RADIATIVE HEAT-LOSS IN GENTOO PENGUIN (PYGOSCELIS-PAPUA) ADULTS AND CHICKS AND THE IMPORTANCE OF WARM FEET

Adult penguins and their chicks differ considerably in their apparent body insulation. The chicks are covered in down, whereas the adults ha ve the short, hard body feathers characteristic of the family, so mech anisms of heat loss may vary considerably between the two groups. We e xamined radiative heat loss by measuring body surface temperatures of gentoo penguins (Pygoscelis papua) in Antarctica. At the time the bird s were considered to be in their thermoneutral zone, and there was lit tle or no wind. Measurements of infrared emission were made on breedin g adults and in large downy, and thermally independent, chicks in rela tion to environmental temperature. All 28 external body surface sites measured were positively correlated with ambient temperature, although there was considerable intersite variability in the relationship betw een site temperature and ambient temperature. Foot temperature increas ed most rapidly per degree ambient temperature increase, followed by t he flippers, followed by the trunk. This pattern was particularly pron ounced in the chicks, indicating that the exceptional heat-loss capaci ties of the feet may counteract for the reduced capacity of the flippe rs. Net heat transfer by radiation was examined using Stefan-Boltzmann 's law and preliminary data on the surface area of a gentoo penguin bo dy. This showed that between ground temperatures of 5 degrees and 15 d egrees C overall heat transfer remains essentially constant, although radiative heat loss from the trunk decreases, this being counteracted by increasing heat transfer from the flippers and feet. Over the same temperature range the specific radiation heat transfer of the feet inc reased approximately 100 times faster per degree ambient temperature i ncrease than did that of the flippers. This and the bimodality in foot temperature found in the study birds even under constant ambient temp eratures indicate that within the thermoneutral zone heat loss by radi ation in gentoo penguins is primarily executed using the feet, through which the blood circulates in pulses.