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You may have thought that crocodile tears were just a figure of speech, but it turns out they're real, and may help those of us with dry eyes.

Hosted by: Hank Green

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Sources:
https://academic.oup.com/bioscience/article/57/7/615/238586?login=true
https://www.sciencedaily.com/releases/2007/10/071003151131.htm
https://www.sciencedirect.com/science/article/abs/pii/S1542012412703075
https://www.healthline.com/health/how-many-times-do-you-blink-a-day#blinking-frequency
https://www.nationalgeographic.co.uk/animals/2020/08/crocodile-tears-are-surprisingly-similar-to-our-own
https://www.frontiersin.org/articles/10.3389/fvets.2020.00283/full
https://www.frontiersin.org/articles/10.3389/fvets.2020.00574/full
https://www.nytimes.com/2020/08/13/science/animal-tears.html

Images:
https://commons.wikimedia.org/wiki/File:Paleosuchus_trigonatus.jpg
https://www.storyblocks.com/video/stock/ecu-woman-blinking-view-of-eyes-suxigk1egiwpr0wur
https://www.eurekalert.org/multimedia/pub/239454.php?from=473005
https://commons.wikimedia.org/wiki/File:AmericanAlligator.JPG
https://commons.wikimedia.org/wiki/File:Mugger_crocodile_2.JPG
https://www.istockphoto.com/photo/pouting-crying-businessman-brat-wiping-his-crybaby-tears-gm170223880-17652600
[♪ INTRO].

When someone is faking their sadness, you’ve probably heard it said that they are crying crocodile tears. And that saying comes from a myth that crocodiles shed tears of fake remorse over what they’ve eaten.

But although the “remorse” part is definitely a myth researchers have shown that crocodiles and their cousin do get watery-eyed over meals. They’re not crying in the sense that you and I normally think about it. But in the process of understanding what they are doing, researchers are also learning more about how these animals keep their eyes moist.

Something that may help us develop a treatment for dry eyes in humans. Scientists investigated this in a 2007 study. They didn’t use crocodiles specifically, because crocodiles are extremely aggressive and agile on land.

Instead, they looked at their close relatives: trained alligators and caimans, who have the same facial anatomy. These reptiles had been conditioned to eat on land, which was important, since it wouldn’t have been possible to observe any crying or watery eyes if the animals were in the water and their eyes were thus already wet. And it turned out, there was a lot to see!

The researchers observed watery eyes, as well as frothing and foaming from most of the study animals’ eyes while they ate. Now, as for why… well, it’s important to know that these weren’t strictly tears in the way we normally think about them. In other words, the glands around their eyes weren’t making new liquid.

Instead, this was more like tear film getting… kind of regurgitated. Tear film is the liquid that covers our eyes all the time, and it usually drains into the tear duct. But in these reptiles… as they’re munching away, the exhale coming from their lungs forces their tear film back out of the tear duct.

And the frothing and bubbling is a reaction caused by substances in the tears, such as proteins. It’s not just that these reptiles get all watery-eyed, though. Their tear film is also way more efficient than ours.

Like, humans have to blink 15 times a minute to spread tears across our eyes. But caimans can go without blinking for almost two hours. What makes these films so much more stable hasn’t been fully worked out.

But a paper published in 2020 shows that it’s at least partly caused by what they’re made of. The researchers learned this by studying the crystalline patterns that formed as the films dried, which clued them in as to what was in them in the first place. These patterns are created by large quantities of things, like proteins, electrolytes, and mucus.

All of which are dissolved in the tear film, but get left behind as the water evaporates. Proteins and mucous both help keep the liquid super stable, so it’s able to cling to the eye for a long time, which comes in handy for animals who spend a lot of time in water. Meanwhile, electrolytes may help keep the eye moisturized and protected from inflammation.

A better understanding of crocodylian tear films will help us understand these animals as a whole, and it could give researchers a better understanding of human tears, too. If they can pin down the exact molecules and mechanisms behind these reptiles’ extremely stable films, that may lead to the development of new treatments for things like dry eyes in humans. Not to mention, tears and how they work in general aren’t well understood.

So learning more about them will help improve researchers’ overall knowledge, which is nothing to cry about! Thanks to all of our patrons, who made this episode possible! Patrons on Patreon keep SciShow going, and they allow us to keep diving into research like this and making content of all kinds on our channel.

If you’re not a patron but want to learn more about what it means to become one and how you can help, you can go to Patreon.com/SciShow. It’s simple, it’s fun, it’s cool and it’s always there for you.