If you are a Chelonian (aka turtle), most likely it’s the temperature. For the majority of turtle species, the sex of the embryo developing inside its amniotic egg is determined by the incubation temperature.This is obviously not how sex determination works for us humans (nor for other mammals, birds, mammals, or amphibians) where sex is determined by genetics (e.g. in humans, XY = males, XX = females). Although I’m pretty sure if human parents could travel to the tropics or the poles to influence the sex of their baby, they probably would.
Back to temperature sex determination or TSD. Critical temperatures during incubation dictate whether a clutch of eggs are all females (warmer), all males (cooler), or mixed (intermediate temperatures). These critical temps are not fixed across turtle species or even within a single turtle species. Take our Black Rock friend, Rocky, the snapping turtle. There is geographic variation in critical temperatures such that what temperatures makes a female turtle in NYS will be different than in Florida.
Researchers have known about the existence of TSD for decades. TSD is thought to be the ancestral state in vertebrates, and then genetic sex determination, GSD, evolved in some species. That doesn’t mean they know how it evolved or when. Is TSD adaptive in turtles or is it just ubiquitous due to phylogenetic inertia (aka, it works and there’s no “easy way” for GSD to evolve). Even the genetic mechanisms driving intra & inter specific differences in critical temperature remained elusive. But there are some clues.
A recent study on the snapping turtle identified CIRBP as a candidate gene in part responsible for determining the critical temperatures that results in boy vs girl turtles.The idea is that variation in the CIRBP gene dictate how an embryo responds to temperature. Dr. Brendan Reid and his team, Rosemary and Michael are investigating how genetic variation in CIRBP corresponds to variation in critical temps across and within species. So far, this gene’s importance to TSD has only been demonstrated in the snapping turtle; however, CIRBP is known to be involved in temperature regulation in humans.
Brendan and his team’s research is timely. As climate changes brings warmer temperatures, the sex-ratio of turtles becomes skewed impacting population growth. This already being seen in a population of Florida sea turtles where temperatures were too warm for too many years resulting in mostly female baby turtles. You don’t have to be a demographer to know that’s not good.