Capital breeding and income breeding: their meaning, measurement, and worth

The life histories of organisms can show pronounced variation in the way that the costs of reproduction are financed. To facilitate discussions of this variation, two terms are increasing in use: "capital breeding" describes the situation in which reproduction is financed using stored capital; "income breeding" refers to the use of concurrent intake to pay for a reproductive attempt. We consider the value of the capital and income typology with reference to three functions that it might serve: description, explanation, and prediction. We find that interpretations of the terms have diversified and lack rigidity, leading to subjectivity in their definition. We recognize that time frames of interest will vary among taxa, but we urge consistency of use within those taxa. We also urge consistency in the use of a single metric designed to measure the reliance of an organism on capital. The concepts of capital and income breeding have served well as proximate explanations for behavioral or physiological diversity, but efforts to explain their adaptive value have been disproportionately focused on individual taxa. Mapping cause to effect is difficult in ecology. Nevertheless, further analyses, based on consistently applied measures of reliance on stored capital, may reveal which of the putative ecological, morphological, and physiological drivers have the most consistent and widespread effects. The capital-income typology has yet to be applied to the question of prediction, and thus, it remains to be seen whether these concepts will be of use in identifying the likely responses of different populations to changes in their environment.