Sometimes natural selection doesn’t  work the way we often think it does.

Instead of selecting for the most successful  variation, it selects for variation itself. This is the eyelash viper, a tree-dwelling  venomous snake found in Central and South America.

It’s known for its namesake head scales, and  for coming in a stunning variety of colors. And until very recently, eyelash  vipers were thought to be all one widely distributed species with  a whole lot of color morphs,   but it turns out that they’re more  like five to nine different species. Which might make it seem like  we’ve got them all figured out – if they’re different snakes, of  course they’re different colors.

Except…that’s not it. Each species comes in a rainbow of colors… even within the same litter. So, how does that work? [♪♪ INTRO ♪♪] If you want to support the channel,   the Bizarre Beasts pin club is open  for subscriptions for the whole month!

Sign up by August 20th and the first pin you  will get will be this gorgeous eyelash viper. Humans have struggled with the  identity of the eyelash viper,   also known as the eyelash  palm viper, for a long time. Between the mid-19th and early 20th century,  it was identified as four or five different species,    but over time these were mostly  boiled down to Bothriechis schlegelii,   which was considered a single species that just  happened to have a boatload of visible variation.

And this conclusion was  accepted for many, many years. But new research has split this species back  up again to include not only B. schlegelii and   updated reinstatements of those original  species descriptions from way back when,   but also five newly described species, too. So how did we end up with over  a century of disagreement about   how many different kinds of vipers these were?

Along with rattlesnakes, eyelash vipers  belong to a group called pit vipers,   due to the heat-sensitive pits between their  eyes and nostrils, which help them hunt. They’re ambush predators that typically live in  tree canopies, vines, and other foliage to hide. As a result, their scales are  rougher than on most snakes,   possibly to protect them from poky  branches, or to help them climb.

The scales on their eyes that earned  them their common name are believed   to help obscure the silhouette of  the viper’s head while it hides. These specialized scales are  typical of ambushing snake species,   especially in environments with similar cover. These vipers are also relatively small  for a venomous snake of the Americas,   coming in at only 55 to 82 centimeters.

But what really makes eyelash vipers so bizarre – and what made them so hard  to classify for so long – is the amount of variation they  exhibit, not just between species,   but even between individuals of the same species. The different colors of eyelash vipers are likely  for camouflage, helping the snakes both to hunt   and not be hunted, and these colors can vary not  only within a species, but even between siblings. This makes it understandably  trickier to tell them apart – color alone isn’t enough to get a solid ID.

And, to make things even more complicated,   studies of this snake have relied  on very limited observations, rainforests aren’t the easiest places for  humans to navigate, and trees even less so. So the researchers in the  latest study had to create   their dataset by collecting tissue  and DNA samples from individuals,   both alive and preserved, at over three  dozen zoos and museums around the world. The snakes they sampled varied in color  and other physical characteristics,   but were all labeled B. schlegelii.

The researchers used DNA analysis to divide  them up into separate groups, and physical   characteristics confirmed both similarities within  those groups and differences between groups. And these were enough that the  researchers could be confident   in their description of the groups  of vipers as truly distinct species. This is all well and good for at least settling  the mystery of how many species there are,   but it still doesn’t explain why these snakes  look so dang different even when they are related.

It all comes down to something  called color polymorphism – differences in coloration within the same species. Traditionally, we understand natural selection  to mean a given trait being selected for over generations,   so in a case like this,  we could reasonably expect a specific   color morph to provide a competitive advantage  and thus become more common than the others. Instead, the variation itself has  proven to be the real advantage,   allowing the many different  morphs to stand the test of time.

That’s how you end up with a polymorphic species. Color polymorphism seems to be disproportionately  common in arboreal snake species, including the   eyelash viper, and this might hold the answer  as to why such a feature would stick around. The snake’s canopy habitat is full of  differences in both color and lighting,   so there’s no one-size-fits-all color  pattern to help them blend in everywhere.

This means that individuals with  different color morphs can thrive   in different parts of the same forest more  successfully than if they all looked the same. There’s also the phenomenon of what’s  called frequency dependent selection:   having a trait that’s too common  might make you too recognizable. For example, if your prey or your potential  predators get used to what you look like,   you’re a more obvious mark than  if you can keep ’em guessing.

