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We know the changing climate
is messing with a lot of things
around the world,
like weather patterns
and ocean temperatures.
And it’s no secret that a whole bunch of animals
aren’t going to be too happy about that.
But there’s one side effect of climate change
that you might not have expected -
it could also make a lot more reptiles female.
[intro song]
Now, we typically think of sex in animals
as being largely determined by their genes,
specifically the sex chromosomes.
In mammals,
females have two copies of an X chromosome,
while males have one X and one Y.
In birds, there’s a system with Z’s and W’s,
and it’s the male with two copies of the Z.
But sex isn’t always determined by genetics.
Instead of using sex chromosomes,
some animals rely on an egg’s incubation temperature
to determine the sex of the hatchling!
This temperature-dependent sex determination
is used by all species of crocodilians,
most turtles, and even some lizards.
Now, using chromosomes to determine sex produces
about a 50/50 chance of an offspring
being male or female,
but in reptile species
that use the temperature-based system,
a whole clutch of eggs might end up being mostly just one sex.
In a lot of cases,
the eggs that incubate at lower temperatures
develop into males,
and higher temperatures result in females.
So a nest of eggs that incubates
at temperatures in the middle zone
will likely differentiate t
o an even sex split between males and females.
And sometimes just a few degrees of difference
results in an all-male or all-female clutch.
But there are even some species
where both temperature extremes
result in female hatchlings -
hotter or colder nests -
with males being produced at intermediate temps.
Relying on temperature at all seems
like a risky way to determine
the sex ratios of offspring,
but some scientists believe
this is the way all reptiles originally did it.
It’s possible that the
W, X, Y, and Z chromosomes of birds
and mammals evolved independently
after those lineages split from early reptiles.
Because it’s pretty hard to determine how extinct genes worked,
we can’t say for sure which method came
first or why there’s such variety among modern reptiles.
And to make things even more complicated,
there are plenty of reptiles
that do use sex chromosomes.
What’s weirder still is that there are even species of fish
and amphibians that have sex chromosomes
but where temperature extremes
can override their chromosomes
and determine the sex of the animal.
The sheer variety of sex determination methods
and the examples of what appears
to be an evolutionary intermediate step
between chromosomes and temperature
suggest reptiles could find a way
to avoid getting caught in an all-female world.
Whichever method came first,
there’s no denying that relying on temperature
is a risky move.
And it’s getting riskier.
As the planet becomes hotter, the nests
in which these temperature-reliant
eggs hatch are getting hotter, too.
And that means we’re likely to see
more and more female crocodiles and turtles.
If it only takes a few degrees difference
to change the whole sex ratio of a nest,
a rise in the average temperatures
during peak breeding months
could significantly
change the population’s sex ratio -
and the mating chances for those individuals down the line.
Take the painted turtle.
Painted turtle clutches
along the Mississippi River
produce nearly all males at 23 degrees Celsius,
and almost all females at 25 degrees Celsius.
A change in the average July temperature
of just 1 or 2 degrees
could mean the difference between nests
with even sex ratios
and nests that are 100% female.
And since the Earth’s temperature
is rising about 0.2 Celsius per decade,
that one degree rise could happen in just 50 to 100 years,
which might not be enough time
for evolution to fully catch up.
Some species, like sea turtles,
live much longer than that,
so they may not have generations
and generations of time
to evolve changes in genetics or behavior.
In the short term,
this all means that we’re likely
to see more reptile clutches
that are entirely or mostly female,
at least from the croc and turtle species
where temperature controls sex differentiation.
It’s currently unclear if these reptiles
will shift their breeding grounds
or times to accommodate the changing temps,
like some other animals have done
in response to climate change.
Breeding earlier in the spring,
for example,
would mean cooler temperatures and thus,
more equal sex ratios.
And we know that mother reptiles do
seem to try to select nest locations
with temperatures
to give their offspring the best chance for survival.
In the long term, perhaps more reptile species
will do away with temperature-dependence
and just move toward sex chromosomes.
It’s a move that plenty of
reptile species have made,
like snakes.
We know reptiles have survived
rapid climate change events
in the past 280 million years,
so we know they can adapt
to rapid changes in temperature.
The question is:
how quickly can they do it this time?
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about the future of reptiles,
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