The ontogeny of thermal nociception in domestic fowl: thermal stimulus intensity and isolation effects.

Socially housed leghorn cockerels were confined to a heated grid (55, 57, or 59 degrees C, Experiment 1; 59, 61, or 63 degrees C, Experiment 2) and tested at posthatch ages of 14 days (Experiment 1) and 1, 3, 7, or 14 days (Experiment 2). In Experiment 1, chicks performed a discrete jump response only at 59 degrees C. In Experiment 2, jump latency was inversely related to temperature at each age but significantly so only at 7 and 14 days posthatch. At the highest temperature, jump latency did not differ significantly across ages. At medium and low temperatures, latency increased from 1 and 3 to 7 days posthatch (antinociceptive effect) and decreased thereafter. Chicks raised in isolation from hatch to 7 days posthatch (Experiment 3) did not display the increased jump latency (antinociceptive effect) displayed by socially raised chicks. Developmental increases in jump latency may reflect stress-induced antinociceptive concomitants of neophobia that emerge with age in this precocial species and social experience may be required for the normal development of this stress-induced antinociception.