A Seasonal Influence on Testes Weight and Serum Gonadotropin Levels of the Mature Male Laboratory Rat

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Abstract
A 12 mo. study of mature male rats, maintained under constant laboratory conditions, showed significant seasonality in testes weights of 5 but not 4 mo. old rats, with maxima occurring in Feb. and June. Serum testosterone levels were not significantly different in either 4 or 5 mo. old rats. Serum levels of LH [luteinizing hormone] FSH [follicle stimulating hormone] and prolactin of 5 mo. old animals also showed significant differences, but did not appear to be correlated with testes weight. A 2nd study of 6 different age groups, conducted during periods of low (late summer) and high (late winter) testes weight, showed that testes weight was significantly higher in the late winter in rats 6 mo. of age. Serum LH levels were significantly elevated in 4 to 6 mo. old rats in the late summer. Serum FSH levels were significantly elevated in 3 and 4 mo. old rats in late winter, while serum prolactin levels were not different between the 2 seasons. Ventral prostate weight increased significantly with age and plateaued at 5-6 mo. of age. Body weight increased steadily with age from 3-12 mo. of age. The seasonal elevations in testes weight of mature rats did not appear to be hormonally mediated, with the possible exception of FSH. Five month old rats appeared to be the most responsive to seasonal influence. Male laboratory rats exhibited seasonality in several reproductive parameters even though the animals were maintained in constant laboratory conditions. Similarities of seasonal fluctuations of laboratory rats to feral rats were noted.