Summary. Adult male wild rabbits were exposed to at least 16 weeks of 16L:8D before experiments began. Plasma LH and FSH concentrations increased significantly (P < 0·001) when rabbits were castrated in 16L:8D but declined when rabbits were transferred to 8L:16D. Concentrations had returned to normal for castrated rabbits in 16L:8D by 74 days after the start of the 8L:16D treatment. Treatment of intact male rabbits with an injection of LHRH before and after transfer to short daylengths caused a transient increase in plasma LH which lasted 50–80 min and this produced a concomitant rise in plasma testosterone. The daylength change had no effect on this response even though testicular size declined after the transfer to short daylengths. Rabbits moulted in response to exposure to 8L:16D. This suggests that hypothalamic activity responds to photoperiod and that changes in pituitary responsiveness to LHRH and steroid negative feedback are unimportant.
I. L. Boyd
Summary. Ovaries of lactating grey seals, which had been shot, were measured to obtain the size of each follicle and corpus luteum. Plasma samples were collected by temporarily immobilizing lactating females. The single corpus luteum regressed rapidly after pupping and circulating progesterone levels declined at parturition and remained low throughout most of lactation. A single wave of follicular growth began about the time of parturition and gave rise to a single mature follicle towards the end of lactation. This coincided with high plasma oestradiol-17β concentrations and behavioural oestrus. A corpus luteum inhibited follicular growth in the ipsilateral ovary. The concentration of plasma progesterone increased in some seals late in lactation, indicating that ovulation sometimes occurred before the end of lactation. This was confirmed by observation of ovaries from shot seals.
I. L. Boyd
Summary. Adult male wild rabbits were exposed to artificial daylengths of 8L:16D, 13L:1 1D or 20L:4D after being taken from the wild during September, when testicular size was minimal. In all groups the testes grew and those in 20L:4D grew more quickly than did those in 8L:16D or 13L:11D. Rabbits kept in the 3 daylength treatments for the whole study maintained enlarged testes after the initial growth phase. The testes of rabbits transferred to 13L:11D after 8 weeks in 20L:4D did not regress. A similar transfer after 16 weeks in 20L:4D resulted in testicular regression (P < 0·01) followed by moult, and then testicular regrowth 9 weeks after the change of daylength. No regression occurred at this time in the control group kept in 20L:4D throughout the study. Plasma FSH concentration increased at the beginning of the study and was elevated during the initial period of testicular growth. Values then declined after the asymptote of testicular weight was reached. Plasma testosterone and LH concentrations did not change significantly throughout the study. The results showed that reproductive regression in wild rabbits can be induced by a reduction in the daylength, but that the potency of this reduction is dependent upon previous experience of daylength.
I. L. Boyd
Summary. Progesterone in Antarctic fur seals was undectable from 1–2 days before parturition to 4–6 days after parturition. There was a rapid increase in progesterone to 20 ng/ml between 6 and 10 days post partum and this increase coincided with peak concentrations of oestradiol-17β at the time normally associated with oestrus and mating in this species. Newly formed corpora lutea were present in the ovaries by Day 9 post partum even though the seals had been isolated in an enclosure and not mated. Thereafter, progesterone remained detectable, but at a low concentration (5 ng/ml) throughout embryonic diapause. A similar pattern was observed in unmated females which suggests they enter a period of pseudopregnancy. Progesterone increased to 35 ng/ml between late February and mid-March, indicating activation of the corpus luteum at the end of diapause, and then declined slowly through the remainder of gestation. Plasma prolactin, measured against a human prolactin standard, was elevated from 1–2 days before parturition and peaked at 0–3 days post partum. It then declined slowly throughout the post-partum period and remained at a low level throughout embryonic diapause. Prolactin concentration declined to undetectable at the end of diapause and before the end of lactation. Reduction of prolactin secretion by injections of bromocriptine from Days 3 to 5 post-partum terminated lactation. Mothers, which normally leave their pups to feed at sea on about Day 7 post partum, did not continue to lactate beyond Day 7 although this did not appear to be associated with reduced prolactin secretion. Bromocriptine treatment appeared to prevent the post-ovulatory surge of progesterone although there was no long-term effect of bromocriptine on progesterone secretion during the early stages of embryonic diapause/pseudopregnancy. This study has shown that prolactin is an important hormone for maintaining early lactation in the fur seal and it probably also has a role in the control of ovulation and luteal development. Prolactin does not appear to be implicated in the control of lactation cycles in fur seals. Changes in plasma progesterone during the annual cycle show that the pattern in fur seals resembles that of some carnivores with embryonic diapause.
Keywords: Antarctic fur seal; progesterone; prolactin; post-partum oestrus; lactation
B. M. Hobson and I. L. Boyd
Summary. Chorionic gonadotrophin (CG) measured by bioassay and progesterone by immunoassay were present in the placentae of grey seals (Halichoerus grypus) from about 20 days after implantation until parturition. During this period a small and significant decrease in the concentration and a large increase in the total amount of placental CG occurred. The biological activity of seal placental CG was neutralized with an anti-hCG serum. There was a correlation between fetal length and the total amount of progesterone in the placenta. The corpus luteum persisted throughout pregnancy and there was a significant correlation (P < 0·001) between CL diameter and the weight of the ipsilateral ovary. Hypertrophy of fetal testes and ovaries was observed and adult gonad size was reached at about the time of parturition. We suggest that placental CG is the hormone responsible for the precocious enlargement of fetal gonads in grey seals.