Fertility ratio in male rats: Effects after denervation of two pelvic floor muscles

Fertility ratio is defined here as the proportion of females that a male ca n impregnate after a constant period of in-polygyny living. This ratio was investigated in male rats after denervation of two pelvic floor muscles, th e pubococcygeus and iliococcygeus. Denervation was carried out by transecti ng the somatomotor branch of the pelvic nerve. The lesion did not modify th e sexual behavior of males or their overall fertility, but decreased the we ight of the ejaculated seminal plug. Consequently, the number of days livin g in cohabitation to induce pregnancy was increased in lesioned males (simi lar to 13 days) compared with intact and sham animals (similar to 5 days). These results showed that the fertility ratio was optimal when intact/sham males cohabited with females for two consecutive estrous cycles, but that l esioned males needed up to four cycles to induce most pregnancies. Two hypo theses are raised by our results. The first is that pelvic floor denervatio n decreases the forceful tension required to expel the semen from the prost atic urethra to the vagina, then an incomplete seminal plug is expelled. Th e second is that denervation cut afferent fibers that reflexively promote t he continence of the semen deposited in the prostatic urethra during semina l emission, allowing some to leak out before ejaculation. The latter hypoth esis can also explain the recovery of the fertility ratio in lesioned males . It could be a compensatory mechanism mediated by the pudendal nerve suppl y to the coccygeus muscle, the other pelvic floor muscle. (C) 2000 Elsevier Science Inc. All rights reserved.