| Summary: | This study describes annual hormone cycles in wild and captive harbour seals, considers hormonal changes in wild grey seals during their lactation period, investigates the changing responsiveness of the pituitary-gonad axis throughout the year to stimulation with small single doses of gonadotrophin releasing hormone (GnRH) and addresses the possible influence of the experimental techniques upon the results using plasma cortisol concentrations as an index of stress. In captive harbour seals, LH concentrations through the year were significantly different only in the pregnant female. Plasma FSH concentrations were significantly different throughout the year only in the two non-pregnant females, being highest during delayed implantation. Plasma progesterone concentrations were significantly different throughout the year in both the pregnant and non-pregnant females. In wild harbour seals, plasma FSH concentrations were not significantly different throughout the year in the adult females but were significantly different in the immature females between stage 1 and 4 (post-partum oestrus and late gestation for the mature females). Plasma progesterone concentrations were significantly different throughout the year only in the adult females. In wild male harbour seals plasma testosterone concentrations were significantly different in adults between the pre-breeding and breeding season, being highest in the pre-breeding season. In wild lactating grey seals, plasma progesterone and FSH concentrations were low throughout most of lactation but increased late on. Plasma prolactin concentrations were highest during mid-lactation decreasing at the time when behavioural oestrus was observed. In all captive harbour seals and wild and captive grey seals an increase in plasma LH concentration was measured following GnRH administration returning to pre-GnRH concentrations within ninety minutes. In lactating female grey seals the magnitude of the LH response was smaller than in the other animals but was more prolonged.
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