The southern elephant seal pup (Mirounga leonina) experiences a dramatic drop in its environmental temperature at birth, from approximately 37° to an average ambient temperature of 4.1°. The newborn pup receives no protection from the cow, has negligible physical insulation (such as blubber) , the fur has very limited insulation value as it is wettable and pups do not employ behavioural means of conserving body temperature, such as huddling or curling up.
The newborn pup therefore has the potential to be thermally stressed at birth or during the first week of life until it acquires an insulating blubber layer. The thyroid gland increases metabolic rate and is known to be active in newborn animals. The presence of brown adipose tissue (BAT) has not been reported in newborn southern elephant seals. The pineal gland has been shown to be very large, secreting extraordinarily large quantities of melatonin in the newborn pup and it has been proposed that the pineal may be involved in thermoregulation. The work reported in this thesis describes investigations of several physiological mechanisms which may be employed by the newborn pup during the immediate 24h post-partum to maintain body temperature.
Thirty seven pups of known age, representing eight groups, were sampled at random during the pupping season from early September to early October, 1985. The groups were Oh (4 pups), 2h (3 pups), 6h (5 pups), 24h (6 pups), 2 days (5 pups), 5 days (5 pups), 10 days (5 pups) and 20 days (4 pups). Serial blood samples were collected from an additional six pups followed for varying times and included in the T4, T3 and melatonin analysis.
The thyroid gland was the principal agent investigated. Thyroid epithelial cell height was significantly increased during the first 48h post-partum. Morphological indicators of increased secretory activity, namely pseudopodia and colloid droplets were observed in all pups during the first 24h post partum. Plasma T4 concentrations were significantly elevated at 6h (41.7 ng/ml) after birth as were T3 levels at 24h (195 ng/ml). The high plasma concentrations of T4 and T3 are endogenous, due to the active involvement of the thyroid gland in the newborn pup. The elevated levels of T3, the physiologically active thyroid hormone, may play an important role in maintaining body temperature by increasing the metabolic rate in the newborn southern elephant seal.
Central core body temperature (rectal) was examined four times during the first 24h, followed at daily intervals to 20 days throughout the breeding season. Pups were found to be endothermic, maintaining body temperature between 36.5° and 39.1°. They were not ectothermic at birth as previously described by Laws (1953).
Male southern elephant seals were found to be significantly heavier (3kg) at birth than females. Birth weight did not vary throughout the breeding season, mean weight 41.02±5.3 kg. Length and girth together are better predictors of weight than either alone. A significant relationship using length and girth as predictors of weight was found (r2=0.95). The limited evidence suggests that the pattern of growth of pups at Macquarie Island has not altered significantly with time.
No significant increase in blubber layer (subcutaneous fat) occurred until 10 days of age. Subcutaneous fat examined by electron microscopy from the scapular and sternal regions of the pup revealed the presence of tissue resembling BAT. However physiological or biochemical tests need to be conducted to confirm the thermogenic capability of this tissue in the newborn southern elephant seal.
The pineal gland is large (mean weight, 4.71 + 0.35 gm, range, 1 to 9.3 gm) and actively secreting melatonin at birth. Melatonin concentrations were extremely variable, yet very high in pups during the first 24h post partum. Mean melatonin plasma concentration for pups Oh to 24h was 17632.8 ± 5723.8 pmol/1 (4090.8 + 1327.9 pg/ml), ranging from 126 pmol/1 (29 pg/ml) to 297000 pmol/1 (68904 pg/ml). Electron microscopic examination did not reveal any noticeable changes in pinealocyte ultrastructure suggestive of increased secretory activity during this period. The large and extremely active pineal gland in newborn southern elephant seal suggests that it is actively involved in thermoregulation.
It has recently been shown that melatonin increases the level of type II 5'-deiodinase activity in BAT, thus increasing the conversion of T4 to T3. Elevated levels of T3 at birth act on BAT to stimulate the thermogenic action of this tissue. Thus melatonin may maximise the thermogenic effect of the thyroid output, thereby acting as a secondary stimulant of the pituitary-thyroid axis.
However if the BAT like tissue in southern elephant seal pups is not thermogenically active other sites of major deiodinating activity may be present. The liver or other tissues may increase the type II 5'- deiodinase activity, thus increasing circulating plasma T3 levels and ultimately raising the metabolic rate.
T4, T3 and melatonin concentrations do not peak until approximately six hours, or later, after birth. The pup does not suckle until at least 3h after birth and therefore cannot use energy from the metabolic breakdown of food for thermoregulation. What other sources of energy are available to the pup for thermogenesis in this immediate period after birth?
The liver is extremely large in southern elephant seals, approximately twice that for terrestrial animals of comparable size. Its stored carbohydrates and lipids are a potential source of metabolic energy which the pup uses during the first few hours after birth to meet the thermogenic need before the thyroid and pineal systems attain maximal activity.
In conclusion the newborn southern elephant seal is quite capable of maintaining a constant body temperature, well within the range observed in other phocids. The morphological changes in the thyroid gland and elevated plasma concentrations of T4 and T3 suggest that the metabolic rate is raised during the first week post partum. The thyroid and pineal glands may act in concert to enable the southern elephant seal pup to maintain a constant body temperature until an insulating blubber layer is acquired.