Different outcomes of sperm competition in right and left sides of the female reproductive tract revealed by thymidine‐³H‐labeled spermatozoa in the rat

Abstract

We attempted to record directly the results of sperm competition between two male rats in samples from female reproductive tracts. Gametes of 7 male rats were labeled in vivo by repeated injections of thymidine-6-3H. 33 females were mated successively with one of the injected males and with a noninjected male, or vice versa. Samples of the resulting sperm mixtures were recovered from both the uterine horns and the oviducts of the females. Pure samples of the injected males' sperm were obtained from uterine horns of females mated solely to them. Smears of samples were evaluated autoradiographically. Except for one male, the injections resulted in a broad peak of sperm labeling with maximum intensity on days 56-58 postinjection. This labeling enabled the assessing of contribution of either male to uterine and tubal sperm in the competitive matings. The injections did not influence competitive ability of male gametes in terms of sperm transport to horns or oviducts. The outcomes of sperm competition were significantly different in the right and left sides in most female subjects. Nevertheless, the outcomes were usually similar in a horn and a tube on the same side of the body. In vivo sperm labeling appears to be a useful method in the study of rodent sperm competition.