The pigmentary effector system. X. Relation of colour change to surface absorption of radiation

Abstract

Exposure of frogs in still air to radiation of intensity 100 cal. /sq. m. /sec., either in the range < 1.0μor in the range > 1.0μ, produces a rapid rise of temperature in the lymph space immediately below the skin irradiated. That of frogs withaggregatedmelanophores (pale condition) lags about 1°C behind that of frogs withdispersedmelanophores (dark condition) during experiments of 30-45 min. During such experiments the temperature in the lymph space reached a final level near the thermal death-point. In the course of these experiments the rectal temperature lagged about 5°C behind that of the lymph sac, but no significant difference between mean readings for dark and pale members of a pair were evident at the end of the period of exposure. What difference of water loss results from differential absorption of radiation during the course of similar experiments (< 10μor > 1.0μat about 100 cal. /sq. m. /sec.) continued for 1 hr. was not sufficient to override individual variation due to other circumstances. Rough estimates of the reflective power of skin within the range of the solar spectrum indicate that at least four-fifths of incident radiation gets absorbed by the skin of both pale and dark frogs. Excess absorption by the latter is of the order 10-15% being small com­pared with the total absorbed by the former. Dispersion of melanophores does not appreciably affect chances of survival after exposure to lethal periods of irradiation in the ultra-violet or infra-red range. Within a wide range of relative humidity a frog’s body temperature in rapidly circulating air tallies closely with the wet-bulb reading throughout the entire range 5-30°C. It is unlikely that differential absorption of solar energy by virtue of chromatic response contributes to the chances of survival of frogs. The relation of the chromatic function to temperature regulation of reptiles remains an open question, to which the foregoing observations have no necessary relevance.