Breeding season and embryonic diapause in crabeater seals (Lobodon carcinophagus).

Direct observation of 387 embryos in the early stages of development was combined with observations on breeding behaviour and reproductive biology obtained from the published literature, to estimate the timing of births, oestrus, ovulation and implantation, and to derive estimates of the duration of pregnancy, embryonic diapause and active gestation for crabeater seals (Lobodon carcinophagus). The total duration of pregnancy (conception to birth) is estimated to be 11.3 months (344 days). It is estimated that the pupping season extends from late September to early November, with peak births in mid-October. The estimated mean duration of lactation is approximately 17 days; the mean date of weaning is 31 October (14 October to 17 November); and the mean date of conception is 4 November (18 October to 21 November). Oestrus, ovulation and conception occur approximately 4 days after weaning. Estimates of times of weaning and conception were made assuming that the preimplantation period is the same in all individuals. The mean date of implantation of the embryo is 24 January+/-17 days; the duration of embryonic diapause is 2.7 months (81 days); and the duration of active gestation (implantation to parturition) is 8.8 months (264 days).

Evidence for a sex-segregated migration in the humpback whale (Megaptera novaeangliae).

Existing population models for humpback whales assume that all individuals within a population undertake the annual migration from feeding areas in high latitudes to breeding areas in tropical waters. An excess of males was recorded in the commercial whaling catches near breeding areas in the southern hemisphere, but no account of this was taken in developing population models, because it was believed that this bias was a result of whalers selecting against females with young calves. Here we demonstrate that the sex ratio of migrating humpback whales near a breeding area is highly skewed towards males. A biopsy study carried out in 1992 throughout the northward and southward migrations revealed a sex ratio of 2.4 males: 1 female in the population of humpback whales migrating along the east Australian coast (n = 180). A reanalysis of the catches made during commercial whaling in this and other areas of the southern hemisphere gave a sex ratio of the same order. The most plausible explanation, supported by some evidence, is that some females remain in the feeding areas throughout winter. The results reported here show that existing management models require major revision to take account of these findings.

Plasma melatonin concentration in neonatal northern elephant seals, Mirounga angustirostris.

The development of pineal function in northern elephant seals was examined in an attempt to understand the physiological basis for previously observed high daytime levels of melatonin in neonatal southern elephant seals. Pineal glands from four northern elephant seal pups, estimated age less than 1 week, weighted 3.0 +/- 0.80 g, which was significantly less than that previously found in southern elephant seals (4.6 +/- 0.35 g). Midday concentrations of plasma melatonin in pups averaged more than 3000 pmol/l in the first 5 days post-partum, but declined rapidly to less than 400 pmol/l after day 9. Daytime melatonin levels in northern elephant seals tended to be lower than in southern elephant seals, although they were very high compared with other species. A circadian cycle of plasma melatonin concentration was observed in newborn northern elephant seals, with levels of 3000-5000 pmol/l during the day, rising to more than 10,000 pmol/l late in the dark phase. Soon after weaning at 4 weeks of age, daytime and night-time levels were in the range 60-100 pmol/l and 100-400 pmol/l, respectively. When approximately 10 weeks old, most samples were in the range 100-400 pmol/l with no discernible difference between day and night levels. The results do not support the hypothesis that the pineal gland is involved in thermogenesis in new-born southern elephant seals. Instead, the very active pineal gland may contribute to energy conservation, by lowering body temperature, particularly at night. As physical insulation is acquired by the deposition of blubber, the mechanism is not required and melatonin falls to adult levels.

The pineal gland in newborn southern elephant seals, Mirounga leonina.

In the newborn southern elephant seal the pineal gland is very large, and both pineal and plasma melatonin concentration is elevated. The pineal gland was investigated during the first 24 h, and up to 20 days of age, in elephant seal pups. A primary aim of this investigation was to determine whether there are obvious ultrastructural characteristics of pinealocytes that are exhibiting extraordinarily high levels of activity. Blood and pineal glands were collected from thirty seven pups of known age which were sampled at random from early September to early November (1985) at Macquarie Island. The pineal gland is large (mean weight, 4.71 +/- 0.35 gm, range 1-9.3 gm) and actively secreting melatonin at birth. Melatonin concentrations were extremely variable, yet very high in pups during the first 24 h post-partum. Mean melatonin plasma concentration for pups 0-24 h was 17632.8 +/- 5723.8 pmol/l (4090.8 +/- 1327.9 pg/ml), ranging from 126 pmol/l (29 pg/ml) to 297000 pmol/l (68904 pg/ml). Electron microscopic examination did not reveal any marked 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.

Osteogenesis in two marsupial species, the bandicoot Isoodon macrourus and the possum Trichosurus vulpecula.

