Influence of Month of Birth on the Serum Hormone Concentrations and Weights of the Accessory Sex Organs and Testes during Maturation of the Male Laboratory Rat1

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Abstract
The serum hormone concentrations of testosterone, LH and prolactin were studied in cohorts of male rats born one group/month over 20 consecutive months to determine the influence of month of birth on serum hormone concentrations observed at given ages during maturation. There was a significant effect of both age and month of birth on the concentrations of the three hormones studied. Serum levels of testosterone increased with age and reached maximal levels at 50-70 days of age in rats born in the spring and at 80-90 days of age in rats born in the fall. Serum LH levels were generally elevated at 30-40 days of age and then declined to adult levels by 60 days of age. However, secondary rises in serum LH levels were noted in 70-90-day-old rats born in the spring. Serum prolactin levels increased with age from 30 to 80 days of age. There was a further elevation of serum levels of prolactin in mature animals (120 or 150 day old) in all months of birth except in those born during the spring and early summer.