Though it has been widely predicted that in animals in which reproductive value declines with age, reproductive effort should increase toward the end of the lifespan, analysis of changes in reproductive effort are impeded by fundamental difficulties in measuring the costs of reproduction. Energetic measures may not reflect the effects of breeding on subsequent survival and breeding success, especially in organisms in which body size increases with age, while attempts to estimate reproductive costs directly are complicated by positive correlations between breeding success and parental survival. Though the long-lived birds and mammals are among the most promising organisms on which to test the theory that reproductive effort increases with age, measures of fecundity commonly decline with increasing maternal age. Some recent evidence suggests, however, that offspring survival may improve toward the end of the lifespan.
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