Thermogenesis, vocalization, and temperature preference of 1-day-old chicken hatchlings after cold-exposure in late embryogenesis

Abstract

In a thermal gradient the preferred ambient temperature (T _a pref) of chicken hatchlings is a few degrees lower than thermoneutrality. To investigate whether a correlation may exist between T _a pref and the autonomic thermogenic capacity or not we studied a group of hatchlings (N = 15) exposed to cold at end-incubation, a procedure known to increase their postnatal thermogenesis. Chicken embryos were exposed to cold (34.5 °C instead of 38 °C) at days 18–20 of incubation. By comparison to Controls (N = 15), they hatched a few hours later, with similar body weight, body temperature, vocalization (number of sounds produced per unit time), and oxygen consumption ($\dot{V}_{{{\text{O}}_{ 2} }}$, measured in a respirometer by an open-flow methodology). When exposed to slow cooling these hatchlings had a higher lower critical temperature (LCT) of thermoneutrality and higher $\dot{V}_{{{\text{O}}_{ 2} }}$, and slightly higher vocalization than Controls. In a thermal gradient, T _a pref averaged 34.3 ± 0.3 °C, or 1 °C higher than in Controls (33.4 ± 0.3 °C; P < 0.05), in proportion with their higher LCT (38 ± 0.1 °C instead of 36.7 ± 0.3 °C; P < 0.001), so that the T _a pref − LCT difference (−3.6 ± 0.3 °C) was similar to Controls (−3.3 ± 0.3 °C). In conclusion, in chicken hatchlings T _a pref was lower than LCT irrespective of the magnitude of their thermogenic response. It was estimated that, at T _a pref, $\dot{V}_{{{\text{O}}_{ 2} }}$ was ~20 % higher than at thermoneutrality. Such metabolic increase could carry some physiological advantage and the choice of T _a pref may reflect the hatchling’s needs to maintain $\dot{V}_{{{\text{O}}_{ 2} }}$ slightly elevated.

Appendix

These measurements were conducted to get some insights into a possibility raised in “Discussion”. The goal was to assess the effect of ambient temperature (T _a) on body temperature (T _b) during two protocols of moderate cooling. By the first protocol, T _a was lowered to a target value and maintained constant. By the second protocol, T _a oscillated above and below the target T _a. The hatchling was placed in a respirometer that had ample windows for rapid equilibration of its internal temperature. The respirometer was transferred between a warm container and a cold container every 2 min for a total of 26 min, so as to generate cyclic oscillations in T _a of ~15 °C (between 38 and 23 °C) around the average 30.6 °C. The second test consisted of maintaining the hatchling at this average T _a (30.6 °C) for the whole time. These measurements were performed on four 1-day old hatchlings. In all cases, when T _a oscillated, T _b hardly changed; differently, when T _a was maintained around the average value, T _b invariably decreased (Fig. 5).