Dale D. Feist

Seasonal Changes in the Metabolic Capacity of Red-Backed Voles

By the use of helium-oxygen mixtures it is possible to elicit maximum metabolic effort from small mammals under modest cold stress without training or injury. By this means, levels ranging from 5 to 8 times the standard metabolism (met) have been observed in various captive rodent species. These values are lower than those of 15 to more than 20 met observed in larger mammals such as dogs, man, and horses which can be systematically trained to exercise (Rosenmann and Morrison 1974). However, wild individuals may have greater metabolic demands at times, particularly in winter. The present study examines the influence of seasonal cold on unconfined individuals of a subarctic species in its natural habitat. Besides the maximum metabolism (Mmax), other bioenergetic parameters of interest include body temperature (TB), basal metabolism (MB), thermal conductance (C), low lethal temperature [TB (Mmax/C)], low critical temperature [TB (MB/C)], metabolic expansivity (MmaxMB), and metabolic reserve (Mmx M).

Effects of cold, short day and melatonin on thermogenesis, body weight and reproductive organs in Alaskan red-backed voles.

SummaryThis study examined whether cold, short day or melatonin causes reproductive regression and stimulates nonshivering thermogenesis in a subarctic rodentClethrionomys rutilus. Red-backed voles born and raised at 23°C and 22 h light per day (LD 22: 2) at Fairbanks, Alaska (65°N) were exposed in one of six groups to: 1) long day (LD 22:2), 23°C, injected daily with melatonin or saline 2 h before lights out, 2) long day, 3°C, injected daily with melatonin or saline, 3) short day (LD 8:16), 23°C or 3°C. Voles were tested for nonshivering thermogenesis (NST) prior to and after 8 wk exposure. Body weight, testes weight and female reproductive tract weight were assessed after 8 wk in long day and 12 wk in short day.NST was not altered by short day or melatonin but cold (3°C) caused an increase in NST which was similar in long day and short day.Body weight of males and females was not affected by short day but was decreased by melatonin.Short day did not alter mean testes weight (about 20% voles regressed) but reduced mean female reproductive tract weight (more than 40% voles regressed). Melatonin reduced testes weight and female reproductive tract weight (more than 50% of voles of both sexes regressed).The results suggest that in northern red-backed voles: 1) the pineal does not mediate seasonal changes in thermogenic capacity, 2) the pineal may mediate reduction of body weight and regression of reproductive organs but, in addition to daylength, other cues or factors may be important, 3) populations may exhibit variability in sensitivity of reproduction to photoperiod which could allow for opportunistic breeding.

Changes in hypothalamic catecholamines and serotonin during hibernation and arousal in the arctic ground squirrel

Abstract 1. 1. Hypothalamic norepinephrine (NE), epinephrine (E) and serotonin (5-HT) were assayed and compared in the Arctic ground squirrel ( Citellus undulatus ) in the prehibernation summer season and different phases of the hibernation cycles during the winter. 2. 2. From a high level in the prehibernation season, NE fell (by 35 per cent) to a lower level in the hibernation period, active phase and re-entry phase of the hibernation cycle. In early and mid arousal, NE increased by 50 per cent over that in deep hibernation. By the end of arousal, NE returned to the level found in other phases of the hibernation cycle. 3. 3. During the hibernation cycle, E increased in late arousal and the active phase. The concentration of 5-HT remained similar in all groups except in early arousal when the level was 34 per cent lower than that in hibernation. 4. 4. The ratio of NE/5-HT in the hypothalamus was highest during early arousal from hibernation. 5. 5. The results support the hypothesis that changes in the activity of hypothalamic noradrenergic and serotonergic systems play an important role in maintenance of and arousal from hibernation.