Skeletal development of two marsupial species, the bandicoot, Isoodon macrourus and the brushtail possum, Trichosurus vulpecula, has been examined in whole mount preparations using a technique of clearing and differential staining of cartilage and bone. Both marsupials possess a complete cartilaginous skeleton at birth and ossification begins in the maxilla, mandible and certain bones of the forelimb soon after birth. The general pattern of ossification in these marsupials is similar to that in eutherians but the long bones of the forelimbs, and the thoracic vertebrae, have a precocious development when compared with that of eutherian mammals. This difference may be due to the requirement of the marsupial forearms for the passage of the young from the cloaca to the pouch. Accelerated rib development occurs, and is possibly related to respiratory movements which are initiated at a very much earlier stage than in eutherian mammals.

Adenohypophysis of the elephant seal (Mirounga leonina): morphology and seasonal histological changes.

Changes in adenohypophyseal cell populations over a 12-month period were studied in the seasonally breeding elephant seal (24 adult males, 3 adult females, and 5 neonates) at Macquarie Island. The glands were weighed and fixed in formol sublimate. Selected sections were stained with the oxidation-alcian blue-periodic acid Schiff-orange G technique. Gonadotropic, thyrotropic, lactotropic, and somatotropic cells were readily identifiable; whilst corticotropes, inactive secretory cells of all types, and stellate cells were not stained and were counted collectively as chromophobic cells. Hypophyseal weight was low throughout autumn and winter, but increased significantly during the spring breeding season and the summer. Thyrotropes were distributed evenly throughout the pars distalis, but the other secretory cells showed areas of concentration. Acidophils were common peripherally, particularly lactotropes, while gonadotropes were largely confined to the 'basophilic wedge,' a narrow, central superior zone. In males, lactotropic and gonadotropic cells showed significant seasonal changes in number. Gonadotropes were more common in sexually active males than sexually quiescent ones, while lactotrope numbers were much greater at midsummer than midwinter. This lactotrope cycle appeared to be related to photoperiod but unrelated to breeding.

The pineal gland is very large and active in newborn antarctic seals.

The pineal gland of newborn elephant seals and Weddell seals is larger than in adult females. The gland is considerably larger at birth in Weddell seals than in elephant seals. The former experience greater extremes of temperature. Plasma melatonin concentrations in excess of 2000 pg/ml were recorded in the first days of life, compared with 20-50 pg/ml in adults.

A fluctuation in plasma melatonin level in the Weddell seal during constant natural light.

Blood was collected over a 24-h period from free-living adult Weddell seals that had been exposed to natural constant daylight for some 12 weeks. The plasma melatonin concentration was low throughout the trial (range of means: 30.8-70.4 pg/ml) with the exception of a 3-h period centered on 1700 h local time, when the plasma level rose to a mean of 131.8 pg/ml. The pineal gland of the Weddell seal is thus capable of transient bouts of activity during constant and bright daylight, a situation usually inhibitory to the function of the gland. It is suggested that circadian fluctuations in the intensity of constant light entrain variation in the pineal gland functioning of this seal during summer.

A modified differential stain for cartilage and bone in whole mount preparations of mammalian fetuses and small vertebrates.

Fixation in formol-acetic-alcohol as a prelude to the staining of whole mount vertebrate skeletons with alcian blue and alizarin red S has greatly facilitated the enzyme clearing step of the method outlined by Dingerkus and Uhler. The modified method has been tested on fetal and neonatal mice, and on a variety of vertebrates including bony fish, reptiles, amphibia and birds, and shown to be rapid, reproducible and permanent. The method is not so rapid as that reported by Kimmel and Trammell but is superior at least in certain circumstances. In the present study, optimal results were obtained by fixing in formol-acetic-alcohol for 40 minutes, staining cartilage with alcian blue 8GX, then clearing with trypsin. The time taken to complete the latter step was reduced significantly by incubation at 37 C. The next step was to stain bone using alizarin red S in a weak solution of potassium hydroxide, followed by clearing in a potassium hydroxide-glycerol series.

The mammary gland of the Weddell seal, Leptonychotes weddelli (Pinnipedia). I. Gross and microscopic anatomy.

The Weddell seal cow possesses two subcutaneous, ellipsoidal, abdominal mammary glands with a volume of two to three liters when lactating. Corrosion casts reveal that approximately ten large ducts radiate from a gland cistern at the base of the nipple, and end in a complex system of terminal branches. Each gland has a separate arterial supply, mostly from the caudal deep epigastric with a minor contribution from the deep circumflex iliac. Histologically, lactating, nonlactating (resting), and immature glands resemble those of other eutherians. Sinusoidal blood vessels, not observed in the mammary glands of other mammals, are present within lactating but not in resting lobules in the seal. The terminal pouch and lactiferous sinuses possess circular smooth muscle and elastic fibers in the walls, and accumulations of lymphocytes immediately beneath the epithelium. Sebaceous and sweat glands open into the walls of the nipple and the apex of the terminal pouch. There are extensive networks of blood vessels and longitudinal smooth muscle and elastic fibers in the walls and base of the nipple. The possible functions of these morphological observations are discussed.

