A Blog by Carl Zimmer

Freeing Animals From Our Evolutionary Traps

James Snyder noticed one day that a frog had climbed onto a tree in his backyard in southern Florida and swallowed one of his Christmas lights. He snapped this eerie photo in which the light glows through the frog’s stomach, like a herpetological holiday ornament.

This frog’s behavior seems weirdly stupid. But there’s actually a wisdom of sorts in swallowing a Christmas light–if you’re a Cuban tree frog, that is. For thousands of years, the only glows  your ancestors ever saw on a tree came from luminescent insects. If they responded to a little glow by attacking, they got a meal. They were more likely to survive and have baby frogs. The frogs that didn’t respond? Some of them may have done just fine. But others may have gone hungry. The males might have struggled to attract a mate; the females might have laid small eggs that failed to develop.

Natural selection laid down this response in Cuban tree frogs, in other words. Christmas lights have only recently come into their lives, and natural selection has shown no sign yet of striking the “attack glowing light” behavior off the menu.

Snyder’s glowing frog is one of the prettiest examples of a surprisingly common thing that happens when animals come into contact with humans. We have altered the environment in a vast number of ways, both small and large. And when animals try to read the cues from our human environment, they can get tricked. They can end up doing something that kills them, loses them the opportunity to reproduce, or simply wastes their time. Scientists call these situations evolutionary traps.

In the journal Trends in Ecology and Evolution, Bruce Robertson of Bard College, Jennifer Rehage of Florida International University, and Andrew Sih of University of California, Davis, take a look at a lot of documented examples of evolutionary traps and try to come up with a theory for them. They would like to be able to predict when traps will occur, and find a strategy to prevent evolutionary traps from endangering species.

Some evolutionary traps, like the Christmas lights, play on the visual strategies animals use to find prey. Albatrosses will peck at brightly colored pieces of plastic floating in the water, for example. It’s a response that used to give them energy but now can fill their guts with trash. Some of the species we’ve moved around the planet are tricking native predators. A native North American wasp used to lay its eggs in a native ladybird insect species. We’ve now imported a new species, which the wasp now prefers. Unfortunately for the wasp, the defenses of the alien ladybird are so strong that it can kill the wasp’s eggs.

Evolutionary traps can even fool animals looking for a mate. In Australia, male beetles of the species Julodimorpha bakewelli, are attracted to the gleaming brown surface of female beetles. Beer bottles, it just so happens, look a lot like female Julodimorpha bakewelli, and so male beetles can often be found furiously trying to mate with them.

Artificial light can set evolutionary traps not just by creating the illusion of prey, but by throwing off an animal’s navigation. When caddis flies become adults and are ready to mate, they need to get to a body of water. Without Google Maps to help them, they do what their ancestors have done for countless generations: they take advantage of the fact that ponds and streams change the reflection of moonlight, altering its polarization. Unfortunately, large plate glass windows can polarize light in the same way, with the result that caddis flies will sometimes blanket the glass, mate, and lay their eggs there.

Sometimes, an evolutionary trap is fairly harmless. Cuban tree frogs don’t swallow Christmas lights that often, and Snyder found that his hapless visitor was actually still alive and eventually spat out its mistaken prey. But in other cases, a mistake can be catastrophic. Some beetles lay their eggs in fallen trees. If they make the mistake of laying their eggs in trees that people have cut down for lumber, their offspring will end up dead in a mill.

These evolutionary traps can be especially dangerous when they’re more attractive than their natural counterpart. Beer bottles, it turns out, throw male beetles into a mating frenzy, because they have exaggerated versions of the visual cues on female beetles. Super-attractive traps lure away a greater fraction of a species to their doom. Robertson and his colleagues surveyed 445 scientific studies of evolutionary traps and found that 86 percent of the severe ones involved this combination of danger and heightened attraction.

The scientists also ranked the ways in which we humans create traps. Invasive species top the list. Next comes agriculture and forestry. Some birds, for example, usually prefer to build their nests at the edges of forests, so that they can fly a short distance to find food in open spaces. These birds are attracted to thinned forests and the edges of cleared land. Unfortunately, so are mammal predators that eat them. Buildings and roads create traps, as well as artificial lighting that goes with it. Sea turtles that lay their eggs on beaches near hotels may head inland instead of going out to sea, fooled into thinking hotel lights are the moon over the ocean.

