Physiological studies of arctic animals

Camp. Biochm. Phgsrol.. 1978. Vol. 59A. pp. 327 to 334. Peryomon Press. Primed tn Great Brirmn

MINIREVIEW

PHYSIOLOGICAL STUDIES OF ARCTIC ANIMALS

L. KEITH MILLER

Institute of Arctic Biology, University of Alaska, Fairbanks, Alaska 99701

(Received 17 June 19771

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 receiv- ing 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 Kon- trimavichus (1973, 19745 b).

For the purpose of this review it has been con- venient 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.

HOMEOTHERMWMALL MAMMALS

Use of heliumoxygen (helox) mixtures in closed-b circuit metabolism apparatus has proven to be a valu- able technique for determining maximum metabolic capacity (M,,,) in rodents (Rosenmann & Morrison, 1974a; Rosenmann et al., 1975). Freezing injury is avoided and M,,, is attained rapidly. The tundra vole Microtus oeconomus, because of its high BMR, had a metabolic ratio (M,JM,in) of only 5.2 but expressed as a multiple of standard metabolism the

value was 8.4 met, which exceeded lower latitude rodents. In air M,,, is reached at about -31°C a temperature well below that typically encountered in the voles subnivean environment (see Fig. 1). In the red-backed vole Clethrionomys rutilis the helox technique has demonstrated marked seasonal changes in M,,, (Rosenmann et al., 1975) with winter values about 70% higher than summer. This value of 15 met approaches the maximum achieved in larger mammals. The mean metabolic rate at any given temperature is lower in winter, which means that the thermo- regulatory load is much lower in winter. Activity metabolism in the red squirrel, Tamiasciurus hudsonicus, a typical taiga species, has also been studied utilizing incline running (Wunder & Morrison, 1974).

Rosenmann & Morrison (1975) have compared metabolic responses of highland and lowland rodents to combined high altitude and cold. Even at much lower latitudes such dual forms of environmental stress are undoubtedly important, but have received little study. A striking finding is that hypothermia amounting to a 7°C drop in T, during hypoxia in the tundra vole does not modify oxygen consump tion or respiration, although heart rate is strongly depressed (Rosenmann & Morrison, 19746).

Rapid acclimation to cold has been shown to occur in the Arctic lemmings Lemmus trimucronatus and

TA I” T

Fig. 1. Oxygen uptake of the tundra vole in air and in He-O, at different temperatures. Upper and lower dotted lines indicate observed M,,, and M,i., respectively. Verti- cal dotted lines from intercept M,,, and Mai, indicate extrapolated air temperature for M,,,. Vertical bars give range in values. He-O, increased conductance by a factor

of 2.12 (from Rosenmann & Morrison, 1974).

C.B.P. 59M-A

328 L. K~irn MILLI:K

Dicrostonyr groenlandicus (Berberich & Folk. 1976). Collier ct n/. (1975), using data from several sources, conclude that in populations of the brown lemming (L. trimucronatus) winter survival of smaller indivi- duals is favored. A relationship between snow depth and population size has been reported for the lemming Lotnmus lemmus (Maclean rr al.. 1974) with low reproduction occurring in winters of scant snow cover. Andrews er ut. (1975) have presented further evidence of endocrine jnvolvement in the dynamics of lemming ~pulatjons. Their evidence indicates that density related pituitary-adrenal changes interfere with reproduction and are involved in renal disease. Whitney (1973) found low survival in males and nestlings of subarctic tundra voles (Microtus oeconomus)

during periods of increasing populations. From the standpoint of water balance the collared

lemming (Dicrostony.x groenlandicus) has a higher total body water content (719, vs 61’+“) than the tundra vole. Water turnover is slightly faster in the vole but both species have much higher turnover rates than low temperate desert species that have been studied (Hollernan & Dieterich, 1973).

