Regulation of Reproduction and Swimming Ability in the Male Marsh Rice Rat (Oryzomys palustris)

Photoperiod and melatonin regulate reproduction in mammals. The rice rat (<i>Oryzomys palustris</i>) is reproductively photoperiodic and on a long photoperiod responds to melatonin administration with inhibited reproduction. I examined whether photoperiod, melatonin, and the removal of gonadal steroids via castration affected growth, reproduction, pelage, and swimming ability. Juvenile males were housed on 14L:10D or 11L:13D and at 68 days of age rice rats were weighed, allowed to swim for 10 minutes while being videotaped to quantify swimming and floating times, and then reweighed. One day later, these rice rats were euthanized and the testes, seminal vesicles (SV), Harderian glands (HG), and spleen were weighed. The shorter photoperiod significantly decreased body, testes, SV, and HG masses only. The pelage of rice rats absorbed more water on 14L:10D than on 11L:13D, despite no difference in pelage lengths. Melatonin implants decreased body mass, the masses of the testes, SV, and HG, and pelage water absorption, but increased pelage length. Lastly, castration decreased SV and HG masses, but increased body and spleen masses. Swimming and floating times were not affected by any treatment. Taken together, these results show that all treatments affected growth and reproduction in rice rats, but did not affect swimming ability. Thus, the physiological changes characteristic of winter are not sufficient to alter aspects of swimming ability as swimming ability appears to be independent of reproductive condition.