The Seasonal Decline in the Litter Size of Meadow Voles

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Abstract
Variation in litter size was examined in meadow voles (Microtus pennsylvanicus) during a 14-month period in 1979–1980 near Ann Arbor, Michigan. Mean litter size declined sharply each autumn, at the end of the breeding season. This pattern of autumnal decline in litter size is widespread in the genus Microtus. We examined four hypotheses that might might explain the seasonal decline in litter size: the decline reflects possibly adaptive changes in the birth mass of offspring; the decline reflects differential investment in offspring of one sex; the decline reflects a decrease in the mass of mothers; and the decline reflects a change in the average age or parity of mothers. To evaluate these hypotheses, we examined changes in litter size, masses of young and mothers, the sex ratio of litters, and associations among these reproductive variables by bringing live-caught pregnant females into the laboratory and observing their litters. Mean neonatal mass, total mass of litters, and mother's mass varied significantly during the breeding season, but preliminary estimates of age and parity of mothers did not. The number of young in a litter was correlated strongly and positively with the postpartum mass of the mother and correlated weakly and negatively with the mass of neonates. No single hypothesis adequately accounted for seasonal changes in litter size. At best, the first and third hypotheses together could account for about half of the seasonal variation in litter size.