John Krog

Insulation of furs in water.

Abstract 1. 1. The mode of heat transfer through furs in water was examined for the polar bear and the harp seal. 2. 2. The woolly hairs of the underfur act to keep an inner water layer stagnant. 3. 3. The harp seal retain a 2 mm thick and the polar bear at 10 mm thick layer of stagnant water in the underfur. 4. 4. The insulative value of the fur in water is a significant part of the total unregulated insulation of the polar bear and of minor significance for the harp seal.

Heart rate in resting seals on land and in water

Abstract 1. 1. Heart rate of unrestrained resting seals was recorded under three different conditions: (a) on land, (b) in water, and (c) with only the head above the water. 2. 2. A comparison of the heart rates obtained showed that there was no significant difference in the bradycardia during diving and apneic periods on land. 3. 3. In both cases the heart rate decreased to 30% of the rate during breathing. Nor was a difference found in the time course of the bradycardia in water and on land. 4. 4. The observed bradycardia seemed to be correlated to a Valsalva-like maneuver, which is known to cause a change in intrathoracic pressure. 5. 5. Changes in the intrathoracic pressure, caused by muscular contraction, might occur during the dives. 6. 6. Such changes could then explain the variation in heart rate seen during some of the dives and apneic periods. 7. 7. A correspondence between ventilation and variation in heart rate was found, but only in the young seals while they had an unusually low ventilation rate.

Seasonal changes in levels of growth hormone, somatomedin and thyroxine in free-ranging, semi-domesticated norwegian reindeer [Rangifertarandus tarandus (L.)]

Abstract 1. 1. Semi-domesticated reindeer of both sexes from two adjacent herds (A and B) in the Lodingen district of Norway were studied. From Herd A jugular blood samples were collected in summer (1974), and from Herd B both in summer and in winter (1975). 2. 2. Serum levels of growth hormone, somatomedin and thyroxine were measured, and, for Herd B, conspicuous seasonal changes in the three growth promoting hormones were found, both for the calf and the adult groups. 3. 3. Levels of growth hormone were reduced in winter as compared to summer: 21.6 ± 1.6 ng/ml vs 31.9 ± 2.2 ng/ml (calves), and 20.3 ± 0.9 ng/ml vs 25.5 ± 1.5ng/ml (adults). The same was found for thyroxine: 144 ± 9 mmol/1 vs 249 ± 10 mmol/1 (calves), and 157 ± 5 nmol/1 vs 223 ±7 nmol/1 (adults). Levels of somatomedin, on other hand, were higher in winter than in summer: 1.11 ± 0.10 U/ml vs 0.67 ± 0.07 U/ml (calves), and 1.02 ± 0.09 U/ml vs 0.53 ± 0.04 U/ml (adults). 4. 4. The observed seasonal changes may reflect one aspect of the physiological adjustments involved in the adaptation of the reindeer to arctic living conditions.

Poikilothermy and cold tolerance in young house martins (Delichon urbica)

Summary1.The poikilothermic state of house martin nestlings has been studied by measuring body temperature (Tb), oxygen uptake (VO2), heart rate (HR), breathing rate (BR), and shivering at different ambient temperatures (Ta).2.TheTb of nestlings less than 4 days old followedTa within 0.5°C, and the nestlings survived at least 6 h atTb about 10°C.3.There was no difference in either cooling or warming rate between living and dead 3-day-old nestlings.4.The Q10 of VO2 and HR was 2.1–2.5 atTb above 35°C but increased progressively as theTb was lowered.5.When the nestlings were cooled, breathing stopped atTb=10°C; upon rewarming it was resumed at 15°–25°C.6.We conclude that the cold tolerance of nestlings less than 1 week old is an important part of their survival strategy. Metabolically, they behave like true poikilotherms with the exception that their metabolic function decreases more sharply at low temperatures and that they stop breathing and die at prolonged exposure atTb below 10°C.