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There's a reason you don't see hybrid animals like ligers running around very much. Not only are they separated by geography, but the genes of different species don't usually play well together.

But recently, researchers in Hungary accidentally crossed two very different fish: the Russian sturgeon and American paddlefish. Their research was published in the journal Genes, documenting hybrid fish that, according to what we know, were not supposed to be possible. These scientists didn't /set out/ to make a hybrid, though.

They were originally trying to create sturgeon offspring that had only their mothers' DNA through a process called gynogenesis. Usually, this process involves fertilizing egg cells with deactivated sperm cells from another species, making it possible for only the mother's genetic material to be passed on. In this experiment, the researchers crossed sturgeon eggs with paddlefish sperm because they thought it wouldn't create hybrid offspring.

These fish are fairly distantly related. And until now, nobody had been able to make a sturgeon-paddlefish hybrid -- and they'd tried. So that was finding number one.

It turns out it is possible. Then, the researchers got to work analyzing the anatomy and genetics of the offspring. And what they looked like depended on what kind of chromosomes they inherited, or really, how many.

See, at some point in evolution, this group of sturgeons acquired another two copies of their chromosomes, giving them /four/ copies of each chromosome where paddlefish have two. And in the world of fish hybridization, when parents with mismatched sets of chromosomes reproduce, their offspring can inherit different numbers of sets. And, sure enough, the offspring in this experiment could be split into two different groups.

Some were triploid, meaning they had two copies of the sturgeon mom's chromosomes and one copy from paddlefish dad, for a total of three full sets. Or they were pentaploid, with four sets from mom and one set from dad. Now that's a little strange, because in birds, mammals, and reptiles, having more than two sets of chromosomes can lead to serious errors in cell division, and it's usually fatal.

It's more common in fish — it may have even been an advantage over their evolution. But only in even sets of chromosomes -- two four, six, eight and so on. Organisms with three, five, or seven sets of chromosomes usually run into fertility problems.

In this experiment, the number of sets of chromosomes in the hybrid offspring depended on what they inherited from their parents. The offspring with three copies of their chromosomes looked like a pretty even split between sturgeon and paddlefish, while the offspring that got more sturgeon chromosome copies looked a little bit more sturgeony. Now if these two fish weren't supposed to be able to reproduce, how'd they pull it off?

Well, while these two families of fish split a long time ago, the researchers think that their relatively slow rate of evolution meant their genes and anatomy hadn't actually diverged that much. And the fact the sturgeon had four chromosome copies might have set up these two fish for increased chances of making successful babies. That's because past fish hybridization experiments have shown that when the mother has more chromosome copies than the father, the hybrid offspring have better odds of survival.

So now, the researchers are looking into filling up fish farms by making more of these sturgeon hybrids -- just, on purpose, this time. Another critter that caught our attention this week was the yellow-rumped leaf-eared mouse. And not because it's cute, which it super is, and not because it has a great name, which it super does.

It's because research published in the journal PNAS reported one living at a higher altitude than any mammal we've ever seen before. Not a lot of vertebrates live at extreme altitudes. They have to fight low oxygen concentrations and freezing cold temperatures, among other things.

But back in 2013, a mountain climber recorded a video of a little mouse on a volcano in. Chile at 6200 meters up. At that point, that was the highest altitude any person had spotted a wild mammal.

That video inspired a research team to head for that mountain region and look for mice. And they found record-breaking mice all over the place. All in all, they collected 80 specimens across four species, all of which lived above 4000 meters.

But it was the yellow-rumped leaf-eared mouse, which they spotted at 6700 meters above sea level, that broke the record for highest living animal. What we don't know yet is how they do it. Like, what do they eat?

At that altitude, there aren't a lot of plants or insects for the mouse to feed on. And what about the mouse's physiology makes it cut out for living on top of a mountain? Scientists hope to someday compare its genes and physiology with other mice of the same species raised at sea level.

That might give them clues as to how these mice are able to survive at altitude with low oxygen. And could these little guys exist even higher? The mouse in the study lived at the very summit of the volcano, so if the volcano were taller, would the mouse live even higher?

The authors of the paper say their finding means we may have been totally wrong about just how high up small mammals can live. Which means now we get to figure out how they're doing it. And that means more science, so sign us up.

Thank you for watching this episode of SciShow News, and thank you to this month's. President of Space Harrison Mills. Your support really does make a huge difference in allowing us to keep making these videos, so thank you.

If you want to help out SciShow and have a shot at becoming President of Space yourself, check out patreon.com/scishow. [♪ outro].