Changes in Plasma Testosterone Levels During the Annual Reproductive Cycle of the Hibernating Bat, Myotis lucifugus lucifugus with a Survey of Plasma Testosterone Levels in Adult Male Vertebrates

Plasma testosterone concentrations were measured by radioimmunoassay in a wild population of the male hibernating bat, Myotis lucifugus lucifugus, (Chiroptera;Vespertilionidae) throughout the entire annual reproductive cycle. Baseline levels (2.5 ± 0.3-11.7 ± 3.2 ng/ml) occur during the early phases of spermatogenesis (May to mid-July), during the breeding period (September to May) and throughout hibernation (late September to April). These levels are similar to those reported for most adult breeding mammals. During the latter part of spermatogenesis (mid-July to late August) and just prior to maximum hypertrophy of the sex accessory glands, plasma testosterone rises to very high levels with a peak (59.1 ± 9.2 ng/ml) occurring in mid-August. This peak is one of the highest known values for adult mammals. The data suggest that a testosterone surge is required to stimulate the accessory glands fully, but lower levels of androgen are able to support them (as well as epididymal sperm and libido) during the breeding and hibernation seasons. However, these “lower” levels apparently are not sufficient to maintain the accessories when the bats emerge from hibernation in the spring, because these glands involute soon thereafter.