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When you see a black cat, you might think of witches and goth bands, but they're also a great example of a melanistic animal, and they're not the only ones!

Hosted by: Stefan Chin

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Sources:

Pygmy Grasshoppers
https://doi.org/10.1038/srep22122
https://doi.org/10.1016/j.tree.2003.09.006
https://doi.org/10.1007/s00442-007-0876-y
https://doi.org/10.1111/j.1558-5646.2011.01324.x
https://doi.org/10.1002/ece3.338
Black “Panthers”
https://doi.org/10.1016/S0960-9822(03)00128-3
https://doi.org/10.1371/journal.pone.0050386
https://doi.org/10.1371/journal.pone.0170378
https://doi.org/10.1371/journal.pone.0226136
https://doi.org/10.1111/j.1469-7998.2010.00731.x
https://doi.org/10.1016/j.biocon.2012.12.016
Gray Wolves
https://doi.org/10.1126%2Fscience.1165448
https://doi.org/10.1126%2Fscience.1147880
https://doi.org/10.1093/molbev/msy031
https://doi.org/10.1007/s10344-013-0703-1
https://doi.org/10.1093/jhered/esu024
https://doi.org/10.1126/science.1209441
https://pubmed.ncbi.nlm.nih.gov/26988852/
Hornworms
https://doi.org/10.1007/978-1-4020-6359-6_3055
https://doi.org/10.1126/science.1118888
https://doi.org/10.1371/journal.pone.0011563
https://doi.org/10.1016/j.ibmb.2009.01.008
https://doi.org/10.1126/science.311.5761.591a
Ayam Cemani
https://doi.org/10.1371%2Fjournal.pone.0173147
https://doi.org/10.1534/genetics.111.136705
https://doi.org/10.1186/s40104-018-0272-y
https://doi.org/10.25226/bboc.v137i1.2017.a9
[ INTRO ].

Whenever a black cat crosses your path,   you should… not worry at all because  that’s just a silly superstition. In fact, black cats may be the  coolest of all cats, but I digress.

I bring them up because they’re  a great example of a melanistic   animal—an unusually dark variety of a creature. Melanistic forms—especially  ones that are all black—are   considered rare overall, but they  occur in a bunch of animal groups. And there are lots of ways that animals  can get that super-dark coloration.

But what’s really cool is that  understanding the biology of   each goth variety can teach us  about the animals themselves. Up first, pygmy grasshoppers.  They come in a variety of colors,   from gray to brown to totally black. The darker colors come from melanin pigments— the same stuff that makes  that black cat’s hair so dark.

But while we have a decent idea of  how birds and mammals make melanin,   precisely how insects do it remains unclear. What we do know is that populations  of pygmy grasshoppers tend to go goth   some years more than others. Sometimes, up to half of the grasshoppers are  black.

Other times, they’re less than 10 percent. And that’s because black pygmy grasshoppers are an  example of what scientists call “fire melanism.” It’s not that they’re actually charred, though   charcoal is the key: after a fire, much  of the grasshoppers’ habitat is black. And non-black grasshoppers stand  out against this background,   which makes them easy pickings for predators.

All of this means, most of the grasshoppers  you see after a fire are black. This doesn’t last, though—and  why is still up for debate. There seems to be some complex interplay between  coloration, reproduction, and environmental   factors like temperature, which ultimately leads  to fewer black grasshoppers in each generation.

Whatever the reason, all this ultimately  means that in years without fire,   the balance shifts back in the other direction. from Bagheera to T’Challa,   we all love black panthers. But if you’re talking  about a wild feline, well, there’s no such thing. When we say “black panther,” we’re usually talking  about one of two completely different creatures:   black jaguars, which live in  the Americas, or black leopards,   which live on the other side of  the globe in Asia and Africa.

And not only are these two different species  of cats, they don’t turn black the same way. Jaguars are born black when they have a chunk   missing from the gene for  melanocortin-1 receptors. These receptors live on the surface  of cells that produce melanin.

