Heat exchange and metabolic response to gradual cooling in developing chick embryos

(1) The large heat conductance of incubating eggs indicates that heat loss upon step-function exposure to even mild cold (1-2℃ difference) exceeds metabolic heat production of developing embryos. (2) Determination of O2 consumption during gradual cooling of the egg suggests that a weak metabolic response emerges at about day 18 of incubation. (3) After external pipping, the metabolic response to gradual cooling is stronger. The embryo need not emerge from the egg for the compensation to occur. (4) Thiourea which antagonizes the metabolic effects of thyroid hormones impaires this metabolic response. (5) We suggest that precocial hatchlings in ovo may exhibit incipient endothermic homeothermy. Full homeothermy may be prevented by the low gas conductance of the eggshell, effectively “throttling" the embryo\u27s heat production capacity. This is a constraint altricial birds probably never experience