Prolonged winter hibernation has proven to be a successful strategy in the Arctic ground squirrel (C&~//US ~~~~4~ar~s) and hoary marmot (Mnrmora

hroweri). Measurement of temperature and 02/C01 levels in artificial dens has demonstrated that ex- tremely large changes in respiratory gas concentrations are tolerated by marmots. (CO2 levels of 13.5*, and O2 of 4”,,) while a maximum of I”;, CO, has been found in ground squirrel dens. Marmots and squirrels tolerated den temperatures of -25’ and -- 12’ C. respectively (Williams & Rausch. 1973). Mor- rison & Calster (19753 suggest a standardized ter- minology for hibernation patterns in Cirelfus and pro- vide detailed information on seasonal weight changes and hibern~ition~active periods in subarctic and Arctic races. Seasonal changes in body composition are reported elsewhere (Galster & Morrison. 1976). The importance of shivering thermogenesis to the arousal process in the suslik Cireflus rrrqjor has heen noted by Yakimenko & Popova (1976). These workers found that injection of 5-hydroxytryptophan reduced shivering and delayed the rewarming process. There is evidence that changes in hypothalamic NE and serotonin in the ground squirrel are important in the hibernation cycle (Feist & Galster. 1974). Peripheral nerves in Cireiius have been shown to undergo func- tional changes during hibernation and arousal (Miller, 1974). Whole blood serum taken from both species of arctic hibernators during hi~rnation will induce summer hibernation when transfused into the 13lined ground squirrel (Spurrier ef L~I., 1976). Serum from active marmots. non-hibernating hypothermic marmots or hypothermic Arctic foxes failed to induce summer hibernation. Although they do not hibernate in the physiological sense. beavers living under ice during winter occupy a restricted habitat. and have a free running circadian rhythm of activity with a period of about 27 hr. compared with a summer period of about 24 hr (Bovet & Oertili. 1974).

Seasonal responses to cold have been followed in the snowshoe hare. Lepus americanus (Feist & Rosen- mann, 1975). In winter M,,, averaged eight times the standard metabolic rate but in summer M,,, was

only 6.4 times the standard rate. Measurements 01 norepinephrine (NE) excretion in Lc,pus and NE induced thermogenesis in Clrrhriorrom_rs suggest that seasonal acclimatization involved enhanced non-shivering thermogenesis (Fcist & Rosenmann. 1975. 1976). The importance of obtaining mformation on recently captured animals is demonstrated hy the finding that wild voles show different patterns of NF storage and release than laborator> voles (F&t. 1976). Protein and maintenance energy req~~iremcnts for the snowshoe hare have been determined (Holter et al.. 1974). The daily intake of digestible energy needed to maintain energy equilibrium at 19 C‘ is 122 kcalikg.

The versatile coyote (C’LIII~S lutrutrsj ranges into arctic Alaska. Shield (1972) reports a lower critical temperature of - 10°C in three coyotes acclimatized to Fairbanks temperatures. Oxygen consumption at -70°C was only 2.4 times the thermoneutral metabolic rate. The precise control of foot temperature in the wolf and Arctic fox has been clearly demonstrated by Henshaw er ui. (1972). In a - 35 C bath foot temperature is maintained just above freezing. which mini~zes heat loss but prevents freezing damage. Mechanisms involved in the maintenance of foot temperature are not clear. but sympathetic adrenergic control seems to have been ruled out (Swan & Henshaw. 1973). Underwood (1971) has clearly demonstrated that seasonal changes in msula- tion in the Arctic fox are associated with lower critical temperatures during summer and winter that arc. respectively, equal to and cu. 15 ‘C colder than the avcr- age ambient temperatures.

HOMEOTHERMS LARGE TERRESTRIAL

MAMMALS

Physjologi~l studies of the larger Arctic land mammals have dealt almost exclusively with the domestic reindeer or its wild counterpart. the caribou. Research before 1972 is summarized in the Proceedings of the First International Reindeer and Caribou Symposium (Luick et al., 1972). More recently White (1975) has discussed environmental and physiological/biochemi- cal aspects of nutrition in two Arctic herbivores. the brown lemming and the caribou. Other nutritional information on caribou at Prudhoe Bay, Alaska. IS contained in the report by White CT al. (1975). The composition of milk secreted by reindeer is compared with that of milk of other terrestrial mammals (Luick et al.. 1974). Polar bear milk has been found to con- tain 3346 fat and both polar and grizzly bear miik is high in protein and caloric content (Jenness tar ul.. 1972).

