The reproductive cycle of female Molossus fortis, a molossid bat endemic to Puerto Rico (18°N), was studied from specimens collected throughout the year and immediately preserved in the field. The reproductive tracts were subsequently dissected out in the laboratory and processed for light microscopic observation of their histological status.
Observations revealed that the species is monotocous and bimodally reproductively active, conceiving in March and June/July and giving birth in June and September. Anoestrus occurs between September and February. The data strongly suggest that Molossus is polyoestrous, conceiving a second time each year as the result of a post-partum oestrus in June/July. Gestation occupies about three and a half months in each instance and the young are born at approximately 30 mm crown-rump length. Lactation ensues for about six weeks.
Molossus fortis displays dextral dominance of the reproductive tract and lack of advanced follicular growth stages in the left ovary, features that are characteristic of Molossidae.
This is the first description of the reproductive cycle of a wild population of a tropical molossid from the New World. Polyoestry has been described in some Old World tropical Molossidae; a post-partum oestrus per se has not been described in any molossid, but no doubt will be recognized in other species once more detailed studies are made.