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Changes with age in sexual fertility in male rats of the Wistar-Imamichi strain were investigated. Litter size and numbers of implantation sites in females mated with males aged 14 to 44 weeks tended to decrease with increased aging of males. These indicators in females mated with males aged over than 44 weeks decreased, compared with that aged 14 to 44 weeks (P &lt; 0.05 to 0.001). In addition, all females mated with males aged 95 weeks were not pregnant. In male copulatory behavior with aging, incidences and numbers of mounting at 67 and 104 weeks of age were 100%, 42.6 +/- 15.3 (Mean +/- S.D.) and 42%, 1.8 +/- 2.8, respectively. The intromission and ejaculation were observed at 67 weeks, but not at 104 weeks of age. These incidences and frequencies were 86%, 10.3 +/- 10.2 for intromission, and 43%, 1.0 +/- 1.3 for ejaculation. Mount latencies (ML) at 67 and 104 weeks of age were 535.4 +/- 607.9 and 3,822.0 +/- 1,753.4 sec, respectively. There were significant differences in ML between two groups (P &lt; 0.05). At 67 weeks of age, intromission and ejaculation latencies, and post-ejaculatory intervals were 2,563.0 +/- 2,216.3, 1,633.7 +/- 977.6, and 657.3 +/- 320.6 sec, respectively. The pregnant rate and numbers of implantation sites in females mated with males aged 67 weeks were 43% and 5.4 +/- 6.8, respectively. In females mated with males aged 104 weeks, all were not pregnant. The weights of tests, epididymides, ventral prostate and seminal vesicles at 104 weeks were light, compared with those at 67 weeks of age (P &lt; 0.05). From these results, the copulatory behavior in male rats of the Wistar-Imamichi strain declined with aging. Sexual fertility in males aged 67 weeks decreased in less than 50%, and males aged 104 weeks showed no intromission and ejaculation.

The Change with Age of the Copulatory Behavior of the Male Rats Aged 67 and 104 Weeks