Seasonal sympatho-adrenal and metabolic responses to cold in the Alaskan snowshoe hare (Lepus americanus macfarlani)

Abstract 1. 1. Winter-acclimatized snowshoe hares achieved a significantly greater maximum metabolic response to cold (Mmax) than summer hares. 2. 2. Summer hares exposed to +13°C and winter hares to −20°C excreted similar levels of urinary norepinephrine (NE) and epinephrine (E). 3. 3. Cold exposure of summer hares to −20°C and winter hares to −45°C (conditions which elicit the same metabolic rate in both groups) caused significantly greater NE and E excretion in summer hares. 4. 4. The results suggest that seasonal acclimatization involves enhanced non-shivering thermogenesis, increased sensitivity to NE and increased Mmax in winter hares which enables retention of a constant annual metabolic range for activity.

Terrestrial Mammals in Cold

Terrestrial mammals in temperate, subarctic, and arctic environments must adapt to cold in order to survive winter. At high latitudes winter may last for 8 months and mammals may be exposed to extreme air and ground surface temperatures of −50°C. Similar conditions may exist at high altitudes in temperate regions. In deserts during winter, subfreezing temperatures are common at night. At temperate and arctic latitudes it is not unusual to find a small vole (20 g) and large moose (400 kg) occupying the same habitat. Both of these mammals maintain relatively constant high body core temperatures and remain active throughout the winter (i.e., they do not exhibit torpor or hibernation). This is remarkable since body size influences the level of heat production at thermoneutrality, the inherent insulative and metabolic capabilities for combating cold, and the nature of behavioral responses. As a result, the solutions employed by small mammals for adapting to seasonal cold may be different than those used by large mammals. In addition to making seasonal adjustments, mammals that reside in cold-dominated regions such as the arctic may show climatic adaptations which distinguish them from mammals at lower latitudes.

Seasonal changes in metabolic capacity and norepinephrine thermogenesis in the alaskan red-backed vole: Environmental cues and annual differences

Abstract 1. 1. Wild red-backed voles (Clethrionomys rutilus) were tested for maximum metabolic rate (Mmax) and for metabolic response to norepinephrine (MNE) in September, November and January. During the same period, voles born and raised in the laboratory were acclimated (for 3.5 months) in the following groups: (1) +20 C and 24 hr light daily (LD 24:0): (2) +20 C and LD 4:20: (3) gradual change from + 5 C and LD 14:10 to −5 C and LD 4:20; (4) gradual change from +5 C to −5 C and continuous LD 4:20, and tested for Mmax. 2. 2. During acclimatization of wild voles from September to January Mmax increased 42% to 23.05 ± 1.19ml O2·g−1 · hr−1 and MNE increased 59% to 14.12 ± 0.86 ml O2·g−1·hr−1. Peak winter Mmax and MNE in these wild voles were lower than found in a previous winter. 3. 3. In voles acclimated to +20 C and either long or short daily light period Mmax remained unchanged. In voles acclimated to increasing cold and decreasing or short light period Mmax increased 39% to 20.0 ml O2·g−1·hr−1. 4. 4. The results indicate that the magnitude of seasonal change of Mmax and MNE may vary from year to year and suggest that cold is essential to stimulate these seasonal changes in red-backed voles.

The Effect of Cold on Norepinephrine Turnover in Tissues of Seasonally Acclimatized Redpolls (Carduelis flammea)

Summary1.The response of the avian sympathetic nervous system during cold exposure was examined by injecting3H-norepinephrine and estimating norepinephrine (NE) turnover rate in skeletal muscle, heart, liver, and brain of seasonally acclimatized redpolls (a granivorous passerine) in interior Alaska.2.Thermoneutral NE turnover rate (ng NE/g·h) was greater in winter (January) than in summer (July) for muscle (12 ng/g·h, +115%) and heart (65 ng/g·h, +62%). This may be correlated to higher resting metabolism of winter redpolls.3.Acute cold exposure (24 h) necessary to increase metabolism 2 x caused increased NE turnover, compared to thermoneutral NE turnover, in muscle (10 ng/g·h, +95% in summer; 18 ng/g·h, +57% in winter) and heart (88 ng/g·h, +117% in summer; 108 ng/g·h, +65% in winter).4.Endogenous NE concentration declined during cold exposure of summer redpolls by a mean of 22% in muscle, liver, and brain, but not in winter birds.5.Thus, sympathetic nerve activity was stimulated by cold exposure in both seasonal groups, but winter acclimatization may involve an increased capacity to maintain steady state NE levels.6.A peripheral NE injection (2 mg NE/kg body weight) in winter redpolls depressed metabolism by a mean of 31% from resting metabolism of 7.2 ml O2/g·h. This may be due to the effect of NE on thermoregulatory centers in the brain.

