Photo via Flickr user Terry Ross
A study published on Sunday in Nature Climate Change gave us the news that climate change is bringing about a higher proportion of female sea turtles to males, thanks to a seemingly idiotic genetic quirk called temperature-dependent sex determination (TSD), also found in a few other species. Essentially TSD gives the turtles a baseline temperature of 84.2 degrees, at which there are 50/50 odds of being male or female. Beyond a certain threshold of heat, too few males will exist to sustain the population.
The study, Effects of Rising Temperature on the Viability of an Important sea Turtle Rookery, didn't involve any radical new biological revelations about the turtles' genes or physiology. It just required extensive geographical analysis, cross referenced with thousands of data points about climate and population numbers. Frustratingly though, it didn't say why turtles reproduce this dumb way.
The authors of the study found that In the short term, turtle numbers are actually going to increase. The study looked at one species: loggerheads, on the island chain of Cape Verde off Africa's west coast, and modeled what's going to happen to them during 150 years of irrevocable temperature increase. The good news is that the increasing number of female turtles—who do the risky work of carrying the eggs to the island of their birth and laying them in hidden nests—will increase the overall number of turtles for the next 30 years.
Then, the males will start to seriously decrease in numbers. Professor Graham Hayes spoke to The Guardian about the long-term effects. "Once you get 100 years or more into the future, then things start to look serious. You have so few males left that it’s likely to be a problem. There will be heaps of females but not enough males to fertilize all those eggs." Then they, y'know, go extinct.
Certain lizards, crocodiles and even a type of bird, most of which are native to Australia, all have their sex determined by the temperature, rather than by having sex chromosomes like our X and Y, or the Z and W chromosomes found in birds. Turtles are just a particularly vulnerable example, and I had to know why they have a trait that puts them at such risk. I know evolution doesn't make plans, but what would the advantage be?
I tracked down a podcast on which a TSD researcher named Ido Pen talked about his research, and I transcribed his main point for your reading pleasure: "If the fluctuations between years are very large, that will select against temperature dependent sex determination. And the reason is that if you have big fluctuations between years then you will also get years with either a lot of females or a lot of males being born that year. There is selection against that."
Pen's reasearch is like a roundabout way of proving that TSD is weird. It's a painfully obvious observation, and it's almost totally unhelpful.
An older study on the subject from the Journal of Evolutionary Biology, by F.J. Janzen and P.C. Phillips, provided at least some clues. They listed the following totally unproven hypotheses:
- "Phylogenetic inertia – TSD is the ancestral condition in this clade and is simply maintained in extant lineages because it is currently adaptively neutral or nearly so."
- "Group adaptation – TSD promotes adaptive control of sex ratio to promote group fitness."
- "Inbreeding avoidance – TSD minimizes inbreeding by producing single-sex clutches."
- "Differential fitness – TSD provides sex-specific fitness benefits such that some developmental temperatures are better for one sex."
But most disturbingly of all, the report suggests that the most likely explanation is the first one, Phylogenetic inertia. In other words, turtle reproduction through TSD worked hundreds of millions of years ago, and now it's just an obsolete, ad hoc solution that needed to be upgraded a long time ago, like ATM transactions that still move data over the phone lines.
This is just the latest setback for sea turtles, a group of species that already had the deck stacked against them by the hellish realities of the wild kingdom. Not only do they lay eggs only once every three years, but they do so by dragging their delicious, meaty bodies out of the sea to plop them on hostile dry land, and then abandoning the eggs in an unsupervised nest. In the event that the eggs survive long enough to hatch, the baby turtles—bite-sized turtle nuggets to predators—have to scramble across that same hostile beach into the sea, and then fend for themselves in perhaps an equally hostile aquatic environment.
If that weren't enough, now humans come along and steal their eggs en masse for a totally unnecessary snack, sometimes wiping out an entire population. Or, upon hatching, the babies will mistake the bright lights of a city for stars reflecting on the ocean, and walk the wrong fucking way, only to dry out in a parking lot and die. Or they'll head toward the sea, but the sand we warmed with our CO2 will cook them. Or they'll make it to the sea, only to chomp down on what they think is a nice jellyfish, and have it turn out to be a grocery bag.
According to ConserveTurtles.org, sea turtles, and their equally doomed pals the manatees, are among the only species that eat sea grass, something that's critical to the life cycles of sea grass beds. I'll take their word for it. Even if the sea beds didn't need them, I think we all love watching sea turtles swim with their majestic, wing-like front legs, and it'll be a drag to lose them.
Many turtle species do show a propensity for homosexuality in a single-sex environment. So if they go down, they're going to go down fucking. If there's any consolation prize for the turtles, let it be that.
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