An Explanation of Geographic Variation in Litter Size

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Abstract
Our explanation of the latitudinal and altitudinal variation in litter sizes of small mammals invokes the effect of length of season and parental mortality related to reproduction. It may be assumed that a portion of the maternal mortality rate varies directly as the size of litter produced. Short seasons limit the maximum number of times a female can reproduce in her lifetime and give an advantage to phenotypes producing large litters. Long seasons favor producers of small litters. The contribution to the total rate of increase of the litters produced in the additional time afforded by long seasons is greater for producers of small litters because a larger proportion of parents of small litters survive to produce throughout the periods. The increment provided is sufficient to overcome the initial advantage of parents producing large numbers of young in their first litters.

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