The Effects of Castration and Testosterone on Thermogenesis and Pelage Condition in White-Footed Mice (Peromyscus leucopus) at Different Photoperiods and Temperatures

White-footed mice (Peromyscus leucopus) were exposed to long day (16L:8D) and warm (23 C) or cold (5 C) or short day (9L: 15D) and warm or cold. Each of these groups was subdivided into (1) controls: sham castrated + sham implanted; (2) castrated: castrated + sham implanted; and (3) high testosterone: castrated + implanted with testosterone. Mice were tested for nonshivering thermogenesis (NST) at 4, 8, and 12 wk and examined for reproductive and pelt condition after 12 wk. Cold increased NST in all groups by 4 wk. In controls, short day decreased weight of testes and serum testosterone, caused molt to winter pelt, but did not increase NST. Castration resulted in no detectable serum testosterone, facilitated molt to winter pelt, but reduced coldenhanced NST. High serum testosterone from implants prevented short day-induced molt to winter pelt and reduced NST in long day warm mice. The results suggest (1) a linkage between gonadal regression and molt to winter pelage in the fall but (2) no connection between gonadal regression and enhancement of NST, which is stimulated by cold via a separate pathway, and (3) some chemical/neurochemical factor altered by castration, other than low testosterone, may be necessary for maximal cold enhancement of NST.

Hematology of alaskan snowshoe hares (Lepus americanus macfarlani) during years of population decline

Abstract 1. 1. The hematologic profiles of snowshoe hares near Fairbanks. Alaska were determined during 2 years of decline from peak population density. 2. 2. Erythrocytic indices and total white blood cells underwent little change during the population decline. 3. 3. During the high population period a relative neutrophilia, lymphopenia and eosinopenia occurred indicating a possible adrenal corticosteroid stress induced reaction.

Norepinephrine turnover in brown fat, skeletal muscle and spleen of cold exposed and cold acclimated Alaskan red-backed voles

Abstract 1. 1.After subcutaneous infection of l -[ 3 H]nonrepinephrine ([ 3 H]NE) into Alaskan red-backed voles ( Clethrionomys rutilus dawsoni ), the NE turnover rate (μg NE/g.h), an index of the sympathetic nerve activity, was estimated in interscapular brown fat, skeletal muscle and spleen. 2. 2.In 20°C acclimated voles at 20°C, NE turnover in brown fat and spleen was similar (0.125 μg NE/g.h) and 6 times higher than that in muscle. In 20°C acclimated voles at 0°C, NE turnover was increased 1.4× in brown fat and 1.8× in muscle. 3. 3.After acclimation to 5°C for 2 months, NE turnover was increased in muscle and spleen but not in brown fat (compared to 20°C acclimated voles at 20°C). In 5μC acclimated voles at 0°C, NE turnover in brown fat was increased to the same level as in 20°C acclimated voles at 0°C. 4. 4.Endogenous NE (μg NE/g) remained unchanged in tissues of all groups exposed to cold. 5. 5.The results indicate that red-backed voles show increased sympathetic activity in certain tissues during cold exposure which resembles that of rats and hamsters. However, the capacity of warm acclimated red-backed voles to maintain a steady state level of NE in these tissues during acute cold exposure suggests an adaptation to colder latitudes which may not be shared by temperate zone small mammals.