Acute, early thermal experience alters weaning onset in rats.

We hypothesized that first ingestion of solid food (weaning onset) would be accelerated in young rats with advanced thermoregulatory development. To manipulate the pups' thermoregulatory development, we exposed rat pups, but not their dams, to a Cold (10 degrees C), Moderate (21 degrees C), or Warm (31 degrees C) ambience for 2 h/day from postnatal Day 2-14, expecting that early exposure to cooler temperatures would accelerate development of thermoregulatory capabilities and thus accelerate nest egression as well as onset of feeding. Contrary to expectation, cold exposure was associated with a profile of developmental delays in both growth and maturation. Pups in the Cold condition began feeding later than pups with Moderate or Warm thermal experiences. We then evaluated thermoregulatory status (mechanisms for heat production and temperature conservation) on Day 15-16 (just prior to weaning onset). Thermogenesis, measured by oxygen consumption, was unaltered by the thermal manipulation. In contrast, pelage development (insulation) was altered. Pups in the Warm condition had greater fur density and an increased frequency of longer hairs relative to pups in the Cold condition. Although the developmental response to early cold exposure was in the direction opposite to our predictions, the hypothesized relation of thermoregulatory development to weaning onset was supported: Thermoregulatory status correlated with weaning onset.