Reproductive life history correlates of early and late sexual maturation in female Mongolian gerbils (Meriones unguiculatus)

Age at vaginal introitus is bimodally distributed in female domesticated Mongolian gerbils; some exhibit vaginal perforation before eye-opening (day 16), others after weaning (day 25). We found early- and late-maturing female gerbils to differ significantly in reproductive life history. Early-maturing females first reproduced when younger, had more litters, with more young per litter, and consequently had more than twice as many offspring as late-maturing females. In comparison with late-maturing females, early-maturing females gave birth to and weaned a greater proportion of females per litter and a higher proportion of early-maturing daughters per litter. Further, early-maturing females exhibited reduced maternal behaviour; they spent less time nursing their young and retrieved fewer offspring displaced from the nest than late-maturing females. Even under constant laboratory conditions, there were significant, correlated, circannual rhythms in female fecundity, the proportion of males per litter and the proportion of early-maturing daughters per litter. Review of the field literature suggests that wild Mongolian gerbils may exhibit similar variability in reproductive pattern.