SUMMARY
Chronic exposure to a low incubation temperature clearly slows the development of poikilothemic chicken embryos (or any other poikilotherms), but little is known about the more subtle developmental effects of temperature,especially on physiological regulatory systems. Consequently, two populations of chicken embryos were incubated at 38°C and 35°C. When compared at the same development stage, incubation temperature had no significant impact on embryonic survival or growth. Moreover, the relative timing of major developmental landmarks (e.g. internal pipping), expressed as a percentage of development, was unaffected by temperature. The ability to maintain the rate of oxygen consumption(V̇O2) during an acute drop in ambient temperature (Ta) improved from Hamburger–Hamilton (HH) stages 39–40 to 43–44 in the 38°C but not the 35°C populations. Late stage (HH43–44) embryos incubated at 38°C could maintain V̇O2(approximately 27–33 μl g–1 min–1)during an acute drop in Ta to approximately 30°C. However, at the same stage 35°C embryos acutely measured at 38°C were unable to similarly maintain their V̇O2, which fell as soon as Ta reached 36°C. Thus, while hypothermic incubation does not affect gross development (other than would be predicted from a simple effect of Q10), there is a significant delay in the relative timing of the onset of thermoregulatory ability induced by hypothermic incubation.