Ironically, evolutionary restoration projects can also create evolutionary traps for the very species conservationists are trying to save. To increase the eggs laid by Coho salmon, conservation managers have attracted the fish to streams where farmers later draw down the water for their crops. The salmon that hatch from the eggs get stranded and die.

Any explanation of evolutionary traps has to account not just for why some species fall into them, but why so many don’t. Fortunately, there’s been a lot of research on how animals respond to different stimuli, and so Robertson and his colleagues have adapted these findings to the question of when evolutionary traps work. They predict that evolutionary traps are more likely to work the more they resemble a cue animals relied on in the past–especially if that cue was reliable. Some cues are especially important for animals to respond to, the scientists also point out. Rejecting them can have disastrous consequences. If an ecological trap resembles one of these essential cues, animals will be less likely to reject it.

Once animals fall into a trap, they may go in one of two directions: escape or doom. Some animals may be able to learn from experience that a cue they used to rely on now brings them grief. Of course, some lessons are easier to learn than others, and some animals are better at learning than others.

Evolution can also spring animals from an evolutionary trap. If an animal is born with genes that lower its preference for a cue, it may be less likely to die–and more likely to pass on its genes. Even if animals don’t evolve this new behavior, they may still persist. There may still be enough good habitat where they can reproduce, so that their entire population doesn’t get sucked into a trap. But if a trap is too potent and animals can’t lose their attraction to it, extinction is a real risk.

One way to reduce that risk is to get rid of the trap. Take away the beachfront lights that fool sea turtles, for example, or block them by restoring sand dunes. Move lumber out of forests before rare beetles try to lay their eggs in them.

It’s also possible to take away an evolutionary trap’s allure. Solar panels, for example, turn out to be very attractive for aquatic insects because their polarized dark surfaces resemble water. But scientists have discovered that all it takes is a thin white border around a solar panel to make it unappealing to the insects.

To save some species, in other words, we may need to learn how to break our spells.

14 thoughts on “Freeing Animals From Our Evolutionary Traps

  1. Fascinating. I’ve been thinking a lot about this recently while watching my mother-in-law’s many birdfeeders posted around the house. One of these is adhered to a window, so we can watch the birds come up to feed from our living room. Some of them are easily scared away by our movements, while others have a shorter flight distance. It reminds me of the domestication of dogs, and where the less-frightened birds are now getting the most feed:


  2. This is a great article, and the TREE paper is also excellent. My only beef is that I question the idea that these “traps” must always be negative. It seems to me that they would be equally likely to produce “positive” effects as negative effects. E.g. bats learn to feed around street lights, or disturbance loving plants that do very well in cities.

  3. I love it. It’s about time people started trying to find ways to help animals out of our evolution. We are not the only animals put on this planet & call it home, so it not fair that will kill off other species or endanger this lives as they know it.

  4. I want to know if we have helped that frog to get the bulb out… Please do let us know. It’s all about love in the end, if we creatures on earth love and respect each other’s existence, we will have less evolutionary traps… God bless…

  5. Very interesting article, it shows how simple it could, sometimes, be to reduce our impact on the wildlife. Unfortunaly, not many are the people who know about it.

  6. It’s funny, I was thinking about the poor beetle mating with beer bottles and realized, “Aren’t humans being caught in our own evolutionary traps?” Isn’t online pornography an evolutionary trap? Humans all over the world are addicted to pixels that are arranged to provoke human sexual desire.

    There’s also the influx of cheap calories into our environment, promoting the epidemic of obesity in America. There’s also the influx of social information through sites like Facebook, which triggers our need for social “gossip” and similar data that was important for when we started to gather into tribes.

  7. In the 1980’s in the Midwest, hummingbirds were attracted to red insulators for electric fences. The insulators looked like very large bilaterally symmetrical flowers with a tube leading to nectar. If the hummingbirds probed the tube in the insulators, they touched the nail holding the electric wire to the fence. If they also touched the wire itself, they died. Evolutionary trap. Fortunately, the company stopped producing red insulators. They still produced other colors, and many of the farmers who had put up the red insulators replaced them.

  8. I second Jerry’s comment – that human artifacts and actions can create environments that promote the propagation of a species. Plastic is now everywhere in the environment which means that microbiology with its rapid mutation rate will at some point evolve to consume plastics. Evolution is blind to what humans call “good” or “bad”, it simply works with what is given.

