Humans are pretty special among mammals.
Like all the others, we have hair on our bodies and feed our young breastmilk.
But we have one anatomical quirk that sets us apart.
And, I’m not talking about our big brains.
Or the “walking on two legs” thing either.
I’m talking about a muscle that sits just under the skin of most mammals …
I mean, most of them except us.
The layer of muscle in question is called the panniculus carnosus,
which translates to “fleshy cloth” in Latin .
True to its name, it’s a thin sheet of muscle attached to the skin and connective tissus
across the trunk, back, and upper parts of the limbs in the animals that have it.
The muscle primarily just twitches really fast, which jiggles the skin above it.
This is the muscle that lets a horse twitch the skin
at the base of the neck called the withers, for example.
Those twitches help to dislodge insects, birds, or other foreign objects
so that animal doesn’t need to use its limbs or turn its head to get rid of hitchhikers,
which is especially great if you’re an animal that’s somewhat lacking in shoulder mobility.
Not only that, but having a big sheet of muscle could help with thermoregulation
thanks to those fast twitches, and might even speed up wound healing.
And echidnas have taken the panniculus carnosus muscle and really rolled with it.
Echidnas are monotremes, which are the group of mammals that lay eggs.
They have sharp spines all over their bodies, kind of similar to porcupines.
But unlike porcupines, echidnas have adapted to roll into a tight ball
and point their spikes outward as a defensive strategy
which they can do thanks to their panniculus carnosus muscle.
So if this muscle is so great for other mammals, why don’t we have it?
Although we don’t have skin-twitching muscle sheets,
anatomists think we have several muscles derived from panniculus carnosus,
including some in our scalp, face, neck, arms, trunk and scrotum.
They don’t really do much though, and most of us can’t move them at will.
And how much of those panniculus carnosus remnants are left in us varies
from person to person, which further tells us that it’s just not that big of a deal for humans.
It turns out that if we want to know more about why we don’t have this muscle,
we can look at our relatives, the apes.
Most other primates do have this muscle, including lemurs, lorises,
and several species of monkeys.
But it seems like somewhere along the evolutionary line of apes, most of us have lost it.
Dissections have shown that most gorillas don't have it,
so for those that do, it's probably vestigial or unimportant, like it is for us.
And as it turns out, there’s a pretty simple reason that we apes
don’t really need to get rid of bugs with skin-wiggling.
Another thing that apes have in common is our flexible forelimb and shoulders.
We can reach more parts of our bodies with our upper limbs,
which means that we can swat at bugs more easily than, like, a horse.
And if having a big sheet of muscle no longer increased those apes’ chances of survival,
natural selection wouldn’t keep that expensive tissue around - it’s use it or lose it!
So the next time you have to put something down to swat at mosquitoes,
you can thank evolution for taking away the muscly fly-swatter
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