THE GENERAL BIOLOGY OF PENGUINS
183
rather than aquatic conditions. Flipper area is enhanced, mainly by
broadening, in species of at least two genera which may need to shed
heat when exposed to strong insolation. Flipper shape, expressed as the
ratio of length to mean breadth, may be a product of either trend, and
has different ecological significance in different species.
c. Plumage Length and Insulation
Unlike most other birds, penguins have a complete covering of body
feathers - an adaptation minimizing heat losses in birds which spend
long periods in cool or cold water. Brood patches develop seasonally in
the mid-ventral line of the abdomen. Spheniscid penguins (particularly
Magellanic and Peruvian) have patches of bare skin about the beak and
eyes, more or less subject to seasonal change (Murphy, 1963, p. 468),
which flush when the birds are active or hot and probably help to shed
surplus heat (personal observation). The bare skin at the base of the
beak in some eudyptids (Royal, Rockhopper, Snares Islands) may serve
a similar purpose.
Body feathers are small (2-4-5 cm long), lanceolate, and densely
packed. Lowe’s count (1933, p. 487) of 300 feathers per inch2 (6.452 cm)
of a young Gentoo indicates a growth stage, not an adult condition; in
adult Emperor, Addie, Yellow-eyed and Northern Blue penguins I have
counted only 70-80 inch2, in surprising uniformity. Each rachis is flattened dorso-ventrally and curved caudally. The tips, usually marked
with pale blue in the dark dorsal feathers, form overlapping diagonal
rows which neither water nor moderate winds penetrate. Each feather
has an aftershaft of downy filaments (Pycraft, 1907, p. 4) which together
pack the spaces around the feather bases, forming a dense and waterproof undershirt. Pairs of opposing muscles control the position of each
feather. When the feathers are raised the spaces between the tips open,
thus reducing the effective insulation of the plumage by allowing outside
air to penetrate. The downy undershirt does not appear to be variable,
and its presence must prevent rapid shedding of heat from the body
surface.
Measured by the methods described above (p. 173), dorsal feathers are
usually slightly longer than ventral. The difference is barely detectable
in some species, averages 3-4 mm in most and reaches 6 mm (25%) in
White-flippered penguins. Plumage lengthens slightly with body size,
i.e. large penguins tend to have longer feathers than small ones (r = 0-27,
s.e. = 0.20). Rate of increase of feather length with body size is probably
affected by the diminishing need for insulation as decreasing surface-tovolume ratios favour heat conservation; thus there iaa clearer and more
significant positive correlation between the 8 power of mean plumage
length and the cube root of body weight (Fig. 1Oc: y = 0 . 4 5 ~ + 1.40:
183
rather than aquatic conditions. Flipper area is enhanced, mainly by
broadening, in species of at least two genera which may need to shed
heat when exposed to strong insolation. Flipper shape, expressed as the
ratio of length to mean breadth, may be a product of either trend, and
has different ecological significance in different species.
c. Plumage Length and Insulation
Unlike most other birds, penguins have a complete covering of body
feathers - an adaptation minimizing heat losses in birds which spend
long periods in cool or cold water. Brood patches develop seasonally in
the mid-ventral line of the abdomen. Spheniscid penguins (particularly
Magellanic and Peruvian) have patches of bare skin about the beak and
eyes, more or less subject to seasonal change (Murphy, 1963, p. 468),
which flush when the birds are active or hot and probably help to shed
surplus heat (personal observation). The bare skin at the base of the
beak in some eudyptids (Royal, Rockhopper, Snares Islands) may serve
a similar purpose.
Body feathers are small (2-4-5 cm long), lanceolate, and densely
packed. Lowe’s count (1933, p. 487) of 300 feathers per inch2 (6.452 cm)
of a young Gentoo indicates a growth stage, not an adult condition; in
adult Emperor, Addie, Yellow-eyed and Northern Blue penguins I have
counted only 70-80 inch2, in surprising uniformity. Each rachis is flattened dorso-ventrally and curved caudally. The tips, usually marked
with pale blue in the dark dorsal feathers, form overlapping diagonal
rows which neither water nor moderate winds penetrate. Each feather
has an aftershaft of downy filaments (Pycraft, 1907, p. 4) which together
pack the spaces around the feather bases, forming a dense and waterproof undershirt. Pairs of opposing muscles control the position of each
feather. When the feathers are raised the spaces between the tips open,
thus reducing the effective insulation of the plumage by allowing outside
air to penetrate. The downy undershirt does not appear to be variable,
and its presence must prevent rapid shedding of heat from the body
surface.
Measured by the methods described above (p. 173), dorsal feathers are
usually slightly longer than ventral. The difference is barely detectable
in some species, averages 3-4 mm in most and reaches 6 mm (25%) in
White-flippered penguins. Plumage lengthens slightly with body size,
i.e. large penguins tend to have longer feathers than small ones (r = 0-27,
s.e. = 0.20). Rate of increase of feather length with body size is probably
affected by the diminishing need for insulation as decreasing surface-tovolume ratios favour heat conservation; thus there iaa clearer and more
significant positive correlation between the 8 power of mean plumage
length and the cube root of body weight (Fig. 1Oc: y = 0 . 4 5 ~ + 1.40: