184
B. STONEHOUSE
r = 0.65, s.e. = 0.12). Species of cold climates tend also to have longer
feathers irrespective of body size, so the regression line clearly separates
many of the tropical from temperate and polar species.
As might be expected, Emperors have the longest plumage. Kings
have surprisingly short plumage, predictable neither from their size nor
their latitudinal range. Addlies follow closely behind Emperors, and
provide an interesting contrast with the relatively short-feathered
Chinstraps and Macaronis. The wide range of plumage lengths in pygoscelids contrasts with the narrow range in eudyptids; Macaronis have
plumage only slightly longer than Rockhoppers and Snares Island penguins of temperate waters - the three subspecies of Gentoo alone show
a greater range in length than the six species of eudyptids. Where the
eudyptids tend to have long plumage (above the line), spheniscids tend
to have short, closely following the trend of the regression line (cf. body
length) ; Galapagos and Peruvian penguins, the northernmost representatives of the genus, are well outside the departure of two standard
errors. The eudyptulid penguins show predictable but interesting trends;
Southern Blues have longer plumage than Northern Blues, and the
Chatham Island subspecies again show the influence of their warmer
environment in having shorter plumage than their parent stock (cf.
flipper size, p. 181 above).
Although little research has been done on insulating capacity and
properties of plumage, comparable information is available for mammalian fur. Hammell (1955, p. 372) showed that the insulation of wellcovered mammals approximated t o $ the value of an equivalent layer
of still air; air conduction and convection provided the main avenues
for heat transfer through dry pelts. Conduction through the substance
of the hair was negligible. From its shape and double layering, penguin
plumage is likely to be remarkably efficient in reducing air movement
between the feathers, especially at the downy layer close to the skin.
Tregear (1 965, p. 796) found that heat losses by radiation and evaporation
were negligible in densely-furred mammals (e.g. rabbit Oryctolagus
cuniculus), and that total heat losses were not increased by winds which
failed to penetrate and disarrange the fur. Again, the penguin feathers
seem especially well adapted for resisting wind movement. The short,
stiff rachis of adjunct feathers, overlapping like tiles on a roof, are pressed
more closely together rather than parted by strong winds, and are likely
to be more efficient than mammal fur in retaining an undisturbed
stratification of air close to the skin. Madsen and Wingstrand (1959,
p. 16) noted similar wind-resisting properties in the contour feathers of
birds wintering in Greenland.
Plumage insulation is reinforced by sub-dermal fat which is found in
all healthy penguins but reaches its greatest thickness in Emperors
B. STONEHOUSE
r = 0.65, s.e. = 0.12). Species of cold climates tend also to have longer
feathers irrespective of body size, so the regression line clearly separates
many of the tropical from temperate and polar species.
As might be expected, Emperors have the longest plumage. Kings
have surprisingly short plumage, predictable neither from their size nor
their latitudinal range. Addlies follow closely behind Emperors, and
provide an interesting contrast with the relatively short-feathered
Chinstraps and Macaronis. The wide range of plumage lengths in pygoscelids contrasts with the narrow range in eudyptids; Macaronis have
plumage only slightly longer than Rockhoppers and Snares Island penguins of temperate waters - the three subspecies of Gentoo alone show
a greater range in length than the six species of eudyptids. Where the
eudyptids tend to have long plumage (above the line), spheniscids tend
to have short, closely following the trend of the regression line (cf. body
length) ; Galapagos and Peruvian penguins, the northernmost representatives of the genus, are well outside the departure of two standard
errors. The eudyptulid penguins show predictable but interesting trends;
Southern Blues have longer plumage than Northern Blues, and the
Chatham Island subspecies again show the influence of their warmer
environment in having shorter plumage than their parent stock (cf.
flipper size, p. 181 above).
Although little research has been done on insulating capacity and
properties of plumage, comparable information is available for mammalian fur. Hammell (1955, p. 372) showed that the insulation of wellcovered mammals approximated t o $ the value of an equivalent layer
of still air; air conduction and convection provided the main avenues
for heat transfer through dry pelts. Conduction through the substance
of the hair was negligible. From its shape and double layering, penguin
plumage is likely to be remarkably efficient in reducing air movement
between the feathers, especially at the downy layer close to the skin.
Tregear (1 965, p. 796) found that heat losses by radiation and evaporation
were negligible in densely-furred mammals (e.g. rabbit Oryctolagus
cuniculus), and that total heat losses were not increased by winds which
failed to penetrate and disarrange the fur. Again, the penguin feathers
seem especially well adapted for resisting wind movement. The short,
stiff rachis of adjunct feathers, overlapping like tiles on a roof, are pressed
more closely together rather than parted by strong winds, and are likely
to be more efficient than mammal fur in retaining an undisturbed
stratification of air close to the skin. Madsen and Wingstrand (1959,
p. 16) noted similar wind-resisting properties in the contour feathers of
birds wintering in Greenland.
Plumage insulation is reinforced by sub-dermal fat which is found in
all healthy penguins but reaches its greatest thickness in Emperors