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Go to Brilliant.org/SciShow to check out their course on Statistics Fundamentals. {♫Intro♫}. Look at pictures of whales --  especially right whales -- and you might notice characteristic white  patches on their heads and faces.

You might think these are just weird skin  growths. Which, they are, but there’s more   to them. Those white patches are only white  because they’re crawling with crustaceans.

And believe it or not,   these weird little dudes can tell us a  lot about the whales they rode in on. Those white spots are called callosities  because, well, they’re basically calluses. These are places where dead skin  gradually builds up -- instead of   drying and shedding away like it  does on the rest of the animal.

Weirdly, callosities form on the  heads of right whales in the same   places we humans have facial hair: above  their eyes, along their lips and jaws,   and even between their blowholes and the tip of  their snout. Like the whale version of a mustache! Baby whales are born without any callosities,  but they start to develop within a few months.

We don’t really know what they’re for,  any more than we know what mustaches   are for. But just like a mustache,  these patterns are very distinctive. The exact pattern is unique to each whale, almost  like a fingerprint, which is super useful because   it helps scientists identify individual whales  and keep track of how their populations are doing.

But here’s the thing: whale callosities are naturally gray. They only look white because…   well, they’re covered in lice. Because even the ocean isn’t safe from lice.  Sort of, anyway.

We call them whale lice,   but they’re not true lice — they’re  a kind of crustacean called a cyamid. The head of a single right whale can have  tens of thousands of cyamids living on it. They find their way to young whales  within a couple weeks after birth,   settle in, and start munching  away on their dead skin.

They can’t swim, so scientists  think they’re transferred by   close contact between whales -- like  from mother to calf during nursing. Once they’re in place,  their flat bodies and sharp,   grasping claws help keep them from washing away. Scientists are still trying to figure  out whether whale lice are just along   for the ride, or if the whale actually  benefits from having them on board.

One possibility is that they actually  help their host whale locate food. Callosities are often scattered with  stiff hairs that might help whales   sense the movements of prey in the water  around them -- kind of like whiskers. And it’s been suggested that the  lice might enhance that ability.

See, we know that crustaceans  closely related to whale lice   stand up when they sense tiny prey animals  passing by -- in order to catch and eat them. So it’s possible that the whale lice that  hang out on the whales’ sensory hairs   are moving around in a way that  helps the whale tell where food is. Scientists aren’t yet sure if that’s the  case, though.

But even if it’s not true,   whale lice don’t seem to cause  much of a problem for their hosts. And strangely enough, they provide a benefit for  us humans -- the ones who study whales, that is. Different whale species often  carry different whale lice species.

In fact, right whales alone are  home to multiple kinds of cyamid. The two that live on healthy right whales  each have their own little microhabitat.   One lives in deep pits in callosities, while  the other one hangs out in more open spots. There’s also a species that’s orange  instead of white and usually only   infests whales that are very young or  sick.

Which means looking out for these   orange cyamids is one way scientists  can tell how healthy a right whale is! That’s not all they’re good for, either.  Because they’re literally stuck to their hosts,   whale lice can tell researchers a lot about  what goes on when they’re not looking. For example, looking at genetic differences  between the lice that hang out on the three   species of right whale has helped scientists  figure out a lot about those species’ history.

Cyamid populations closely track the size and  structure of the populations of their host   whales -- but because each individual whale  can host thousands and thousands of cyamids,   there are way more individual  cyamids than there are whales. This means there can be a lot more genetic  variation among whales’ cyamids than among   the whales themselves. And that means more  information for scientists to pore over.

From tracking the genetic variation  of their lice, we’ve inferred that the   three species of right whale split from  each other around six million years ago. The two species in the northern hemisphere  are now endangered, thanks to whaling. But   lice genetics also show that they were once as  abundant as their southern hemisphere cousins.

And in one of the most unusual cases of  all, researchers once found a beached   right whale calf that was covered in a species  of cyamid normally found on humpback whales. That suggested the right whale  had been cared for by humpback   whales -- it was raised by a different species. So not only are those white  patches on whales alive,   they’re also really useful to biologists who  study whales and work on their conservation.

It turns out that these “lice” aren’t just gross  creepy crawlies -- they have stories to tell. The more we understand   about the world, the more we can unravel stories  like these. And the courses at Brilliant can help.

Like their course on Statistics Fundamentals,   which is all about the math we  use to help decide what’s real. Brilliant has loads of other courses too,  in math, science, engineering, and computer   science. They’re all designed to be hands-on,  with guided problems and interactive quizzes.

The first 200 people to sign up at  Brilliant.org/SciShow will get 20% off an   annual Premium subscription. And by checking  them out, you’re also helping support us,   so thank you. {♫Outro♫}