We tested the suitability of fecal sexual steroid hormone metabolites for use in endocrinological studies of free-ranging, small bats. We monitored estrogen and 20-oxo-pregnane metabolites in fecal samples collected daily from 19 free-ranging female Saccopteryx bilineata during the assumed mating season. Median fecal estrogen metabolite levels equaled 0.09 ng/g for the pre- and postovulatory periods and 4.2 ng/g for the follicular phase of the estrous cycle in female S. bilineata. Median fecal 20-oxo-pregnane metabolite levels were 50.9 ng/g in pre-estrous, 88.8 ng/g in estrous, and 190.2 ng/g in postestrous females. Reliability of fecal steroid metabolite analysis was supported by results of video observations. Two successful copulations for a single male–female pair were recorded; mating coincided with peak fecal estrogen levels (7.0 ng/g) in the copulating female. In contrast, 2 observed mating attempts in a 2nd male–female pair occurred approximately 19 days after the female's physiological estrus and fecal estrogen metabolites in the rejecting female were only 0.2 ng/g. Fecal 20-oxo-pregnane levels were almost 3 times lower in fecal samples collected from the copulating female than those from the rejecting female (157 ng/g versus 410 ng/g). All females that were still present in the colony 6 months later gave birth to a single offspring between late May and early June. Gestation length averaged (mean ± SE) 169 ± 2 days. During the 20 days preceding parturition, fecal 20-oxo-pregnane metabolites were approximately 100 times higher than during the mating period. Fecal 20-oxo-pregnane metabolites of lactating females were comparable to those during the mating season. Noninvasive fecal steroid analysis can be a valuable tool for monitoring reproductive events in colonies of free-ranging small-sized bats.
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