The effects of age and sex on pain sensitivity in young lambs

Abstract

Lambs routinely undergo painful husbandry procedures including ear tagging, tail docking and castration. Pain early in life is important because it can compromise lamb welfare. Studies of altricial species, such as rodents and humans, suggest that baseline pain sensitivity varies with age and sex. While age effects on behavioural or physiological responses to pain induced by injury have been assessed in lambs, the effects of age and sex on baseline pain sensitivity have not. The aim of this study was to investigate the effects of age and sex on baseline pain sensitivity in lambs using a remote laser thermal stimulus. Thermal nociceptive thresholds were measured in male and female lambs aged between one and 12 days old. There was no main effect of sex or age on lambs’ latency to respond to the thermal stimulus. However, there was a significant interaction effect of sex and age, with male and female pain sensitivity diverging with increasing age. The latency to respond increased with age in males but did not change significantly with age in females. This suggests that older males were less sensitive to thermal pain than younger males. The ontogeny of pain processing appears to differ between male and female lambs. This may be explained by sex-specific changes in peripheral components of the pain processing system (e.g. population, distribution or sensitivity of receptors) or in central integration of nociceptive signals. Alternatively, there may be sex-specific changes with age in other physiological processes relevant to thermal nociception, e.g. peripheral thermoregulation. Future research should look beyond 12 days of age to see whether sex differences in pain sensitivity persist, thus providing a more complete picture of the ontogeny of pain processing development in precocial species.