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At first glance, river otters' hearts don't look too different from ours. But if you look at enough of them, especially older ones, you might find patches of bone around some of the internal walls of the heart and around the major heart valves. It's a structure called the os cordis, os for bone and cordis for heart -- literally the heart bone.

Now, river otters aren't the only creatures to grow bone tissue in their hearts, but their aquatic lifestyle can help us understand why they do, and it seems like a healthy response to stress.

So first of all, why would hearts need bones? Our human hearts actually do have a structure called the cardiac skeleton. But despite its name, it's made of tough connective tissue, including cartilage, not actual bone. Its job is to help anchor certain soft parts of the heart together, like the valves between the upper and lower chambers. Plus, it helps keep the heart's electrical signals flowing in the right direction. And it's a good thing that it's made of cartilage, since we want our cardiac skeletons to be at least a little soft and forgiving.

When we do see calcium deposits, which are precursors to bone, show up in our cardiac skeletons, those otherwise soft structures turn stiff. So bone in our hearts is considered a sign of heart disease.

And the same thing may be true for other primates. A study published in 2020 showed that chimpanzees developed os cordis alongside certain heart diseases, which made the researchers think the bone might be a sign of chronic heart stress.

But what's dangerous in our hearts is a feature for other animals. Over the 20th century, we've learned that certain animals, especially ruminants, like sheep and goats, may have some bone tissue here and there throughout their cardiac skeletons.

In the year 2000, researchers publishing in the Journal of Anatomy found that otters often had heart bones as well, especially older individuals. They think that over the otters' lives, bony tissues would begin to develop in the cartilaginous parts of their hearts, thanks to a process called endochondral ossification. That's the formation of bony tissues out of cartilage. It happens in humans as our bones are forming in the womb, but also when broken bones are repaired.

See, as far as connective tissue goes, cartilage is a pretty tough material. But if the body finds itself constantly under physical stress or strain, it can reinforce itself further. If the area in question already has some cartilage, it will add more cartilage on top of that. If that doesn't do the trick, it can add bone. And this process happens in pretty much any skeleton-having animal you'd care to name, from cows to hamsters. But as to why this would happen, it usually comes back to reducing physical stress.

So what does reducing stress do for the otters? Scientists still aren't totally sure, but they have some ideas. For starters, river otters have huge hearts compared to their body size. Our human hearts are around six tenths of a percent of our body weight, while most aquatic mammals' hearts are closer to nine tenths of a percent. And river otter hearts in particular are on the upper end of that, coming in at nearly a full percent of their body weight. And they need those big hearts, since they're constantly swimming, and swimming fast.

If you've ever been to a zoo that has these little guys, you know what impressive athletes they are. Not only are they always moving, but they're doing it in fresh water, which is less buoyant and supportive than salty ocean water. Not coincidentally, river otters have bigger hearts than their salt water counterparts. 

Now, some scientists think that the increased stress from their high-performance hearts gets distributed across the fibrous parts of the cardiac skeleton. Heart muscle cells anchor into this structure, so stiff bone might provide them more support. Plus, their hearts are more cone-shaped than ours, which contributes to more stress in the exact area that they have heart bones. 

That leaves us with a question, though. Is this just a side effect of aging when you're an otter, or is it a beneficial adaptation to swimming? Calcium deposits show up in the heart cartilage alongside healthy, living cartilage cells, which makes it seem like this ossification is a healthy process of reinforcing that tissue, not a sign that things are breaking down.

But the placement of the bits of bone is somewhat random. They're not consistently found on the same side, for instance. Which means scientists haven't found a brand new bone that all otters will always have in the future. Instead, they're more common in older otters, which suggests that they grow in response to stress over time.

But regardless of exactly why river otters develop these bony hearts, it shows us how the body isn't a static thing. It's constantly adapting to the stresses placed on it, to the point where entirely new anatomy might emerge.

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