[♪ INTRO] Cold-blooded animals, including insects, reptiles, and fish, don’t really have cold blood.

Rather, they rely on the environment to help maintain a stable body temperature, what researchers call ectothermy. These organisms tend to do well in warmer environments.

Because, warmer temperatures support a host of biological processes, and pretty much every living thing has evolved some way to stick to the temperatures they need to live. But that doesn’t mean hotter always equals better. These creatures have a limit to their temperature tolerance, and a new study published in Nature this week suggests that Earth’s changing climate is quickly pushing them up against it.

In all living creatures, temperature plays a major role in supporting biological processes. That begins at the molecular scale of biochemical reactions, and goes all the way up to complex activities like metabolism and reproduction. For ectotherms, their bodily functions are directly affected by the temperature of their environment.

These types of creatures do best and increase their numbers quickly in environments where the temperature is just right to support important processes like digestion and reproduction. But they can get in trouble if temperatures in their environment get too toasty, because they have no way of internally regulating their temperature like warm-blooded or endothermic creatures do. Many ectotherms are adapted to handle swings in temperature for short periods of time, like when there’s a heat wave.

But they are in danger if exposed to the wrong temperatures for too long. Even if warm is good, that doesn’t mean hot is better, and ectothermic animals start to experience heat stress or even death if those limits are reached. Of course, heat stress on a global scale is kind of looking more likely these days.

A group of researchers based in Denmark wanted to better understand how rising temperatures caused by the climate crisis will affect ectotherms worldwide. They published their findings in Nature this week, and the results are pretty grim. They found that for some ecotherms, the temperature window between life and death is very small.

They started out by analyzing data published in over 300 publications of 314 ectotherms, representing several groups of species including fish, reptiles, crustaceans, shellfish, and insects. They looked at everything from these creatures’ heart rates, to how fast they move, to their metabolisms. And what they found was that even the more modest increases in global temperature, as predicted by the Intergovernmental Panel on Climate Change, will be extremely stressful for these creatures.

They dug further into the data by reviewing over 120 published studies on heat failure for 112 ectothermic species. They found that, for every one degree Celsius increase, the median risk of heat failure or death in these animals increased by 110%. As the climate changes, more ectotherms will be exposed to the upper limits of their temperature thresholds for longer periods of time, because those temperatures will become the new normal, not just a short swing into the danger zone.

By 2100, the researchers estimate that if Earth’s temperature increases by 2.7 degrees Celsius, as predicted by one of the three warming scenarios presented by the IPCC, this may increase heat related deaths in terrestrial ectotherms by a median estimate of 774%. For aquatic ectotherms, the estimate is “only” 180%. And even in the more modest suggested temperature increase of just under 2 degrees Celsius, which is pretty much locked in at this point, the impact of these temperature changes on ecotherms is going to be much more significant than previously expected.

Which matters because species don’t exist in a vacuum. They’re part of complex food webs, and we’re going to have to account for their absence in future climate scenarios. Speaking of cold-blooded creatures, when we picture the Jurassic period, we generally imagine dinosaurs.

But another paper published in Nature this week describes an early relative of squamates. Squamates are diverse a group of scaled reptiles that today makes up more than 10,000 living species, ranging from snakes to lizards. All squamates descended from a common ancestor who lived around 240 million years ago, during an era that was ruled by dinosaurs.

That common ancestor would have contributed many features to squamates we know today. But the fossil record is pretty incomplete for this group, so there is a lot of uncertainty around the evolution of their anatomy and which species diverged when. But this well-preserved fossil, discovered in Scotland, can help fill in some of the gaps surrounding the evolution of the squamate body plan and where they lived on Earth during the Jurassic era.

In the new study, researchers were able to take a look at which traits were ancestral, or inherited from the common ancestor of all squamates. That helps us understand what hand every creature in the group has been dealt. In the case of this creature, the roof of its mouth and spine are similar to a more ancient squamate.

It also had some newly acquired or derived traits that it didn’t get from earlier squamates, like some features of its head and shoulders, which we also see in later squamates going forward. Researchers believe that this particular species is a stem squamate, meaning an early branch near the base of the snake and lizard family tree. Its body plan sheds some light onto the evolution of squamates that we are familiar with, like the way their skull bones move in relation to their jaws, which plays a role in how they feed and capture prey.

Think about a snake opening its mouth way too wide to swallow prey, for instance, that’s more in the skull than the jaw, and this fossil’s skull can tell us a bit about how features like that got started. Also, this fossil may help untangle a little of the convolution surrounding squamates, because of the diverse amount of creatures that fall into the squamate category, everything from snakes to chameleons to geckos to extinct marine lizards. There’s a lot of debate about whether certain groups, like iguanas, come earlier in that family tree, or are more derived.

This new fossil’s body features may help resolve some of this ambiguity. Not only that, the researchers believe that it was coexisting with crown squamates, or ancestors of species that are still alive today, potentially hanging around them until the middle of the Cretaceous period. We have a lot left to learn about squamates, and ectotherms in general.

Even the fossil record might yield clues to help us understand how our actions are endangering species today. Thanks for watching this SciShow News video! All of our incredible patrons on Patreon helped us make it happen.

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