Diurnal cycle of core temperature in huddling, week-old rat pups.

Lean Zucker rat pups were reared at 25 degrees C on four staggered 12:12 light:dark cycles with lights on at 0100, 0700, 1300 or 1900 hr. Core temperature (Tc) of 5- to 8-day-old pups was measured repeatedly at the rearing temperature of 25 degrees C between 0700 and 1900 hr using a procedure that minimally interfered with spontaneous nursing cycles and huddling of the pups. Tc of litters measured 30 to 60 min after the mother spontaneously left the nest was low and variable (32.2-35.4 degrees C) during the first few hours after lights-on, but was uniformly high and much less variable (35.5-36.5 degrees C) around the time of lights-off. Pup Tc was phase-advanced to the nocturnal elevation of maternal Tc, and lagged several hours behind the diurnal elevation of nursing activity. Cyclic variations in Tc of pups huddling in the nest thus reflect a diurnal rhythm in the ability and/or drive for body temperature defense that is independent of heat derived from the mother during nursing.