Testosterone and Avian Life Histories: Effects of Experimentally Elevated Testosterone on Prebasic Molt and Survival in Male Dark-Eyed Juncos

Abstract

Male Dark-eyed Juncos (Junco hyemalis) that breed in Virginia begin their prebasic molt after breeding has ended, usually in August. Almost all males caught in late October have completed the molt. In 1989, we obtained anecdotal evidence that males whose testosterone (T) we maintained at artificially elevated levels beyond the end of the breeding season postponed or suppressed prebasic molt. To test the effect of T experimentally, in spring 1990 we implanted some males (T-males) with testosterone and others (Cmales) with empty implants, and we released both groups to breed. We caught some members of both treatment groups in October and removed their implants. The T-males had delayed their prebasic molt, while the C-males were molting on schedule. Other implanted T-and C-males were not caught; these carried their implants into winter. Next spring we examined surviving males whose implants we had removed in October as well as males whose T-and C-implants had not been removed. T-males whose implants we had removed had molted completely, despite their delayed start, whereas T-males whose implants we had not removed had not molted. Still-implanted C-males had molted. We compared the minimum overwinter survival (i.e., return rates in spring) of the treatment groups. T-and C-males whose implants we had removed in October returned at the same rate, but among males whose implants we had not removed, significantly fewer T-males than C-males returned. The transition between reproduction and molt of male juncos apparently can be blocked by preventing the normal seasonal decline in T. This suggests a physiological basis for a possible trade-off between time allocated to reproduction and time allocated to molt. Our results indicate that males could maintain high T and prolong breeding, possibly into October, and still molt completely with no adverse effects. We consider why such a modification of schedule has not occurred. However, postponement of molt beyond some date in autumn, possibly late October, suppresses it altogether, as indicated by the failure to molt of the returning T-males whose implants we did not remove. This treatment group apparently suffered higher overwinter mortality, and we consider possible reasons.

Adaptation, Exaptation, and Constraint: A Hormonal Perspective

“…Removal as late as October also did not influence survival (50% T vs. 50% C; Nolan et al 1992). However, when implants were not removed, T-males were significantly less likely than C-males to return (20% T vs. 43% C; Nolan et al 1992). We conclude that suppression of molt or some other consequence of elevated T in autumn resulted in higher mortality of T-males.…”

“…Thus, the consequence of experimentally elevating testosterone has been to shift male reproductive effort away from care of offspring toward other forms of reproductive effort, such as singing and sexual and aggressive behavior (e.g., Beletsky et al 1995; see citations in table 1). In some cases, elevation of testosterone has also compromised male sur- Nolan et al 1992 vival or produced phenotypic changes that would seem likely to have that effect (e.g., ; see citations in table 1).…”

“…Survival from year to year (measured as return rate in the year following treatment) of T-and Cmales depended on whether and when the implant was removed in the year of treatment. When implants were removed in late summer, the usual procedure, T-males were at least as likely as C-males to return and breed the next year (58% T vs. 48% C; Nolan et al 1992;Ketterson et al 1996). Removal as late as October also did not influence survival (50% T vs. 50% C; Nolan et al 1992).…”

“…When implants were removed in late summer, the usual procedure, T-males were at least as likely as C-males to return and breed the next year (58% T vs. 48% C; Nolan et al 1992;Ketterson et al 1996). Removal as late as October also did not influence survival (50% T vs. 50% C; Nolan et al 1992). However, when implants were not removed, T-males were significantly less likely than C-males to return (20% T vs. 43% C; Nolan et al 1992).…”

Rate of moult affects feather quality: a mechanism linking current reproductive effort to future survival

“…Nilsson & Svensson (1996) suggested that the reduction in over-winter survival and subsequent breeding success of late-moulting blue tits were caused by increased thermoregulatory costs due to poorer feather quality. However, Nolan et al (1992) found no di¡erence in over-winter survival of dark-eyed juncos, Junco hyemalis, in which moult was delayed by testosterone implants, although, not surprisingly, survival was markedly reduced in birds that were prevented from moulting. The subsequent breeding performance of the late-moulting juncos was not reported.…”