Or, I should say, “melanins,” because  melanin actually comes in two varieties. And when melanocortin-1 receptors are activated, they tell those cells to  produce the darker eumelanin   instead of the yellowish to reddish phaeomelanin— turning hairs, and sometimes skin, black. Now, it’s not clear if this deletion  turns the receptor on full time,   or messes with one of its off switches.

But either way, even if only one of a jaguar’s  two copies of this gene behave this way, the entire cat goes black—making  the mutation dominant. And this happens often enough that nearly 11  percent of jaguars end up with black coloring. Now, mutations like this are fairly common—as  far as melanistic variations go.

Something   similar is found in species like  cows, horses, and squirrels. But it is not how melanism  happens in black leopards. In leopards, melanism happens when a single nucleotide gets changed in  the gene for agouti signaling protein.

This protein acts as an antagonist  to the melanocortin-1 receptor— so, it’s one of the “off” signals I mentioned. And that single, small change   makes it so the agouti signaling  protein can’t turn the receptor off. So, even though melanism in leopards comes  from a totally different mutation in a   totally different gene than melanism  in jaguars, the effect is the same: melanocortin-1 receptors get to throw  a rager without being shut down.

But, since this is a loss-of-function  thing, to go fully black, a leopard needs both copies of the gene to  be this way, which makes the trait recessive. Still, black leopards account for  roughly 11% of the global population— similar to the prevalence of black jaguars. And leopards aren’t the only animals  that become melanistic this way.

For instance, in some parts of Europe,   up to a quarter of roe deer are  black thanks to a similar mutation! Also, there are a number of other  cats with this type of melanism. A totally separate change messes up agouti  signaling protein in Asian golden cats, for example, and there’s  another one in house cats.

Which brings us to a bigger point:  regardless of how it happens, melanism overall is kind of a cat thing. So far, scientists have found  melanistic forms in 14 of the   40 cat species. The trait has evolved  independently at least five times!

We don’t know exactly why melanism is such a big  deal for cats, but part of it could be camouflage,   since cats often hunt at night or in  dark environments like dense forests. But it has to be more  complicated than that. Because, if black coloring were a huge  advantage for nocturnal hunters,   then basically all cats should  be black.

And they’re not. A 2019 study suggests that’s because  they need visual communication. See, many cats have small,  white marks on their coats, which help them spot and recognize each  other when there’s very little light.

So, if those go black with the rest of the  animal’s fur, some things become harder— like keeping tabs on cubs around dawn and dusk. And cubs that get too far from  their mother might get eaten   or hurt. Because they’re just little babies.

This tradeoff also explains some of the  stark geographic patterns we see in melanism— like, that in Africa, almost  no leopards are melanistic,   while on the Malay peninsula,  nearly all of them are! That’s likely because there are lots of  tigers there. It turns out leopards that   share their territory with tigers  are more active during the day – probably to avoid competing  with their larger cousins. [optional: Because if The  Jungle Book taught us anything, it’s that a black leopard just  can’t win against a tiger.] This switch to daytime means most of their  activity takes place in brighter conditions, when the white spots don’t  make as much of a difference.

So their black color just makes them  stealthier in the shadows of the jungle. The name “gray wolf” is kind of a  misnomer, because not all wolves are gray. Some are brown, some are white and,  of course, some of them are black.

Black wolves get their color from  a mutation in a β-defensin gene. The protein this version of the gene produces  competes with agouti signaling protein   for melanocortin-1 receptors. Except, it  doesn’t turn off the receptor when it binds.

Instead, it prevents agouti from getting  in there and shutting things down,   leading to an increase in eumelanin. Since it’s an “if present” thing, blackness in   wolves is dominantly inherited.  But it’s not found everywhere. In some parts of North America, up  to two-thirds of wolves are black.

But in the rest of their native range— which extends through Europe and into Asia— black coats are rare. Part of the reason for that is  probably because those North  . American wolves got their blackness from dogs.

Researchers aren’t sure exactly when or  where the gene jumped from dogs to wolves, but they think it was in the Yukon Territory area, upwards of 7 thousand years ago! And there’s a more recent  example of this happening. In 1976, melanism showed up  in a pack of Italian wolves, and researchers traced the trait to local dogs.