  9. I think there are a couple important misunderstandings to dispell:

    Evolution “works with what it is given”, but those new traits that it “works with” only occur by random chance. There is no “bacteria WILL evolve to eat plastic” because an environmental stimulus cannot cause a new trait. New traits occur by mutation, and mutations randomly; they do not occur when they are “needed”. Evolution is blind, period, because evolution is simply the change observed in subsequent populations; evolution is not the process by which that change occurs. Some bacteria can metabolize plastic, but there’s no way to guarantee that all bacteria exposed to plastic will pick up those necessary genes, such as is possible through transformation. It’s unlikely to such an extent as bordering on impossibility that bacteria would independently mutate “in order to” digest plastic just because it’s there. By that same logic, turtles “should evolve” a way to distinguish moonlight from hotel lights.

    To say that “traps can be positive” I think misinterprets the meaning of the term being used here; “trap” implies that it is having a negative effect. True, it can’t be said that human influence has no positive effects, but if a particular human influence benefits an animal population, does it still fall under the provided definition of a “trap”? If you factor in the potential for getting hit by traffic into the learned behavior of seeking out roadside lightposts as food sources, does it still count as a positive effect of human influence?

  10. I was wondering along the same lines as Ryan Somma, there must be human evolutionary traps. Maybe the taste for sweets and carbohydrates when caloric needs are now so easily met? Pornography could actually stimulate more sexual behavior. And an alien scientist observing the use of condoms might conclude it was an evolutionary trap, when what’s really needed is a greater understanding of human society. Who knows–maybe the beetles knew full well that they were mounting bottles, but for them it was like watching pornography.

  11. Ryan Somma is correct, there are evolutionary traps that humans are susceptible to. Among them:
    Eating high fat/high sugar food – humans have evolved to prefer these in an era they were precious scarce and unpredictable resources. In the modern world though we’re literally having them thrown at us 24/7 and we simply haven’t evolved to say “no” to “do you want awesome food?”. And this in a nutshell is why diets fail, the people who need them most are also the ones they’re least suited for them.
    Adoption – how is adopting babies an evolutionary trap? Simple, when they’re unrelated to us raising them causes us to waste precious energy on something that not only will not result in increasing our evolutionary fitness but may actually decrease it if it results in us investing less in our kin. In the past if a child showed up on our doorstep it was probably a relative, maybe our sister died and nobody else was available to raise her progeny. Now though you have institutions that specialize in throwing cute kids at potential parents without regard to how closely related they are.
    Addiction in general is the result of evolutionary traps, and it comes in many forms. Usually this is because it gives us the sensations we instinctively yearn for without us having to do the work in real life. We’re wired to take the easiest route to happiness we can find and dammit if that’s through a joint so be it.
    Artificial lights can be just as confusing to us as they are to bugs. Too much light throws off our circadian rhythm so we have trouble sleeping. It’s all to easy and common for people to fall into a positive feedback loop where they stare at a screen which emits light in wavelengths that tell your brain it’s day. Then when they stop to go to sleep their brain won’t let them since obviously it doesn’t just go from being noon to night immediately. This leads to them fidgeting in bed and picking up the same device that was telling them it was still day and continuing the illusion. Young people are particularly susceptible to this trap due to widespread adoption of handheld gadgets with backlights like mobile phones.
    Unrealistically attractive artificial people – Not that entirely effective in many cases, but can cause people to waste time pursuing a mate that doesn’t exist simply because that character’s design exuded high quality mate cues to a higher degree than real people. Women have long groused about how a normal woman cannot hope to compete with a super model, especially now that they’re photoshopped to truly unnatural hotness. Girls are also susceptible to chasing artificial men, like those Twilight boys or Final Fantasy 7’s Sephiroth. Though it’s hard to think of a good reason to dissuade teenager girls from this particular trap if it keeps them from getting pregnant.
    Games and other forms of escapism can turn into traps if they meet our psychological needs better than real life does. Though, I suppose there are times when this is a blessing, real life doesn’t always offer people that which they truly desire. For example lets say someone has genes that code for them wanting to go off the edge of the map and carve out a territory wild west style – we’ve pretty much explored all the habitable areas of the planet, so unless you’re an exceptional person this path will never appear for you, but games can still simulate the experience. Likewise lets say you’re an absolute toad of a man who can only get a girl by paying for one, isn’t it better to indulge your fantasies with porn than to live a life of frustrated lust? You’re not going to reproduce either way so you might as well take the more attractive road.

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