Peripartum body temperatures in free-ranging ewes (ovis aries) and their lambs

We measured core body temperatures of three pregnant ewes and their fetuses by means of chronically implanted temperature-sensitive data loggers. In standard laboratory conditions (dry-bulb temperature 20–22°C) core body temperatures of pregnant sheep and their fetuses during the last trimester of pregnancy demonstrated a constant feto-maternal temperature gradient, and a daily thermal circadian rhythm for the mother and fetus with a small amplitude (<1°C). When the same pregnant ewes were transferred to a natural field habitat where dry-bulb temperature varied between 7°C and 27°C in a 24 h period, there was a marked increase in the amplitude of ewe nychthemeral temperature rhythms (∼2°C). There was similarly an increase in fetal temperature rhythms, but the variations were smaller. Immediately after birth in the field, the core temperature of newborn lambs fell by up to 4.5°C. Thereafter, newborn lambs increased their core body temperatures and appeared to regulate core temperature and daily temperature rhythms at the same levels that they were as fetuses. Only in the second month post-birth do lamb core temperatures and temperature rhythms begin to resemble those of adult sheep. We have shown that fetal thermal protection, which is evident in ewes housed in in-door conditions, persists in natural field conditions also, where fetuses appear to be more protected from night-time falls- than daytime rises in maternal core temperature.