Reprinted from PHYSIOLOGlCAL ZOOLOGY

Oxygen consumption, heart rate, and thermal tolerance were measured in week-old embryos of Heermann's gull ( h r u s heermanni). Heart rate and oxygen consumption are temperature independent between 30 and 40 C and average 120 beats/min and 1.87 cm3 Ot/(egg-h). Below 30 C, these rate processes become strongly temperature dependent, and the heart stops beating when cooled to 7-13 C. The beat recovers when the embryo is rewarmed, even after 1 h a t 6 C. The heart beat stops when the embryo is heated above 40.0-41.6 C but recovers with cooling. However, the beat does not recover after the embryo has been heated to 43 C for 1 h. The thermal independence of embryonic function over the 30-40 C range minimizes disruptions associated with variation of temperature during incubation. Unattended eggs a t night may chill to temperatures which cause cessation of heart beat, but such exposure is not lethal if the eggs are rewarmed. Exposure to radiant heat during the day can rapidly raise egg temperature to levels which kill the embryo. Thus, a n important aspect of incubation by adult Heermann's gulls is shading of the eggs and prevention of overheating.