The Effect of Litter Size and Litter Size Manipulations on Energetic Costs of Lactation in Grasshopper Mice.

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Reproduction is energetically costly and, in mammals, the cost is born entirely by mothers through gestation and lactation. Maternal efforts increases with litter size, but at slow rate such that individual offspring in large litters receive a smaller percentage of available resources than offspring are tied to the level of maternal care, so there is a tradeoff between litter size and offspring quality. Unfortunately, assessing this tradeoff is not simple because mothers may differ in their efficiency of producing offspring. This study will manipulate the number of embryos and pups in northern grasshopper mice to examine the relationship between litter size and the energetic cost of lactation. Energetic data will be used to test for individual optimization of litter size and to test the hypothesis that the modal litter size produces the largest offspring per unit effort. Reproductive efficiency and costs will be determined in litters where the mother is rearing natural-sized litters, and in artificially larger and smaller litters. The results of this project will provide foundation data on the energetic cost of lactation and the evolution of litter size and litter size variability