These factors might explain the “why” of eyelash  viper coloration, but how about the “how?” Color is often tied to clusters of similar  genes that have important, non-color functions. By being tucked away in  these more necessary genes,   the colors are likely to stick  around even after many generations. Scientists have begun to identify some of  these possible genes in Asian vine snakes,   and the pretty minor mutations  within them that can lead to   such major variation in what color the snakes are.

But all this variation raises a question: if  so many of these vipers look so different,   how do members of the same species  even recognize each other to mate? They’re probably better at telling  species apart than we humans have   proven ourselves to be, but how do they do it? Snake vision depends on the lifestyle and  environment of the species in question,   and for the eyelash viper, that means very  sharp eyesight for its arboreal niche.

So… can they just see the  different colors really well? Well, they do use their sense of sight in mating,   but mostly for the purpose of watching  a prospective partner’s dance moves – males compete in a display called  the “dance of the adders” in order   to intimidate one another  and win the chance to mate. The difficult step – finding mates in the first place – is more about chemical sensing and smell.

This is the real key to these vipers  recognizing members of their own species   in spite of the dramatic superficial  differences that stump us humans. And although there aren’t any  studies of it in the eyelash viper,   female preference in some species can actually  help reinforce such pronounced polymorphism. It’s a phenomenon called “disruptive selection”,  and it’s when individuals prefer mates at the   extreme ends of the color ranges, and  not just those who are in the middle.

Scientists are still working to better understand  how color polymorphism works in general,   but the eyelash viper at least presents  a nice case study for learning more. So what do we take from all of this? That this snake is cool looking,  and that it confused scientists?

Sure, the eyelash viper’s uniquely   huge color palette has made species  identification pretty challenging. But this fact may actually have major  implications for viper conservation. A healthy array of different color morphs might  be a sign of an especially diverse gene pool,   which could give a species an edge when  it comes to surviving extreme change.

Evolutionarily older snake species are also more   likely to be polymorphic than species  that have branched off more recently,   so variety is believed to have at least  helped them make it through tough times. But the discovery that the eyelash  viper isn’t just the one species we   thought it was tells us that it might  not have quite so big a toolkit for   surviving the accelerating human-caused  habitat loss that they’re now facing. A single species with tons of  variation and a wide geographic range?

Even with deforestation in parts of its habitat,   that’ll still earn you a “least  concern” status on the IUCN Red List. But if that geographic range instead houses  multiple smaller populations of different species,   all independently experiencing  habitat loss and fragmentation? Suddenly we’re looking at potential  extinction risk for almost half of them.

And that means that something as  seemingly esoteric as the accurate   taxonomic classification of a technicolor  tree snake may actually be the first step   toward protecting these beasts so  they can keep on being bizarre. Sign up for the pin club at  BizarreBeastsShow.com to keep the channel going. If you want the viper to be your  first pin, sign up by August 20th.

The pin art for the rest of the  year is SO GOOD, so don’t miss out! We've also go fun new eyelash viper socks over at Complexly.store where a big summer sale is going on now. You can get up to 40% off Bizarre Beasts merch like these signed Emily Graslie prints, our Kakapo 3-D Printed Figurine, and this Locust Sticker Sheet.

And now for some bonus facts… [♪♪ BONUS FACTS ♪♪] We mentioned earlier that the eyelash  viper’s rainbow of colors helps these   snakes blend into a variety of different  micro-environments within their forest homes… and some of the specific  examples of this are pretty wild. Like, one of the places that yellow eyelash  vipers like to hang out is on banana plantations. The fruits attract small  animals like birds and bats,   allowing the vipers to just  sit and wait to ambush them.

Their bright yellow scales blend in  perfectly with the bunches of bananas   and they’ve even been accidentally  shipped out in crates of bananas. Which, no thanks, I prefer my bananas snake-free. Red eyelash vipers, on the other hand, have  been observed hiding in red bromeliads,   waiting to prey on small amphibians.

Another extra fact we learned while making this   episode is that eyelash vipers can  live up to 20 years in captivity,   though they’ve only been estimated to  live up to about 10 years in the wild. And lots of zoos have eyelash vipers, because they  are, frankly, just a really great-looking snake. Have you seen one in your local  zoo or, even cooler, in the wild?

Let us know! [♪♪ OUTRO ♪♪]