how seabirds catch food | 175utes if its body processes continued to function as they do when it is
breathing air. This ‘limit’ is comfortably exceeded by recorded dives
lasting some 20 minutes. Just as a breathless athlete striving for the fin-
ishing line builds up lactic acid, so the underwater penguin builds up
lactate, principally in the muscles. This is then flushed out when it re-
turns eventually to the surface.
Another key adaptation to diving is a reduction in heart rate under-
water, exactly as also occurs in diving seals and whales. Detected via at-
tached electrocardiogram (ECG) recorders, the heart rate of a resting
Emperor Penguin is around 70 beats/minute. This value roughly doubles
immediately before the dive. If the dive is short, under five minutes, the
underwater rate is about the same as when resting. If the dive is long,
heart rate drops off dramatically, and may reach as low as three beats/
minute. Just before the penguin surfaces, the rate accelerates. It can be
around 200 beats/minute when the penguin surfaces and can breathe
once more to replenish its oxygen stores.^25
Remembering that even in tropical seas, the water temperature below
200 m is probably no higher than 5°C, a further physiological problem
faced by seabirds underwater is potentially that of cold. Penguins and
auks have tight plumage that retains air close to the skin. This assists
heat retention, albeit by creating buoyancy that hinders the downward
dive. The situation is different in cormorants and shags. Their plumage
is notoriously wettable. Think of the classic pose of a perched cormo-
rant hanging out its wings to dry after a spell of swimming. If the water
has reached the skin, the cormorant will have lost more heat than an-
other seabird whose skin remains dry.
In fact the paradox is more apparent than real. The Great Cormorant
is one of the most widespread of all seabirds. Its breeding range extends
from Australia via south- east Asia to Europe and Greenland. Whether
in northern France or Greenland it requires less food per day than other
seabirds of comparable weight.^26 The birds are evidently not leaking
heat, and not needing to take in extra food. How they retain heat be-
came evident when European researchers looked at the plumage more
closely. All four subspecies studied, living in sub- Arctic to subtropical
climes, retained an insulating air layer in their plumage, which was,
however, much thinner than for other species of diving birds. Detailed