The kidney of Leptonychotes weddelli (Pinnipedia: Phocidae) with some observations on the kidneys of two other southern phocid seals.

The structure of the kidney of Leptonychotes weddelli was examined using corrosion casts, India ink injection, and histological methods. Some observations were made on the kidney of the crabeater seal (Lobodon carcinophagus) and the elephant seal (Mirounga leonina). The kidneys in all three species are reniculate, as in many other marine mammalian species. Features that have not been described previously in a phocid seal are a peripyramidal muscle, and venous drainage characterized by a large extrinsic system and a small intrinsic system. Examination of specialized fornices, relative medullary thickness, and the volumes of juxtamedullary relative to peripheral glomeruli (all of which relate to urine concentrating ability) revealed that each reniculus of Leptonychotes is similar to the unilobar kidney of a small mammal that produces only moderately concentrated urine. The high glomerular volume to cortical volume ratio may be related to high glomerular filtration rates after feeding observed in marine mammals. It is concluded that reniculation is more likely to be related to the large size of most marine mammals than to some factor related directly to the marine environment.

Summer and winter cycles in plasma melatonin levels in the elephant seal (Mirounga leonina).

Plasma melatonin concentration of immature male elephant seals was determined by radioimmunoassay. Comparison of concentrations during two 24-h periods, one in midsummer and one in midwinter, showed that there was a marked circadian cycle in winter which was greatly modified during the long day length of summer. It is suggested that in summer there was sufficient ambient lighting during the night hours to depress the nocturnal rise in plasma melatonin. The complexity of pineal cycles in the natural environment is stressed, and in this regard the polar regions are of particular interest due to the extreme seasonal changes in day length there.

Arteriovenous anastomoses in the skin of seals. I. The Weddell seal Leptonychotes weddelli and the elephant seal Mirounga leonina (Pinnipedia: Phocidae).

The structure, distribution, density and innervation of arteriovenous anastomoses (AVAs) have been compared in the skin of Weddell and elephant seals, as part of a study of the structure and function of arteriovenous anastomoses in mammals. In both genera AVAs were coiled vessels with the segmental structure typical of "epithelioid" anastomoses and possessed a dense peripheral adrenergic innervation. In both Weddell and elephant seals there was no statistically significant difference between the mean density of AVAs in body skin and flipper skin. The majority, approximately 71% of AVAs occurred superficially in the dermis, fewer (23%) occurred in the deeper dermis, and 6% were present in the thick hypodermis (bubber). The density of AVAs in Weddell and elephant seals is approximately eight times greater than that reported in other animals and the superficial position of anastomoses over the whole of the body surface is characteristic of phocid seals. When open, AVAs in Weddell and elephant seals allow maximal heat loss from the skin surface by passing large volumes of blood into the superficial veins. AVAs in seals are important in dissipation of body heat, particularly when the animals are out of the water, and the entire surface area is thermoregulatory rather than specific regions such as the flippers.

Arteriovenous anastomoses in the skin of seals. II. The California sea lion Zalophus californianus and the northern fur seal Callorhinus ursinus (Pinnipedia: Otariidae).

The structure, distribution and density of arteriovenous anastomoses (AVAs) were studied in body and flipper skin of a California sea lion and a nothern fur seal. In both animals AVAs consisted of arterial, intermediate and venous segments, and were generally larger and more tortuous in the sea lion than in the fur seal. In the sea lion the majority of AVAs (72%) occurred in the deeper region of the dermis, and the density was significantly greater in the flippers than in the body. In the northern fur seal most AVAs (76%) occurred in the superficial region of the dermis; the density of AVAs in flipper skin was significantly higher than in body skin, and the density in the hind flipper was significantly greater than in the foreflipper. Arteriovenous anastomoses are important in the regulation of body temperature in seals; when these animals are on land, AVAs function to dissipate body heat, and vascular thermoregulation occurs in the flippers but notover the general body surface. Due to differences in distribution and density, AVAs play a more significant role in thermoregulation in the northern fur seal than in the California sea lion.

Arteriovenous anastomoses in the skin of the Weddell Seal, Leptonychotes weddelli.

Arteriovenous anastomoses of epithelioid type were demonstrated in Weddell seal skin. The majority occurred just beneath the epidermis and among the hair follicles. There was no significant variation in density of these anastomoses between body and flipper skin. These observations suggest that arteriovenous anastomoses are important in thermoregulation in the Weddell seal, particularly as heat dissipating structures when the animal is out of the water, and that the entire body surface is involved rather than specific regions such as the flippers.