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The contribution of observations on early pregnancy in the little sac-winged bat, Peropteryx kappleri, to an understanding of the evolution of reproductive mechanisms in monovular bats.

The reproductive biology of Peropteryx kappleri was investigated histologically in females collected during the onset of a synchronized breeding season. Many had recently given birth, although the interval between parturition and a new ovulation appeared to be variable. Successive ovulations in this monovular species tended to alternate between the ovaries, but this was restrained by partial dominance of the left ovary. Evidence of reproductive failures was noted in several post-partum females in which ovulation had not alternated. Although development usually proceeded to the blastocyst stage in the oviduct, shedding of the zona pellucida was deferred until the embryo passed into the uterus. Implantation was of the central type and always occurred at the cranial end of the uterine horn ipsilateral to the new corpus luteum. Immediately prior to and after ovulation the oviduct on the active side of the reproductive tract appeared preferentially stimulated. Its epithelial cells were usually larger and often contained more cytoplasmic glycogen than those of the other oviduct. During the tubal journey of the ovum, endometrial development was most pronounced at the prospective implantation site and was minimal in the opposite uterine horn. Many of the oviductal epithelial cell nuclei in all of the adult bats examined appeared extensively vacuolated. Histochemical tests established that this was attributable to the intranuclear presence of glycogen. The ability of P. kappleri (and of some other bats with bicornuate or duplex uteri) to alternate ovulations and to control the uterus via local pathways probably serves to decrease the interval required to recycle the female tract after either a reproductive failure or a previous pregnancy. This could be advantageous to species with low reproductive potentials and restricted breeding seasons. Retention of the ovum in the oviduct until the blastocyst stage probably favors subsequent successful development of the embryo in the uterus, only a small portion of which is maximally stimulated. The bats with simplex uteri appear to have evolved another means of efficiently recycling the uterus in the event of a nonfertile cycle. They eliminate much of the aging endometrium at the prospective implantation site by menstruating and regenerate a new one.