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 seasonal changes in reproductive effort, adult female voles, maintained in long photoperiods (14 h of light/day), were injected daily with 10 micrograms melatonin 2 h before light offset to extend the duration of the nighttime melatonin pulse. At parturition the number, sex, and weight of offspring were assessed. The number of ovarian corpora lutea (CL), an index of potential litter size, was used to calculate rates of prenatal survival (i.e., pups per CL). Prenatal survival rates were reduced in female but not male pups of dams that had been injected before blastocyst implantation (Days 1-6 of pregnancy) with melatonin as compared with saline. Melatonin injections initiated after blastocyst implantation (Days 7-21 of pregnancy) did not affect prenatal survival, nor were birth weights of pups affected by either pre- or postimplantation melatonin treatment. We conclude that sex-specific prenatal survival is a labile feature of vole reproduction that may be under proximate control of photoperiod and melatonin before blastocyst implantation.