Variation in ejaculate quality in dark-eyed juncos according to season, stage of reproduction, and testosterone treatment

Abstract

To assess natural variation in ejaculate quality of male Dark-eyed Juncos (Junco hyemalis), as well as to measure any effect of experimental treatment with testosterone (T), we used cloacal massage to collect sperm samples from captive (1993) and free-ranging (1994, 1995) populations. We made three measurements of ejaculate quality in males, approximately half of which had been treated with T: (1) volume, (2) sperm concentration, and (3) total number of sperm per ejaculate (ejaculate size). Ejaculate volume and concentration varied by year, but ejaculate size did not; therefore, we used ejaculate size as our primary measure of ejaculate quality. Control (C-males) and hormone-treated (T-males) males from the captive population did not differ in any measure of ejaculate quality, but in the free-ranging population, C-males produced larger ejaculates than T-males. Independent of treatment, ejaculate size varied significantly with season and stage of reproduction in the free-ranging population. C-males had significantly fewer sperm at the beginning of the breeding season than in midseason and thereafter, but reserves in T-males did not differ significantly with season. For males in both treatments, ejaculates were smallest when their mates were fertile and increased significantly when their mates were incubating and when the pair was feeding nestlings. We then asked whether the observed patterns were more likely attributable to differences in rate of sperm production or rate of sperm utilization (i.e. copulation frequency). The finding that free-ranging T-males had fewer sperm than C-males, whereas captives did not, suggests that treatment with T may have led to differences in utilization (i.e. by copulation). The observation that' in both T- and C-males ejaculates were smallest when their mates were fertile also suggests that frequent copulation depletes sperm reserves.