Reproduction in Stephens' Woodrat: The Wages of Folivory

Abstract

The reproductive pattern of Stephens' woodrat (Neotoma stephensi) reflects strong selection for energy conservation. We hypothesize that the major selective force is this woodrat's dietary specialization on juniper foliage, a food rich in defensive, digestibility-reducing chemicals. Compared to most other rodents, and to other species of woodrats, Stephens' woodrat has small litters (usually a single young); each litter comprises a small percentage of the mother's weight (8%), young grow slowly and are weaned late, and there is broad temporal overlap of suckling and feeding on solid food. In addition, females attain reproductive maturity late (typically at some 10 months of age), commonly suffer severe weight losses during lactation, and many individuals recover this weight slowly after lactation. Most natality occurs in the period from March through May; of the females that live to reproduce, only 15% survive to reproduce a second season. Dietary selectivity—the use of foliage from specific trees with relatively low levels of defensive chemicals—is probably learned in the long pre-weaning period and may allow females to maximize milk production, growth rates and survival of her young, and, by minimizing lactation weight losses and the time necessary to regain weight, may shorten the interval between litters. Wide variation in reproductive performance among females may be associated with differences among den sites with respect to access to junipers with low levels of defensive compounds.