Some adaptations of cattle to arctic conditions are discussed by Vistavnoy (1973) and by Kuschnin & Rauschenbakh (1973).

HOMEOTHERMS-. MARINE MAMMALS

Marine mammals are abundant m Arctic w-aters and many Arctic pinnipeds move seasonally with the sea ice. Due to the large heat capacity of water and the ease with which heat is transferred from skin to water, peripheral insulation in the form of fur or biub- ber is an im~rtant means of reducing heat loss. Hair

Physiological studies of Arctic animals 329

seals, with short wettable pelage (except for whitecoat pups), utilize a combination of peripheral heter- othermy and subcutaneous blubber to reduce heat flow (Miller & Irving, 1975; Miller et al.. 1976). Reten- tion of a stagnant layer of water in the fur is relatively unimportant in hair seab but is of major importance to the polar bear (0ritsland, 1970; Frisch et al., 1974) and undoubtedly to the sea otter (Morrison et al., 1974) and fur seal (Ohata & Miller, 1977~). Whitecoat hair seal pups are protected from cold when dry by their thick coat, which retains much of its effective- ness even in wind (0ritsland & Ronald, 1973), but has the disadvantage of being wettable. Blubber thickness in adult harp seals can reach 12 cm on the dorsal torso, and the daily increase in blubber thickness in newborn harp seal pups averages 2.8 mm (Morshtyn, 1973).

Grav et al. (1974) have shown that brown fat is present in considerable amounts in newborn harp seals where it may serve as a mjaor heat source while blubber is being built up (Grav & Blix, 1976). Heat transfer from brown fat to blood via the large venous plexuses may be important during diving (Blix et al., 1975). Selective re~st~bution of blood through the venous plexuses in the harp seal is discussed by Hoi et al. (1975). Studies of combined thermal and diving reactions have shown that under some conditions the peripheral vasoconstriction produced by diving can override thermally induced vasodilation (Elsner et al., 1975). Similar to nerves from heterothermous appen- dages of terrestrial animals distal sections of peripheral nerves from seal flippers can conduct at lower temperatures than more proximal nerve segments from warmer body regions (Miller, 1972).

Resting metabolism is high in very young harbor seals but declines considerably (on a tissue mass basis) within 3-4 months (Miller & Irving, 197.5). A high resting metabolic rate is atso present in the sea otter (Morrison et a/., $974). Even with the large sur- face area provided by their bare flippers, fur seals are unable to avoid hyperthermia during moderate terrestrial activity at 10°C (Ohata & Miller, 19776). The basis for the almost universally high basal metabolism in marine mammals is still largely a matter of conjecture, but in any case the critical temperature in both air and water is reduced and the metabolic scope is decreased.

The high protein and fat diet of pinnipeds etimin- ates a need for salivary amylase and it is interesting to note that in the ringed seal only the sublingual gland is active. presumably functioning to produce lubricant for the throat (Messelt & BIix, 1973). Ringed seals comprise the major portion of the polar bear’s diet and the caloric value of whole ringed seals has been found to average 2.3-5.3 kcal/g wet wt (Stirling & McEwan, 1975). Studies of the gross energetics of the northern fur seal indicate that previous estimates of food consumption in the Bering Sea/North Pacific are too low, perhaps by a factor of 1.5-2 (Miller, 1977h).

Much of the current knowledge of the physiology of arctic birds has recently been summarized by West

& Norton (197.5) including information on metabolism, reproduction and nutrition in three Arctic resi- dents [snowy owl (Nyctea scandiuca), ptarmigan (Lugopus) and redpoll (Acanthis)] and also on migra- tory sandpipers (Calidris spp.).

One of the most conspicuous and interesting resident arctic birds, the raven Coruis corax, has until recently received little study. Compared with the snowy owl (see Gessaman, 1972) or ptarmigan (West, 1972a) the plumage of the raven seems poorly suited to Arctic conditions, yet ravens can be seen in Right during the coldest weather. Recent studies indicate that metabolism at - 50°C is only increased by a factor of about 1.5 over the resting thermoneutral level. and that cold tolerance depends largely on a high resting metabolic rate (Schwan & Williams, 1976).

