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1. 1.|Temperatures from 11 different sites representing core, trunk periphery, and hindflippers were measured in 86 harvested northern fur seals after they had been forcibly driven overland under a variety of terrestrial conditions. This has enabled an assessment of their thermal responses to various extrinsic and intrinsic heat loads. 2. 2.|Hyperthermia resulted from forced terrestrial activity, but the level of hyperthermia varied with ambient conditions. Solar radiation led to overheating, and rain or condensation on vegetation that wet the fur, aided heat loss and helped prevent overheating. 3. 3.|The present investigation quantitatively confirms previous suggestions that the effectiveness of fur insulation over the seals' trunk requires that the large bare flippers, with thine xpandable webbing, assume the major role of dissipating heat. Due to their limited heat loss capability on land, the fur seal's primary response to forced terrestrial activity is involuntary heat storage and tolerance of hyperthermia to approximately 43°C.

A review of the physiology of Arctic animals implies that the effects of, and adaptations to, extreme environmental conditions are of paramount interest. In the minds of many the word “Arctic” is synonymous with “cold”, and the majority of studies concerned with arctic species continue to deal with the effects of low temperature on various body processes. Great seasonal variations in photoperiod also occur in the Arctic, and some recent studies have addressed physiological processes related to this important environmental variable. Other environmentally related factors such as nutrition and water balance are receiving increasing attention, and there is a continuing trend toward studies that combine physiological and environmental measurements. Interest in northern marine environments is presently strong and the literature is beginning to reflect this fact. This review is concerned primarily with literature since 1972 not summarized elsewhere in review articles, although for the sake of continuity some redundancies are present. Several reviews cover the physiology of Arctic homeotherms and some poikilotherms through 1971 (Irving, 1964; Morrison, 1964, 1966; Meryman, 1966; Asahina, 1969; Hart, 1971). and the book by Irving (1972) summarizes information on the physiology of Arctic birds and mammals. Various aspects of adaptation in tundra animals are covered in the recent symposium edited by Vernberg 1975). Recent pertinent Soviet works include the book edited by Chemyavsky (1975), which deals primarily with the ecology of subarctic small mammals, and the series of works on Arctic biology edited by Kontrimavichus (1973, 19745 b). For the purpose of this review it has been convenient to categorize studies as dealing with either homeotherms or poikilotherms and to somewhat arbitrarily form rough phylogenetic subdivisions of those categories based on numbers of papers concerned with a given group.

In the adult tenebrionid beetle Upis ceramboides unusually low cooling rates are required to demonstrate maximum freezing tolerance, and a very slight change in rate can reduce survival from 100 to 0%. Freezing to −50 °C results in 100% mortality at rates above 0.35 °C/min, but no injury is apparent if the cooling rate is 0.28 °C/min. The lower lethal temperature, determined with a cooling rate of 0.17 °C/min, is about −60 °C. The maximum cooling rate which allows full survival is nearly identical to optimal cooling rates previously found for mouse embryos and some lymphocytes, but the striking sensitivity to very slight changes in rate is unique to Upis . Most studies dealing with insect freezing tolerance have utilized rates of 1 °C/min or faster, and the failure of some of these laboratory studies to observe freezing survival may be due to the use of lethal cooling rates.

The adult tenebrionid beetle Upis ceramboides overwinters in the northern taiga forests of North America in a hibernaculum typically just beneath loose tree bark above the snowline. The beetles may be exposed to temperatures as low as −55°C, which is approximately the lower limit of cold tolerance found in specimens collected in mid-winter. Supercooling points average −6.3°C throughout the year and, contrary to expectation, show no seasonal variation in spite of major alterations in haemolymph composition and freezing tolerance. Summer beetles are incapable of withstanding temperatures below the supercooling point but freezing tolerance increases during the fall (September–November) and the lower lethal temperature (LLT) is maintained at ca. −55°C until March, after which it gradually rises to the summer level of −6°C. Changes in freezing tolerance are closely associated with seasonal alterations in the polyhydric alcohols sorbitol and threitol. Neither polyol is present in measureable amount during summer; sorbitol accumulates to an average haemolymph concentration of 0.44 M/l in winter and threitol reaches 0.25 M/l. Summer beetles contain about 14% more water than beetles collected during the other seasons. Upis ceramboides thus undergoes unique seasonal changes in physical and chemical characteristics that enable it to tolerate severe, prolonged subfreezing temperatures.

1. 1. Nerve-muscle function was studied at a variety of temperatures in the most northerly American frog, Rana sylvatica , and in a southern (west-central Mexico) population of Rana pipiens . 2. 2. Maximal twitch tension in isolated gastrocnemius muscles increased with decreasing temperature in R. sylvatica but decreased with decreasing temperature in R. pipiens . 3. 3. R. sylvatica muscles excitable to sciatic nerve stimulation until the freezing point (−3 to −5°C) was reached. Muscles from R. pipiens became inexcitable to either nerve or direct stimulation at about 0°C. 4. 4. No temperature-related differences in nerve function were seen between the two species.

Quantitative paper chromatographic determinations of glycerol content in the haemolymph of adult P. brevicornis have revealed seasonal variations in the concentration of this substance. Mean glycerol concentrations in excess of 22 g% have been measured during winter with the values decreasing to less than 1 g% during spring and summer. Acclimation studies gave similar results. In the artificially warmed winter beetles glycerol content decreased to less than 1 g% within 36 hr at 20°C. Glycerol concentrations were closely correlated with the changing supercooling points and haemolymph freezing points in both the naturally acclimatized and laboratory acclimated specimens. Temperature preferences showed a positive correlation with changes in cold tolerance.

1. 1.|Temperatures were measured at 11 sites representing the hindflipper, trunk periphery, and core in six northern fur seals after they had been shot and brought aboard the ship. 2. 2.|Intrathoracic temperature (37.4 ± 0.6°C) was at the expected level, however reduced rectal (35.8 ± 1.1°C) and sub-blubber (32.9 ± 1.1°C) temperatures reflect a decrease in peripheral temperature during long term exposure to 7.7 ± 0.3°C sea water. Hindflippers were uniformly cool (10.2 ± 1.1°C) suggesting the operation of a physiological heat conserving mechanism (vasoconstriction).

Excised peripheral nerves from several species of mammals from interior Alaska continue to operate when supercooled to temperatures as low as -6 degrees C. In some nerves, spontaneous freezing begins before activity ceases. If the nerves are not allowved to remain in a partially or completely frozen state for a prolonged period they will regain their activity when rewarmed.

The adult carabid beetle Pterostichus brevicornis tolerates freezing under natural conditions. Laboratory tests confirm that winter beetles tolerate temperatures below -35 degrees C, whereas summer beetles die if frozen at -6.6 degrees C. Winter beetles can be cooled to about -10 degrees C before freezing, and they thaw near -3.5 degrees C. Summer beetles thaw at -0.7 degrees C. To avoid freezing damage even in winter beetles, cooling rates must be near 20 degrees C per hour or less.

1. 1. A number of northern mammals have bare or lightly insulated tails in which the nerves are commonly exposed to low temperatures. 2. 2. Caudal nerves that are accustomed to cold have lower action potential extinction temperatures (to −6°C) and generally better low-temperature-operating characteristics than nerves from warm, well-insulated tails. 3. 3. Function can be restored after cold block, even after the nerve has been briefly frozen. 4. 4. Nerve function is maintained in poorly insulated peripheral tissues at low temperature as a result of the special operating characteristics of the nerves supplying such regions.