Reproductive System of the Obese Male Zucker Rat. Reproductive Capacity, Artificial Insemination and Plasma Testosterone Levels1

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Abstract
The sexual behavior of obese and lean male Zucker rats placed with receptive lean female rats was evaluated longitudinally. At 3-4 mo. of age and at 5-8 mo. of age, obese males intromitted less consistently than lean males and fewer obese males ejaculated during the observation periods. By 9-12 mo. of age, none of the obese males were sexually active during the observation period. More obese males ejaculated if left overnight with receptive females. At all ages studied, the pregnancy rate for females left overnight with obese males was lower than that for females left overnight with lean males. Sperm from obese and lean male rats was used to artificially inseminate proestrous lean female rats. The pregnancy rate was similar for females artificially inseminated with sperm from obese vs. lean males. Even sperm from obese males which previously failed to naturally inseminate lean females was successfully used to artificially inseminate lean females. The number of implanted embyros was similar for females inseminated, naturally or artificially, with sperm from obese vs. lean males. These findings support the suggestion that sperm from obese males is normal in fertilizing capability, and suggest that inadequate sexual behavior is the primary factor limiting their reproductive capacity. Plasma testosterone levels in obese and lean rats were measured by radioimmunoassay. Plasma levels were lower in obese males 8-12 mo. of age compared to lean males of comparable age, and compared to obese males 3-6 mo. of age. Reduced plasma testosterone levels cannot account for the reproductive inadequacy of obese male Zucker rats since sexual behavior was inadequate and the natural insemination rate was reduced at 3-4 mo. of age despite normal plasma testosterone levels. Testosterone levels in obese males are adequate to maintain normal spermatogenesis. The reproductive defect in obese male Zucker rats appears to be behavioral rather than functional.

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