Arctic sea birds have also received little attention. but a study of the common and thick-billed murres (Uris aulge inornata and U. lomuia arra, respectively) shows that they resist the cold despite poor insulation by means of a high metabolic capacity (Johnson & West, 1975). Homeothermy in chicks of both species is not achieved until 9-10 days of age.

Aulie & Moen (1975) indicate that some thermoregulation is present before hatching in the fully de- veloped embryo of willow ptarmigan (L. tugopus). Five- to seven-day-old chicks have a T, of 30°C. Be- havior apparently plays an important part in main- taining homeothermy in willow ptarmigan chicks (Myhre et al., 1975). Shivering is important for heat production in the ptarmigan (Aulie. 1976), and ptarmigan chicks undergo an 83-fold increase in the mass of the pectoral muscle fibers during the first 20 days after hatching (Autie & Steen, 1976).

Food utilization by ptarmigan has received considerable attention in recent years. Winter diet and gut lengths are reported by Moss (1974). Using a ma~esium marker technique, Moss (1973) found digestibility of natural foods to be 44, 37 and 45% in, respectively, willow, rock and white-tailed ptarmigan. Daily metabohzable energy intake was about 100 kcaf in rock and white-tailed ptarmigan but about 150 kcal in willow ptarmigan. Seasonal variations in diet, volatile fatty acid production and cecal size occur in rock ptarmigan (Gasaway, 19760). Digestion and water absorption in the intestine and cecum of rock ptarmigan have been studied with tracer tech- niques (Gasaway et al., 1975, 1976). Cellulose digestibility is about 34% (Gasaway, 19766). Methane produced during cellulose digestion was found to be an insi~ificant energy source (Ga~way, 1976~). Infor- mation on energy balance and some aspects of cecal function in 4 species of tetraonids including willow ptarmigan has been presented by Andreyev (1973). West & Pohl(1977) have constructed a model annual energy budget for redpolls at high latitudes. Daily energy requirements are estimated to vary from 23 kcal/bird during winter to a maximum of 33 kcal/ bird in males supplying food to the female and young. White & West (1976) estimate that the redpolls’ eso- phageal diverticulae when filled with 2g (wet wt) birch seeds will sustain an individual for only about half of a winter day. Following an earlier report that sandpipers utilize teeth and bone fragments of lemmings as a source of calcium (MacLean, 1974). Seastedt & MacLean (1977) have found similar usage

330 L KffTH

of lemming hard tissues by Lapland longspurs (Cal- carius lapponicus).

The redpoll is distributed over a wide latitudinal range which includes the Arctic and has recently been used for translocation photoperiodic studies of different latitudinal populations (Pohl & West. 1976). Rcd- polls from interior Alaska (65’N lat) kept outdoors at 65’ and 4X’N were found to have similar activity patterns during the year in response to daylight. When compared with birds from 48“N maintained at that latitude the 6L‘N population maintained at 48’N shows significant differences in timing of circadian activity rhythm and postnuptial molt. and seasonal differences in nightly unrest and body weight. Sun position is a possible zeitgeber during the Arctic summer and evidence has been provided that circling of an artificial “sun” will synchronize locomotor activity in finches (Krull, 1976~). The large, regular daily oscillations in the color temperature of sunlight might also serve as a zeitgebers in the arctic (Krull, 1976b).

Metabolic rates have recently hccn determined ot redetermined for a dozen arctic fishes (Cameron t+ LZ~., 1973: Holeton. 1973. 1974). On the basis of the resulting data Holeton (1974) suggests that the con- cept of metabolic cold adaptation of Arctic fish is an artifact resulting from procedural and interpretational errors. It should bc noted. however. that data

MILLI+

obtained by Holeton for the Arctic cod (Borryudm suida) fit the curve given by Scholandcr et ctl. (1953) for Arctic fishes, and that several other species studied by Holeton have metabolic rates higher than extrapolated values for temperate fishes (Fig. 2). Additional studies are needed, especially emphasizing metabolic rates of arctic and temperate fishes at similar tcm- peratures. wherever possible.

