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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.
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