certain body regions of freely foraging penguins can decrease much more dramatically than in the stomach, which is cooled predominantly by the ingestion of cold prey. These temperature decreases, leading to a depressed metabolism, may give penguins an overall energetic benefit during foraging trips, helping to explain the extraordinary diving performance of king penguins and other marine endotherms. This energy saving may be analogous to torpid periods in hibernators (Handrich et al. 1997).