Many species of arctic fishes encounter very low levels of oxygen in ice-covered lakes. Oxygen dissociation curves for burbot (Lota loto) and pike (ESOT

Irrcitts) indicate these species tolemtc hypoxic conditions by loading and umoading oxygen at low P,,. while stream-living grayling (Thyrnulius arcticus) have higher P,, values in keeping with their more highly oxygenated environment (Cameron. 1973).

Activity rhythms in adult hurbot shift from noctur- nal to diurnal twice a year at high latitudes so that the activity period never exceeds 11 hr during any 24-hr day (Muller, 1973). In contrast. burbot fry become desynchronized in May and establish a noc- turnal rhythm in late July and August (Solem. 1973). In northern Sweden Corrzcs po~c~~upus and C. gohio

undergo a phase shift to day activity in winter (Andreasson. 1973).

The overwintering habitat of the wood frog (Knrict s~+aric~r) has finally been located by the use of radio- active tags (Kirton. 1974). Slight depressions in the ground in forested or shrub-covcrcd areas are used and the frogs are covered only h! ground litter or moss. Preliminary study mdicdtcs that overwintering mortality is high,

*O- “yg f f I f I f I I

-5 0 5 IO 15 20 25 30 35 40

TEMPERATURE ‘C

Fig. 2. Summary of oxygen uptake as ;i function 01’ water temperature in fish from mqor climatic regions. Data normahxd to 5 g fish: data transformations hased on actual regrewon coethclents or an whltrary coetfrcient of 0.X. Numbered circles indicate recent data from Holeton (1974): (1) Borrtyudw saida: (2) Arctic cottids. (31 Arctic zoarcids; (4) Arctic liparids: (5) Sufwfinus nfpiws (Holeton. 1973): (6) if&u pcc~or~rlrs from Bethel. Alaska. cultured at 20 C for “several months” (Crawford. i97li. Short hrokcn l&e lab&d ANT rcfcrs to an Antarctic roarcid (Wohischl~~~. 1963). Figure taken from &leton

(19741


The great majority of studies dealing with Arctic invertebrates have been concerned with insects. Mac- Lean (1975) has reviewed ecological adaptations of tundra invertebrates with emphasis on environmental characteristics, life cycles, timing mechanisms, and metabolic adaptations, so the present review deals mainly with cold-hardiness and studies of aquatic invertebrates.

Insect cold-hardiness has most recently been reviewed by Asahina (1969) and Salt (1969). Freezing tolerance in an adult insect was first d~umented in the subarctic/arc~c carabid beetle Pterost~chus hrevi- cornis (Miller, 1969) and almost simultaneously in a temperate hymenopteran species (Asahina & Tanno, 1968). Freezing survival in Pterostichus is seasonally correlated with high levels of glycerol (Baust & Miller, 1970) and can be induced by laboratory cold a~limation (Baust & Miller, 1972) A marked increase in the ratio of un~turat~/~turated fatty acids occurs in winter, but evidence does not point to fats as the major source of glycerol (Arvey, 1974).

Subsequent studies have revealed that freezing tolerance is widespread in adult Arctic/subarctic insects (Table 1). In less severe climates - 10°C is the Iowest temperature which adult insects have so far been found to survive (Ohyama & Asahina, 1972).

The Arctic/subarctic tenebrionid Upis cerantboides readily tolerates - 60°C although its supercooling point is near -6°C (Miller & Smith, 1975). Freezing tolerance is correlated with seasonal changes in sorbi- to1 and threitol, the latter of which has not been pre- viously reported to occur naturally in animals. Another surprising feature of Upis is that low cooling rates, less than 0.3”C/min, are necessary to avoid freezing damage below -30°C (Miller, in press). So far no freezing tolerant insects have been found in Alaska that do not produce sizeable quantities of gly- ceroi or other polyhydric alcohols.

