Imagine if the sex of your unborn offspring was determined by the climate you lived in while pregnant. Vacation in Cabo? Guaranteed son. Visiting the Northern lights? You’re having a daughter. This method of sex determination would play havoc with human sex ratios: countries like India and China, already on the verge of sex-ratio breakdown would become even more male-dominated, while Canada and the Scandinavian countries would swing towards a female majority (and given the general state of the world when men have been in charge, that’s a fairly appealing thought).
But sadly that’s not how sex determination works in humans — instead it’s a 50-50 chance based on whether the sperm fastest to the egg carries a male chromosome or a female chromosome. However, it is how sex is determined in lizards and crocodiles, which might prove to be a bit of a problem as the worlds climate changes for the warmer.
When mating season comes, and reptile hormones are all in a tizzy, males donate a packet of sperm to a female, which is stored in her cloaca (how romantic). She then uses this sperm to fertilize eggs, and bury them in a nest for incubation. Lizard moms manipulate the sex ratio of their offspring by choosing where to build a nest: if they want more females, they will build a deeper nest in cooler areas — if they want sons, they build a shallow nest in warm habitats.
Why exactly this method of sex determination has evolved is up for debate. Some research indicates that it may be the ancestral state for all amniote vertebrates (animals which lay eggs on land), dating back around 300 million years. It may continue to exist in lizards, crocodiles, and turtles because it is adaptively neutral — that is, it doesn’t necessary convey a great evolutionary advantage, but it also isn’t disadvantageous.
Other scientists argue that temperature-dependent sex determination (TSD) ensures that regardless of the climate or seasonal conditions, the sex best able to cope will hatch. For example, the spotted skink in Tasmania uses TSD: cool incubation temperatures lead to male offspring, and warm incubation temperatures lead to female offspring. In order for newborn female skinks to survive winter, they need ample amounts of time to grow during the summer. Having a brood of female skinks late in the breeding season is a bad idea: they won’t have time to grow, and will likely die over winter, meaning a wasted breeding season for mum. However, because of TSD, this doesn’t happen – females hatch only early in the summer, when temperatures are warm. As the average temperature cools down in mid- and late-summer, any egg laid hatch as males. No matter what time a clutch of eggs is laid, TSD makes sure that the sex that appears is the one best able to survive.
Unfortunately, this can backfire if the climate moves out of the ranges in which that behaviour has evolved to be adaptive (the evolutionary trap that also affects sea turtle behaviour — a behaviour that was previously beneficial becomes negative in light of recent, rapid changes). That’s the possible fate facing the tuatara.
The tuatara is a New Zealand reptile that looks like a lizard, but isn’t. Instead, it is the only living member of an ancient order of reptiles, the Rhynochocephalia, which reached its peak 200 million years ago. Anatomically, they are the most unspecialized amniote, and researchers think they may be good models for understanding the behaviour of dinosaurs. They don’t reach sexual maturity until they’re 20, and can live until well over 100. For millennia they were widespread across New Zealand, until the introduction of rats and cats as invasive predators led to a dramatic decline in their numbers, and eventually extirpation from the main islands. Currently, the tuatara survives in relic populations on the small islands of New Zealand which have never been colonized by predatory mammals.
But even if it can survive the rats, cats, and minuscule gene pool, climate change might get it. Tuatara sex is determined by temperature — warm temperatures lead to males, cool temperatures to females. Like in other TSD species, to some degree the effect of air temperature can be mitigated by changing nest depth. Digging a deeper nest can, in theory, counter-balance increased solar radiation or air temperature to maintain a balanced sex ratio. Unfortunately, the islands of New Zealand that the tuatara inhabit don’t have a soil base deep enough to allow that sort of digging (plus, tuatara arms are pretty stubby, they’d be hard-pressed to dig a deep nest).
Which means that the tuatara might be in trouble. Researchers predict that, if global climate change proceeds according to schedule, by 2080 tuatara’s will be laying nests consisting entirely of male eggs. This might be great for a fantasy football league, but isn’t quite so good when it comes to species survival.
Active intervention by humans might help. Tuatara’s can be translocated to islands with cooler climates. Or, as has been done with sea turtles, volunteers can move nests — reburying them in shadier locations, or at lower depths. But without that help, one of the last living fossils could very well go extinct.
Mitchell, N. J, M. R Kearney, N. J Nelson, and W. P Porter. “Predicting the Fate of a Living Fossil: How Will Global Warming Affect Sex Determination and Hatching Phenology in Tuatara?” Proceedings of the Royal Society B: Biological Sciences 275, no. 1648 (October 7, 2008): 2185–2193. doi:10.1098/rspb.2008.0438.
Refsnider, J. M., B. L. Bodensteiner, J. L. Reneker, and F. J. Janzen. “Nest Depth May Not Compensate for Sex Ratio Skews Caused by Climate Change in Turtles: Nest Depth and Turtle Sex Ratios.” Animal Conservation 16, no. 5 (October 2013): 481–490. doi:10.1111/acv.12034.
 Also turtles, but they’ve got it flipped the other way: high temperatures lead to females, and low temperatures to males. Just to confuse biologists even further, some species have found a third way. Temperature extremes (high or low) lead to female dominated nests, while mid-range temperatures lead to male dominated nests.