Current paradigms may substantially underestimate the complexity of reptilian sex determination. In previous work, we have shown that the sex of a hatchling lizard (Bassiana duperreyi, Scincidae) does not depend entirely on its genes (XX versus XY sex chromosomes); instead, low nest temperatures can override genotype to produce XX as well as XY males [1, 2, 3]. Our experimental studies now add a third mechanism to this list: sex determination via yolk allocation to the egg. Within each clutch, the eggs that produce daughters are larger than those that produce sons. If (and only if) eggs are incubated at low temperatures, removing yolk from a newly laid egg turns the offspring into a male. Adding yolk from a larger (but not smaller) egg turns the recipient egg's offspring into a female. Remarkably, then, offspring sex in this species is the end result of an interaction between three mechanisms: sex chromosomes, nest temperatures, and yolk allocation.