In winged Arctic insects activity associated with reproduction generally occurs only at temperatures

well above 0°C (Dahl, 1970), but neural activity in the beetle Pterostichus continues down to the supercooling point, about - 12°C in winter (Baust, 1972). It appears that muscle or neuromuscular transmission is the limiting factor in flight at low temperature. Very little work has been done on cold tolerance during the Arctic summer, yet freezing temperatures can occur at any date. Collembola in the soil at Barrow do not appear to be freezing resistant (Tanno, 1975). but a small percentage survive temperatures of - 15°C or -23”C, apparently by remaining super- cooled.

Among the aquatic invertebrates the Arctic amphipods Gammarus and ~oe~kus~~zus can tolerate wide ranges of temperature and salinity, even if the changes are abrupt (Busdosh & Atlas, 1975). Filtering feeding rates of arctic Daphnia are highest at the temperature optimum for the species, or 11 “C in D. middendorjr fiuna (Chisholm et al., 1975). This same species exhi- bits a bimodal die1 rhythm of feeding rate with max- ima at 1400 and 2400 hr, or the time of day when pond temperature passes through the mean daily temperature. Thus, the timing of the feeding rhythm appears to maximize food gathering ability (Chisholm et al., 1975). The relict crustacean Mysis relicta requires 2 years to mature in the Arctic and only 1 year further south but it does not exhibit metabolic compensation over its environmen~1 temperature range (Lasenby & Langford, 1972). An interesting finding was that the total calories required to become a reproducing adult are essentially the same in Arctic and temperate populations. The boreo-arctic echinoid stronglyocentrotus droebachiensis demonstrates a true metabolic a~Iimatization to temperature. Metabo- lism during winter is increased at O”, 5” and 10°C (Percy, 1972). A marked seasonal acclimatization with respect to activity has also been demonstrated (Percy, 1973). Measurement of metabolism in selected tissues in vitro has shown that the overall increase in metabolic rate in S. droehuchiensis is a generalized tissue adjustment {Percy, 1974~). Laboratory acclimation to cold induces an increase in whole body metabolism

Table 1. Some freezing tolerant adult, Arctic/Subarctic insects: physical and chemical characteristics

Species Weight

(mg) (Winter)

SCP (“C)l (Winter)

LLT (“C)’ Possible

cryoprotectant Ref.

Coleop tera : tolzcrl Pterostichus brevicornis 7.5 -11 < -87 Glycerol- 1 SY; Miller (I 969) Upis ceramboides 152 -6.2 -60 Threitol-- 3qO Miller &

Sorbitol- 8?0 Smith (1975) Ostoma ferruginea 41 -12 -60 or colder Glycerol- 8y0 Miller (Unpub.)

Lepidoptera : Nymph& antiopa 260 -19 ca -30 Sorbitol- 5% Miller (Unpub.)

Glycerol- 2.5% Polygoniu sp. 100 -25 CQ -35 Glycerol- 97, Miller (Unpub.) Martyrhilda cini$onelia 4.9 -23 -60 or colder Glycerol- 8% Miller (Unpub.)

Diptera: Mycetophilia SQ. 4.2 -33 -60 or colder Glycerol- 14% Miller (Unpub.) Exechia sp. 4.3 -33 -60 or colder Glycerol- 14Y:, Miller (Unpub.)

Hymenoptera : Rogas sp. ca 4 ca -30 -60 or colder Glycerol- ? Miller (Unpub.)

Neuroptera: ~emerobius simukms 5 -14 -60 or colder Glycerol- I 5% Miller (Unpub.)

’ SCP = Supercooling point or temperature at which spontaneous freezing occurs. ’ LLT = Lower lethal temperature. Most determined with cooling rate of 0.5” to l.V/min. li’pis

and Pterostichus at rate <0.2”/min.

232 L. KI:ITH MILLI:K

comparable to seasonally acclimatized urchins (Percy. 1974h).

.‘l~.k,~o\~.I~,ti~~,~n~,~lt.~- I wish to thank <i. C. West, P. R. Morrison and R. Elsner for commenting on various por- tions of the manuscript. This review was undertaken while on sabbatical leave at the Institute of Low Temperature Science. Hokkaido University. Sapporo. Japan. I am grate- ful to I:. Asahina and colleagues for providing facilities and stim~iiating discussions.

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Physiological studies of arctic animals