Chicken hatchlings prefer ambient temperatures lower than their thermoneutral zone

We investigated whether or not the preferred ambient temperature (Tapref  ) of the 1-day old chicken hatchling, a precocial neonate with excellent locomotory capacity, clearly identifiable thermogenesis and independence from maternal care, coincides with the lower critical temperature (LCT) of thermoneutrality and minimal oxygen consumption (V˙O2). Tapref of single chicks measured in a thermocline (N = 16) averaged 33.5 ± 0.3 °C (mode, 33.3 ± 0.4 °C). The same value was obtained in hatchlings studied in pairs. LCT was computed from the ambient temperature (Ta)–V˙O2 relationship, constructed by slowly decreasing the Ta of a respirometer from 38 to 29 °C over 2.5 h, while continuously measuring V˙O2 by an open-flow methodology; LCT averaged 36.4 °C ± 0.3 or 36.8 °C ± 0.4, depending on the method of computation. In all hatchlings Tapref was lower than LCT (P < 0.001), by a magnitude that depended on the method of computation of the two variables, 2.8 °C ± 0.3 (P < 0.001) or 3.9 °C ± 0.5. The Tapref-LCT difference implied that, at Tapref  , V˙O2 was higher than at thermoneutrality. We conclude that in the chicken hatchling thermal preference does not coincide with thermoneutrality, probably because during development what seems optimal from a thermoregulatory viewpoint may not necessarily be so for other regulatory functions.