THE GENERAL BIOLOGY OF PENGUINS
185
(2.3 cm) and Adblies (1-2 cm). Fat has a low coefficient of thermal
conductivity (0.0003-0.00049 cal/cm/sec/”C: Lipkin and Hardy, 1954,
p. 212), which provides effective insulation when the dermal blood supply
is restricted (Miller and Blyth, 1958, p. 17). In penguins partial vasodilation of skin and blubber could control skin temperature, thus helping
to maintain a suitable temperature gradient within the plumage. When
plumage and sub-dermal fat are exerting their greatest insulating efficiency, outward flow of heat is very slight in polar species; thus the long
feathers of penguins in cold climates ensure that, even in extreme
temperature gradients, the outer envelope of the birds (i.e. the layer of
overlapping feather tips) is at or very close to ambient temperatures.
In ambient temperatures 40 to 45°C lower than body temperatures on
calm, overcast days, I was not able to detect any influence of body heat
at the feather surface of resting or incubating Emperor and Addie
penguins. Snow settling against the plumage of resting birds usually
remains unmelted, often forming an outer crust which further protects
the birds from wind and extreme cold; melting occurs only when air
temperatures are close to freezing point, under direct insolation, or when
the birds have recently been active and are shedding heat through the
body surface and plumage.
With the feather surface close to, or at ambient temperatures little or
no heat is lost by radiation. Small perching birds of temperate climates
radiate heat extensively from the surface of head and beak (Veghte and
Herreid, 1965, p. 269), which for lack of insulation are usually warmer
than the rest of the body, and in most species are especially protected
against heat loss during rest, e.g. by covering them with the axillary
feathers. Penguins probably radiate heat from head and neck, which
are thinly feathered and little protected by blubber; all sleep either with
the neck fully retracted or with the beak buried in the axilla. The rather
large heads of tropical species, often with conspicuously bare faces (p.
183), contrast with the small and well-feathered heads of those from
colder regions. The beak of the AdClie penguin is feathered along half its
length, not as a protection against drifting snow (the nostrils are imperforate), but probably to minimize heat losses from this vulnerable area.
The “cooling power” or “pouvoir de refroidissement” of Antarctic
winds, as measured in experiments involving rates of cooling of uninsulated metal cylinders (Siple and Passel, 1939; Sapin-Jaloustre, 1955;
Prbvost and Sapin-Jaloustre, 1964), is largely inapplicable to penguins,
which retain their body heat by very effective insulation. The cooling
influence of strong cold winds, with or without the complication of blown
snow, cannot be great in birds which are able to retain their surface
temperature close to that of the air, and have feathers strong enough to
present an intact, impenetrable surface. Losses of heat to cold water are
185
(2.3 cm) and Adblies (1-2 cm). Fat has a low coefficient of thermal
conductivity (0.0003-0.00049 cal/cm/sec/”C: Lipkin and Hardy, 1954,
p. 212), which provides effective insulation when the dermal blood supply
is restricted (Miller and Blyth, 1958, p. 17). In penguins partial vasodilation of skin and blubber could control skin temperature, thus helping
to maintain a suitable temperature gradient within the plumage. When
plumage and sub-dermal fat are exerting their greatest insulating efficiency, outward flow of heat is very slight in polar species; thus the long
feathers of penguins in cold climates ensure that, even in extreme
temperature gradients, the outer envelope of the birds (i.e. the layer of
overlapping feather tips) is at or very close to ambient temperatures.
In ambient temperatures 40 to 45°C lower than body temperatures on
calm, overcast days, I was not able to detect any influence of body heat
at the feather surface of resting or incubating Emperor and Addie
penguins. Snow settling against the plumage of resting birds usually
remains unmelted, often forming an outer crust which further protects
the birds from wind and extreme cold; melting occurs only when air
temperatures are close to freezing point, under direct insolation, or when
the birds have recently been active and are shedding heat through the
body surface and plumage.
With the feather surface close to, or at ambient temperatures little or
no heat is lost by radiation. Small perching birds of temperate climates
radiate heat extensively from the surface of head and beak (Veghte and
Herreid, 1965, p. 269), which for lack of insulation are usually warmer
than the rest of the body, and in most species are especially protected
against heat loss during rest, e.g. by covering them with the axillary
feathers. Penguins probably radiate heat from head and neck, which
are thinly feathered and little protected by blubber; all sleep either with
the neck fully retracted or with the beak buried in the axilla. The rather
large heads of tropical species, often with conspicuously bare faces (p.
183), contrast with the small and well-feathered heads of those from
colder regions. The beak of the AdClie penguin is feathered along half its
length, not as a protection against drifting snow (the nostrils are imperforate), but probably to minimize heat losses from this vulnerable area.
The “cooling power” or “pouvoir de refroidissement” of Antarctic
winds, as measured in experiments involving rates of cooling of uninsulated metal cylinders (Siple and Passel, 1939; Sapin-Jaloustre, 1955;
Prbvost and Sapin-Jaloustre, 1964), is largely inapplicable to penguins,
which retain their body heat by very effective insulation. The cooling
influence of strong cold winds, with or without the complication of blown
snow, cannot be great in birds which are able to retain their surface
temperature close to that of the air, and have feathers strong enough to
present an intact, impenetrable surface. Losses of heat to cold water are