Chicks from seven early-feathering populations were used in a series of experiments to detail the course of thermoregulation in newly hatched chicks. Lines HN and LN were composed of White Plymouth Rock chickens that had been selected 24 generations for high and low 8-week body weight, respectively. Reciprocal crosses of those lines (HL and LH) and an F2 produced from HL as well as lines originating from HN and LN with the sex-linked dwarf allele (dw), HD and LD, respectively, were also used. Surface (foot pad) temperature, core (rectal) temperature, and body weight were obtained on female and male chicks at 2, 4, 6, 8, 11, 14, 17, 20, and 23 days in all populations and at 105 days of age in HN and LN lines. Although LN and LD chicks generally had lower surface temperatures than those from the other lines from 11 to 23 days of age, surface temperatures of HN and LN birds were not different at 105 days. No significant differences due to the dwarf allele were found in surface temperatures, but there was a consistent tendency for dwarfs to be cooler than normals in the high lines and warmer than normals in the low lines. The LN chicks had lower core temperatures through 23 days of age than the other populations measured, but the narrow range of core temperatures in all populations precluded a definitive analysis to reveal mode(s) of inheritance. Surface-to-core temperature ratios were also lower in LD and particularly LN chicks through 23 days of age, although no difference between HN and LN birds occurred in this trait at 105 days of age.
