Co‐occurrence of multiple, supposedly incompatible modes of sex determination in a lizard population

Sex is determined genetically in some species (genotypic sex determination, or GSD) and by the environment (environmental sex determination, or ESD) in others. The two systems are generally viewed as incompatible alternatives, but we have found that sex determination in a species of montane lizard (Bassiana duperreyi, Scincidae) in south‐eastern Australia is simultaneously affected by sex chromosomes and incubation temperatures, as well as being related to egg size. This species has strongly heteromorphic sex chromosomes, and yet incubation at thermal regimes characteristic of cool natural nests generates primarily male offspring. We infer that incubation temperatures can over‐ride genetically determined sex in this species, providing a unique opportunity to explore these alternative sex‐determining systems within a single population.