Variation in Ejaculate Quality in Dark-eyed Juncos According to Season, Stage of Reproduction, and Testosterone Treatment

--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 freeranging population, C-males produced larger ejaculates than T-males. Independent of treatment, ejaculate size varied significantly with season and stage of reproduction in the freeranging 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 Tand C-males ejaculates were smallest when their mates were fertile also suggests that frequent copulation depletes sperm reserves. Received 3 April 1997, accepted 19 December 1997. THE MALES OF MANY SOCIALLY MONOGAMOUS BIRDS adopt a mixed reproductive strategy and engage in both parental care and extrapair copulations (EPCs; Westneat et al. 1990, Birkhead and Moller 1992). Because a consequence of EPCs is a mixing of male ejaculates in female reproductive tracts, sperm competition among males may be an important selective force in these apparently monogamous mating systems (Birkhead 1987, Birkhead and Moller 1992). Males may benefit from guarding their mates; however, when guarding is ineffective, selection should favor males that dilute a competitor's sperm through frequent copulations and inseminations by larger numbers of sperm (Birkhead et al. 1987, Moller 1988). In passerines, sperm available for copulation is stored in the seminal glomera, or sperm reserves, of the cloacal protuberance (Salt 1954, • Present address: Cornell Laboratory of Ornithology, 159 Sapsucker Woods Road, Ithaca, New York 14850, USA. E-mail: [email protected] Wolfson 1954, Birkhead 1991, Birkhead et al. 1994). The number of sperm in the glomera at any one time is a function of three variables: the glomera's total capacity, the rate of sperm production, and the rate of sperm utilization, or copulation (Birkhead 1991, Birkhead et al. 1994, Birkhead et al. 1995). When full to capacity, the glomera are thought to contain multiple ejaculates. The number of sperm per ejaculate (ejaculate size) decreases with frequency of successive copulations until eventually the sperm are depleted (Ansah et al. 1984, Birkhead 1991, Birkhead et al. 1994). Although the rate at which sperm are replenished probably varies among bird species, maximum ejaculate size in Zebra Finches (Taeniopygia guttata) is not fully restored for up to 24 h after the glomera have been depleted (Birkhead 1991). Variation in any of the variables affecting ejaculate size could greatly influence reproductive success among males. Testosterone (T) is essential to male reproductive behavior and physiology. It stimulates