Condition-dependent ornaments, life histories, and the evolving architecture of resource-use.

Over the past 20 years, researchers have emphasized condition-dependency as a core feature of many sexually selected ornaments. This empirical focus has been motivated by the assumption that condition-dependent ornaments should function as honest indicators of other fitness-related traits. However, evidence remains mixed regarding whether condition-dependent ornaments are consistently correlated with the expression of other key traits such as immunocompetence. I argue that the diversity of the observed relationships between condition-dependent ornaments and other fitness-related traits can be understood, and even predicted, based on attention to the structure of organisms' life histories. More specifically, these relationships are influenced by the relative variation between individuals within a population in their acquisition of resources versus the allocation of those resources to various physiological functions. However, characterizing these two core attributes of life histories requires that researchers quantify condition, a persistently challenging concept to measure empirically. In this review, I first highlight key concepts related to condition-dependency and life history theory. I then outline why measuring the acquisition and allocation of relevant resources is critical for advancing our understanding of sexually selected ornaments. As attempts to tackle these issues have been hampered in the past by empirical challenges, I offer a number of suggestions that aim to identify more tractable approaches to measuring condition, as well as its acquisition and allocation. I conclude by pointing to the broader value of pursuing these concepts empirically as well as to exciting new directions opened by this perspective.