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The venomous mammal club is so small you’d think they’d all be closely related, but it turns out solenodons have been doing their own thing for a very long time.

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Host: Hank Green (he/him)
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Thumbnail Image Credit: Lucy Emery
- [Hank] Just because venom is common, doesn't mean it isn't weird. Like, if venom wasn't a thing, and I told you, "Hey, what if there was an animal that had teeth like hypodermic needles that injected an extremely complex cocktail of chemicals into another animals' blood specifically to harm or kill them?" That would be a wild idea.

But it turns out injecting a substance into another animal to harm them is super useful, so it has evolved a bunch of times. Snakes, spiders, jellyfish, snails, fish, cephalopods, all come in venomous varieties. But just based on sheer numbers alone, you're much less likely to have encountered a venomous mammal, which do exist. There just aren't that many of them, and we're still not exactly sure why that is, though it looks like this adaptation evolved independently in each venomous mammal species. Whatever the reason, being a member of this small club of venomous mammals definitely makes you a bizarre beast.

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Today, our venomous mammal of choice is an enigmatic, shrew-like creature called the solenodon. They might actually be the largest living venomous terrestrial mammal at around one kilogram, and they're pretty rare. One of the two species was even thought to have gone extinct by the 1970s until it was rediscovered in 2003.

Today, they're only found on the Caribbean islands of Hispaniola and Cuba. They're nocturnal omnivores, and they mostly eat arthropods, like insects, but they're not picky. They'll also eat fruit, small vertebrates, and bird eggs, given the option, and some of the things they eat, like millipedes, are even kind of poisonous.

Now, just to help keep things straight, the difference between venom and poison lies in the delivery system. Venom is actively injected, while poison is more passively absorbed, either through the skin or by inhaling or swallowing it. And yes, animals can be both venomous and poisonous. The blue-ringed octopus, for example, will envenomate you if it gets a chance, but if it fails and you eat it, it will also poison you. Unlike venomous snakes, whose fangs can be found in their upper jaws, solenodons inject their venom through specialized lower incisor teeth, which have deep grooves connected to venom glands at the base of the teeth.

So we know the how of the solenodon venom delivery system, but why the solenodon is venomous in the first place is still unclear. It's been suggested that the venom helps them capture prey or avoid becoming prey, or it might even be used against other solenodons for within-species competition of some kind. At the very least, we're pretty sure it works on other solenodons. In one paper from 1959, a researcher reported that Hispaniolan solenodons kept in captivity together had high death rates, apparently caused by biting each other's feet.

A group of researchers recently did a deep dive into the composition of the solenodon's venom to try to figure out what it's made of and whether it's related to the venoms of other mammals. They found that the venom is mostly made up of proteins called kallikreins, enzymes that cut the chemical bonds in certain other proteins. And chopping up longer proteins into shorter proteins is a thing that happens in our bodies all the time, except in this case, those shorter proteins cause low blood pressure in vertebrates.

That doesn't sound so bad, but eventually, blood pressure gets low enough that the blood can't, like, get to the brain, and the animal slows or dies. This supports the idea that the solenodon's venom is used for capturing prey, not for defending itself against predators. Most anti-predator venom works by being immediately painful, not by the slower-acting effect of lowering blood pressure.

Researchers then compared the solenodon's venom and the genes that code for it to those of three different kinds of shrews, which are also members of the ultra exclusive venomous mammals' club to see where it came from. Now, solenodons split off from the ancestors of shrews, moles, and hedgehogs over 70 million years ago in the late Cretaceous period, and those are their closest living relatives. So yeah, solenodon's been doing they're own thing since the days of the dinosaurs.

But when you're an evolutionary biologist, your default hypothesis when you see the same adaptation in several species within a group is that it came from a common ancestor, even if that common ancestor was 70 million years ago, except, in this case, it didn't. Based on the differences in the structure of genes that code for the venom proteins, it looks like the ability to produce venom evolved independently in all three of the shrews and in the solenodon. 

But there's a twist! While the evolution of venom in each genus was an independent event, all four of them converged on the same kind of venom. Because, like in the solenodon, the venom in the shrews is mostly made up of kallikreins, just slightly different numbers and versions of them.

And here's how that probably happened. Kallikreins are actually found in the saliva of lots of mammals, including you. So the ancestors of the solenodon and the shrews likely started off with the same raw material that would eventually become venom. Over time, slightly different random mutations happened in the genes that coded for those enzymes in the lineage of each animal.

That variation meant that natural selection could happen. Like, the solenodon or shrew that happened to make more kallikreins or slightly different ones was better at bringing down prey, which meant that more energy could be turned into babies, who then also could carry those same mutations for more effective venom. And on and on it went like that for generations, because that's how evolution works. There is no ideal form that it's going towards; natural selection can only act on the variations that exist. 

And today, the end result is we have two groups of mammals, separated by 70 million years of evolution, who ended up with really similar weird mammal venoms. They just got there slightly differently.

For more on the evolution of venom, check out the episode of PBS Eons coming out next week, where Bizarre Beasts host Sarah Suta will be talking venomous mammals and their fossil relatives with Blake DiPastino. To celebrate that collaboration, the Bizarre Beasts pin club subscription window will be open from now through March 14th. That means you will have an extra week to get this fantastic solenodon pin.

Sign up now to get your first pin in the middle of the month, and the pins after that will arrive around the time each new video goes live. And be sure to show us your solenodons on Twitter at BizarreBeasts, and on Instagram and Facebook at BizarreBeastsShow. As always, the profits from the pin club go to support our community's efforts to decrease maternal mortality in Sierra Leone.

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