In both cases, once the gene  arrived, it took hold really quickly, which suggests it offers some  kind of strong advantage. That may come from the fact that  defensins are part of the immune system. So, wolves with this particular  β-defensin may be a bit tougher.

And in fact, melanistic wolves do seem to be more  resistant to a disease called canine distemper. Plus, some models suggest black  wolves live longer and are more   likely to survive from one year to the next. But wolves that have two copies of this  melanistic gene have lower reproductive success.

So it seems like there’s  some kind of downside to it — though, as of yet, we don’t know what that is. But still, it’s probably why black  wolves haven’t totally taken over. Now, hornworms can somehow eat an entire  tomato plant while remaining totally invisible.

If only they weren’t the exact  same color as tomato leaves…. Well actually, some of them aren’t. Tomato hornworms—and their close cousins,  tobacco hornworms—can be melanistic.

But unlike everything we’ve talked about so far, their melanism isn’t wholly dependent on genes. They’re examples of polyphenism, which is when an animal’s genes  can produce two or more different   looks under variable environmental conditions. And in the case of hornworms, some can  change color when the temperature changes.

It starts with a hormone called juvenile hormone. Scientists working with the tobacco  hornworm discovered that low levels of it   ultimately lead to high levels  of the neurotransmitter dopamine. And dopamine helps signal  for an increase in melanin   in the caterpillar’s protective  outer layer, turning it black.

So, the less juvenile hormone a  caterpillar has, the darker it is. And, in general, lower temperatures  depress juvenile hormone levels. However, usually, caterpillars  produce so much juvenile hormone   that natural temperature variations  aren’t enough to turn them black.

For blackness to happen, caterpillars  have to have a mutation that lowers   the overall level of this hormone in their bodies. And those caterpillars only remain black if  the temperature stays below 28 degrees Celsius. Any higher, and their bodies  ramp up the production of   juvenile hormone enough to turn them green again.

When you consider how good green hornworms  are at vanishing into their habitat, it might seem a little strange that  black caterpillars exist at all. Because melanin absorbs light and heat,   black caterpillars may have an  advantage in colder temperatures. Because it’s not like a green  caterpillar can put on a tiny,   caterpillar shaped sweater in a cold snap.

No list show about melanism  would be complete without a bird,   because melanism is the most  common color morph in birds. It can be caused by a lot of genes, and it doesn’t  always look the same from species to species. But one bird — called the ayam cemani  — takes this to a whole new level.

For all the elegance of its name,  the ayam cemani is a chicken. They’re native to Indonesia, where they’re prized   not for their drumsticks but  for their bizarre coloring. See, this chicken is an example of  fibromelanosis, where e-ver-y-thing goes black.

We’re talking black feathers, black beaks,  black skin, black meat, and black bones. This is all driven by a gene called endothelin 3. When the animals are developing, activation  of endothelin 3 in different tissues   tells melanin-producing cells  to migrate to those places.

And these chickens have an extra copy of the gene, so their bodies produce  pigment cells in overdrive. They’re not the only ones  with this sort of melanism,   but they’re certainly the most extreme case. The trait probably arose before  the domestication of the species, perhaps 6 to 9 thousand years ago.

But then,   people kept it around and made it more  prevalent though selective breeding. It’s not clear if the extra endothelin 3  gene gives any advantages to the animals. But, despite how weird they look, there don’t seem   to be any consequences for  their health or longevity, .

So it really is just a weird thing that teaches  us more about how chickens get their colors. In the end, whether we’re talking about  all-black chickens or not-so-gray wolves,   it’s clear that melanistic animals  are more than striking to look at. They can also teach us a lot about  genetics and how an animal’s looks   impact its ability to survive.

So the next time a black cat crosses your  path, just remember that it’s not bad luck— it’s got a super goth agouti  signaling protein gene. Thank you for watching this episode of SciShow. Now, if you’re a fan of gothic looks,   you might like our episode on how scientists  made the darkest material we know of.

And if you just love science in general  but you’re not a subscriber yet, just click that little button below the video. We deliver all kinds of awesome science  straight to your YouTube feed on the daily. Thanks for watching SciShow! [ outro ].