Abstract
A commercial strain of rats brought into this laboratory was found to have very low fertility. Individual mating trials indicated lack of male libido as the primary cause of infertility. Testosterone injections at various levels were effective in initiating breeding activity in 11 of 27 males, whereas 15 uninjected controls failed to breed during the same period. Limited data on six induced-breeders indicated a tendency for ejaculations to cease after withdrawal of exogenous testosterone. Induced-breeding males were found to breed as high a proportion of females as spontaneously breeding males. They were also found to be as fertile when compared on the bases of proportion of females pregnant after being bred, proportion of litters with living young and litter size. Autopsy information and the response of non-breeding males to androgen and to the withdrawal of exogenous androgen support the interpretation that production of male sex hormone below the threshold quantity necessary to insure libido may be an important cause of male infertility in the rat. The evolutionary consequence of non-breeding males occurring in natural populations is discussed briefly.
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