Evolutionary biology: sex change and relative body size in animals.

Organisms that change sex during their lifetime use a variety of strategies--they may be female first, male first or even repetitive sex changers. Natural selection should favour those individuals that change sex at a time when it increases their reproductive value. Allsop and West claim that the relative timing of sex change is invariant across all animals, with individuals changing sex at 72% of their maximum body size, and infer that natural selection for sex change must therefore be fundamentally similar across animals. Here we explain why we believe that Allsop and West's claims are not supported by their analysis or by their empirical data.