The effects of dietary and skeletal calcium availability on reproductive performance of mammals
Type of Degreedissertation
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Investment in offspring production often requires the mobilization of endogenous resources, a strategy which may negatively impact maternal condition. In mammals, skeletal ossification in growing offspring requires considerable calcium investment by the mothers, and bone loss has been described in several species as a means of supporting this demand. Although bone loss can have adverse effects, its potential role in a reproductive trade-off has not been addressed. Using white-footed mice (Peromyscus leucopus) and domestic swine (Sus scrofa) I investigated the following questions: 1. Does dietary calcium intake exacerbate bone loss as a cost of reproduction? 2. How do mothers allocate calcium in response to dietary availability? 3. How does offspring production influence bone metabolism independent of diet? 4. How do mothers respond to calcium availability to maximize lifetime reproductive success? I characterized changes in maternal skeletal condition, variation in pup characteristics and overall reproductive output of the mothers. Bone characteristics were quantified via dual energy x-ray absorptiometry, bone marker assays, 3-point flexural tests, micro-computed tomography, and mineral composition (using both ashing and inductively-coupled plasma spectrophotometry). To assess the degree of calcium allocated to offspring, I measured bone morphology and mineral composition of pups. Reproductive output was characterized by litter size, individual offspring and litter mass, sex ratio, and total number of offspring produced. Under low dietary calcium conditions, females reproduced at a cost to their own skeleton. This cost was long term, as mothers that had consumed a low-calcium diet throughout their lives exhibited an overall reduction in bone volume. Offspring production also affected bone metabolic activity independent of maternal calcium intake, and multiparous females responded less drastically to offspring demands relative to primiparous or generally reproductively inexperienced females. Over the course of their lifetime, mothers on a low-calcium produced generally smaller litter sizes as well as female-biased litters, indicating that they had indeed responded to dietary calcium availability and had adjusted investment/allocation accordingly. Thus, I show that calcium availability and bone loss can provide a tractable means for evaluating reproductive trade-offs in vertebrates.