MELATONIN INFLUENCES SEX-SPECIFIC PRENATAL MORTALITY IN MEADOW VOLES

Meadow voles exhibit seasonal changes in litter size, ovulation rates, and prenatal mortality. To investigate the proximate bases of seasona l changes in reproductive effort, adult female voles, maintained in lo ng photoperiods (14 h of light/day), were injected daily with 10 mu g melatonin 2 h before light offset to extend the duration of the nightt ime melatonin pulse. At parturition the number, sex, and weight of off spring were assessed. The number of ovarian corpora Lutea (CL), an ind ex of potential Litter size, was used to calculate rates of prenatal s urvival (i.e., pups per CL). Prenatal survival rates were reduced in f emale but not male pups of dams that had been injected before blastocy st implantation (Days 1-6 of pregnancy) with melatonin as compared wit h saline. Melatonin injections initiated after blastocyst implantation (Days 7-21 of pregnancy) did not affect prenatal survival, nor were b irth weights of pups affected by either pre- or postimplantation melat onin treatment. We conclude that sex-specific prenatal survival is a l abile feature of vole reproduction that may be under proximate control of photoperiod and melatonin before blastocyst implantation.