Seasonal Changes in the Adrenal Glands of Woodchucks (Marmota monax)

Abstract

Adrenal weights of 872 woodchucks (Marmota monax), collected over a 4- yr period from a 10,000-acre area in Pennsylvania, were correlated with several physiologic and behavioral factors. There was a significant positive linear relationship between adrenal weight and body length. Adrenals of females were about 8 % heavier than those of males of equal length. Data for adrenal weights were analyzed for seasonal variations by pooling the animals collected during each weekly period. Mean adrenal weight increased 60 % from the 1st of March, just after hibernation, to the 1st of July, then declined sharply. A 2nd, less marked (48%), sharp increase in weight occurred at the end of August. Mean adrenal weight fell from the 2nd peak to a level in October that was 15 % below the March level. The initial rise in the spring corresponded to a period during which an increasing number of woodchucks were active for longer periods of time each day, the concentration of animals at a given time was increasing, and the level of aggressiveness was high. The beginning of the subsequent decline coincided with the end of the breeding season, when the level of aggressiveness became very low. The 2nd sharp peak in August seems to have been associated with extensive movement of young woodchucks, with a possible increase in conflicts. The summer decline in mean adrenal weight occurred while maximal numbers of animals were active, but when aggressiveness had dropped to very low levels; thus it seems that the lack of aggressiveness was the most important consideration initiating the decline in adrenal weight. The physiologic processes of reproduction had no detectable direct influence on the weights of adrenals, as pregnant females, nulliparous females, sexually active males and immature males all had equivalent adrenal weights if the influence of body size, sex, age and season of the year was adequately considered. The increase in adrenal weight was due to hypertrophy and hyperplasia of all cortical zones, the inner portion of the cortex responding most. The decline in adrenal weight during the 2nd half of the season was brought about by a reversal of the process of increase.