Sex allocation based on relative and absolute condition.

Traditional models predict that organisms should allocate to sex based on their condition relative to the condition of their competitors, tracking shifts in mean condition in fluctuating environments, and maintaining an equilibrium sex ratio. In contrast, when individuals are constrained to define their condition absolutely, environmental fluctuations induce fluctuating sex ratios and the evolutionary loss of condition-dependent sex allocation in short-lived organisms. Here, we present a simulation model of temperature-dependent sex determination (TSD) in fluctuating environments that specifically examines the importance of relativity in defining individual condition. When relativity in condition is allowed to evolve, short-lived organisms evolve switchlike TSD reaction norms and define their condition relative to the annual temperature distribution, thus preventing biased cohort sex ratios in extreme years. Long-lived organisms also evolve switchlike reaction norms, but define condition less relatively and experience biased cohort sex ratios. The predictions are supported by data from painted turtles, where TSD reaction norms exhibit pivotal temperatures of sex determination that partially track mean annual temperature. Examining relativity in amniotic vertebrates provides a conceptual framework for multifactorial sex determination and suggests new ways of exploring adaptive hypotheses of sex allocation by focusing on the importance of frequency-dependent selection on sex.