Ontogeny of the Adrenal and Behavioral Responses of Lambs to Emotional Stress

Abstract

The ontogeny of adrenocortical responsiveness in lambs was studied from 24 hr of age to maturity. Throughout the neonatal period, the adrenal axis was responsive to exogenous ACTH and the stresses of both restraint and exposure to open field testing. Newborn lambs both had higher resting levels of plasma corticosteroids and responded to the test stimuli with greater increases in corticosteroids than did other age groups. From the second week of life, the responsiveness of the adrenal axis was comparable to that of mature ewes. On exposure to an open field arena, lambs vocalized less and initiated movement sooner with increasing age and experience through 35 days of age, after which there was little change. Once locomotion was initiated, there was little difference in the amount of time spent moving in the arena. Open field testing was as potent a stimulator of the adrenal axis as restraint stress or exogenous ACTH.

Temperament does not affect the overall establishment of mutual preference between the mother and her young in sheep measured in a choice test

“…Thirty lambs of the temperament flock (14 born from calm ewes and 16 from nervous ewes) that were used in Experiments 1 and 2 had their temperament measured at 1 week of age in an open field arena. The open field test is a reliable method for eliciting a repeatable emotional stress response in lambs (Moberg, Anderson, & Underwood, 1980) as it measures an animal's level of fearfulness (Boissy, 1995).…”

A sex difference in the cortisol response to tail docking and ACTH develops between 1 and 8 weeks of age in lambs

“…For instance, cortisol was elevated following restraint stress in lambs (of unspecified sex) at 0, 0·5, 1, 2, 3 and 4 weeks of age (Moberg et al 1980), following open field testing in female lambs at 0, 0·5, 2 and 4 weeks of age (Moberg et al 1980), following tail docking using rubber rings or hot or cold knife in lambs (mixed sex groups) at 3-5 weeks of age (Morris et al 1994) and following surgical tail docking of female lambs or surgical tail docking and castration of male lambs at 3-6 weeks of age (Shutt et al 1988). Cortisol was also elevated following injection of ACTH (1 IU) in lambs (of unspecified sex) at 0, 0·5, 1, 2, 3 and 4 weeks of age (Moberg et al 1980). Nevertheless, it is not clear if male and female lambs responded differently to any of these treatments, since no sex comparisons were made.…”

Daughters are more strongly attached to their mother than sons: a possible mechanism for early social segregation

“…Proximity seeking with the mother from 24 h of age has been observed under natural (Shillito- Walser, Walters, & Ellison, 1984) and laboratory conditions such as in two-choice tests opposing mothers and unfamiliar ewes (Nowak et al, 1997a(Nowak et al, , 1997bNowak, Poindron, Le Neindre, & Putu, 1987). When separated from the mother, lambs display distress reactions (uttering high-pitched bleats, increased locomotor activity) and increased adrenal response (Moberg, Anderson, & Underwood, 1980;Moberg & Wood, 1981). There have been some reports that the mothereyoung relationship varies according to the sex of the lamb.…”