Abstracts (first author)
When to shrink? Understanding the evolution of negative growth in ectotherms
Body size is a key trait linked to the main determinants of fitness: fecundity and mortality (1). For many ectotherms, adult body size is plastic and follows the temperature size rule: individuals grow smaller at higher temperatures (2). Adapting the body size to the environmental condition is crucial and species with continuous growth such as fish and ametabolous hexapods benefit from their ability to adjust their size throughout life (3). But growing larger is in general considered to be the only option for theses organisms whereas shrinking could be advantageous especially if the organism has to cope with a temperature increase. We experimentally demonstrate that such shrinking occurs in the Collembola Folsomia candida - an ametabolous hexapode which continues moulting during its whole life - when they undergo a temperature increase. Comparing multiple clonal strains, we demonstrate genetic variability for this ability to shrink suggesting that negative growth can evolve under natural selection. Using a theoretical model fitted to the collembolans life cycle, we found that for predictable temperature variation, shrinking evolves for environmental temperature variations happening at a time scale close to the individual’s lifetime. For stochastic temperature variation, negative growth can also be selected, as a way for large individuals to escape high mortality following a temperature increase and therefore avoid being stuck in a so called “size ratchet”. 1. Kingsolver JG, Huey RB. Size, temperature, and fitness: Three rules. Evolutionary Ecology Research 2008;10(2):251-68. 2. Atkinson D, Sibly RM. Why are organisms usually bigger in colder environments? Making sense of a life history puzzle. Trends Ecol Evol 1997;12(6):235-9. 3. Charnov EL. Size and temperature in the evolution of fish life histories. Integr Comp Biol 2004, Dec;44(6):494-7.