Assistant Professor Duke University durham, North Carolina, United States
Abstract: Organisms cope with increasingly warm and variable temperatures through ecological, behavioral, and physiological mechanisms. These mechanisms shape the Thermal Performance Curves (TPCs) that determine ecological responses across temperatures. Past research has focused on how the TPC shape is determined by mean temperature regimes. For example, whether the thermal optimum (Topt) –i.e., the temperature at which TPCs peak– adaptively tracks mean temperatures (e.g., lower in colder temperatures, higher in warmer ones), or whether the maximum value of the TPC increases or decreases with mean temperature conditions (e.g., colder-is-better vs warmer-is-better hypotheses). While organisms deal with variable environments where temperatures almost never remain constant, how this temperature variability has and will continue to shape and constrain TPCs, is not well understood. Here, I address this issue by asking: if temperature is a random variable and TPC shape optimizes fitness (e.g., minimizes energy expenditure, maximizes births, minimizes deaths), what TPC shape will be optimal under different distributions of the temperature random variable? I subject this optimization problem to commonly assumed constraints in Thermal Ecology (e.g., conserved area under the curve or curve length) through a variational Euler-Lagrange approach.
My main result implies that, under the assumption that TPCs optimize fitness –i.e., TPC shape is driven by physiological or evolutionary processes that maximize TPC values given a temperature regime– TPC shape should be mostly determined by the shape of the temperature distribution. Symmetric temperature distributions (e.g., normal), should lead to symmetric TPCs, while asymmetric temperature distributions (e.g., skewed) should lead to asymmetric TPCs. This result is surprising because while most TPCs are strongly asymmetric around their Topt, temperature distributions are not –or only weakly– asymmetric. Yet, TPCs are often assumed to result from acclimation and adaptation, which should optimize the TPC shape within some constraints, in ways analogous to how this process was modeled here. Our results therefore suggest that either TPCs are not the result of an optimization process like selection or acclimation, which is unlikely, or that there are constraints operating on TPCs beyond those currently assumed, and therefore, known. We are currently testing these interesting theoretical results in a microbial system but I believe that they add an important perspective to a growing literature on the mechanisms through which organisms deal with increasingly warm and fluctuating temperatures in a changing world.
