Costs, constraints and sexual trait elaboration

Much research on sexual selection focuses on evolutionary forces that drive sexual trait elaboration. Yet sexual traits are also shaped equally in their evolution by countervailing factors, namely costs and constraints, which curtail continued sexual trait elaboration. In this essay I consider the roles of these factors, particularly constraints, in curbing sexual trait expression and evolution. First, I review basic definitions of costs and constraints, with emphasis on the distinction between them. Second, I discuss operational strategies for identifying constraints on sexual traits, emphasizing the value of studies on mechanisms that underlie sexual trait expression. Third, I offer new speculations regarding the relative roles of costs and constraints over evolutionary timescales. In particular, I suggest that (1) during initial stages of trait elaboration, costs play dominant roles as countervailing factors, (2) for systems in which trait elaboration proceeds, constraints become increasingly influential as a countervailing co-factor alongside costs and (3) for nature's most elaborate sexual traits, constraints emerge as the most dominant countervailing factor, yet never to the point where costs are eliminated. Finally, I apply this new speculative framework in re-evaluating definitions of two categories of sexual signals that are often regarded as hinging on costs and constraints: handicap signals and index signals.(c) 2021 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.