In a world so thoroughly reshaped by human hands, animals often bump into novel ecological conditions — problems evolution didn’t prepare them for. Plastic items look like food, but they’re indigestible; artificial lights look like stars, but they’re useless in navigation; dead logs look like prime real estate, but they’re often bound for the woodchipper. Natural selection couldn’t foresee all these deadly new surprises, known as “evolutionary traps,” and thus, animals lack the behavioral tools to handle them.
Nature is, of course, always weeding out individuals whose behavior isn’t well-calibrated — the nocturnal possum that comes out an hour early and exposes itself to predators, for example. What’s different about evolutionary traps (and what makes them so dangerous) is that when an animal falls into one, its behavior “is actually perfectly miscalibrated,” says Bruce Robertson, an associate professor of biology at Bard College. “They’re preferring the worst possible thing,” like a possum that whoops in open fields all afternoon.
With enough time and genetic variation, any species can hypothetically escape these pitfalls by evolving better behaviors. But given the breakneck pace of environmental change in the Anthropocene, many can’t adapt fast enough. “Traps will cycle populations toward extinction extremely rapidly,” Robertson says. “They’re like demographic black holes.”
Animals of all kinds, from beetles to birds, are bedeviled by evolutionary traps. Here are some of the strangest and most troubling, along with a few hopeful reminders that we can often undo the damage — at least when we act before it’s too late.
1. Fatal Attraction: Australian Jewel Beetle
One common type of trap arises when an option that decreases evolutionary fitness appears more attractive than the higher-fitness alternative — ice cream for dinner, anyone? Under this category, we find what must be the most tragicomical trap of all.
In September 1981, two entomologists were wandering outside Dongara, a small town on Australia’s western coast, when they stumbled upon an unlikely tryst: On the ground was a discarded beer bottle, and on the bottle was a male Australian jewel beetle, its genitalia protruding. Without getting too much into the steamy specifics of insect reproduction, the scientists later explained that this confused creature was “attempting to insert the aedeagus.”
Over their weekend field trip, Darryl Gwynne and David Rentz witnessed several more amorous encounters. And there was a pattern: Although the beetles had peculiar taste in mates, they were not undiscriminating. They exclusively courted “stubbies,” short bottles with shiny raised bumps that resemble the reflective forewings of real females, only far more desirable.
In the parlance of evolutionary psychology, this is a “supernormal stimulus,” an exaggerated version of some ordinary object or event that prompts a much stronger response. It’s the same dirty trick that makes social media (or candy, or alcohol, or gambling) so seductive. We simply didn’t evolve to cope with such intense psychological rewards, and we are in thrall to them just like the beetles were to their bottles.
Upon mounting one, males were more than delighted. They were entranced. They clung to their beloved with no regard for the consequences. Baked by the summer sun and gnawed by hordes of ants, they held fast — such was the allure of the stubbie. Yet they were wasting their time, ignoring actual females and dying in delusional acts of futile copulation.
Read More: 6 Unusual Traits of Animal Evolution
2. Dangers in Disguise: Death Adder
A second form of trap is sprung when some formerly beneficial behavior suddenly turns destructive — but without warning. The death adder (a highly venomous Australian snake) found itself in one of these predicaments, giving its name an ironic twist.
For untold generations, the species has survived by charming frogs to their demise with a method known as caudal luring. They wiggle their tail to mimic a worm, and when some unfortunate creature approaches, thinking itself the hunter, they ambush. That strategy worked great with native species, whose chemical defenses degrade quickly after death. The snakes developed a formula: see amphibian, wiggle tail, get meal. But then came the invasion.
In 1935, non-native cane toads were released in Australia to control agricultural pests. They’ve since overrun the continent, numbering more than 200 million, and in 2006, a team of researchers at the University of Sydney reported that death adders lure these newcomers in addition to their historical prey.
The problem is that cane toad toxins take longer to degrade. Snakes eat them too soon, and up to half die as a result. “Adaptations that enhance fitness in one situation,” the researchers write, “can become liabilities if circumstances change.” We humans — routinely poisoning ourselves with sugary foods that our ancestors only encountered in small doses — can sympathize.
The death adder’s plight illustrates another important point: In a 2013 review of the relevant scientific literature to date, Robertson and his colleagues concluded that exotic species are the single largest cause of evolutionary traps.
Read More: Rapid Evolution Changes Species in Real Time
3. Worst of Both Worlds: Olive-Sided Flycatcher
Jewel beetles and death adders may have had a tough break, but the stars are precisely aligned against olive-sided flycatchers, a species of sparrow-sized birds found across Canada and the western U.S. In their case, the trap isn’t just more attractive or more harmful than the alternative — it’s both at once.
Olive-sided flycatchers nest in sparse forest clearings, which are typically a product of wildfire. But, selective logging (removing some trees while leaving others) creates superficially similar habitat, with one critical difference: It draws twice as many predators as natural ecological disturbances. Nevertheless the birds seem to prefer logged areas, perhaps because they offer more suitable nesting trees.
In a 2007 study, Robertson compared survival rates for eggs and young birds between two sites — one logged, one burned — near Glacier National Park. The statistics were grim: In the logged plot, just 30 percent of nests yielded a fledgling (a bird that has learned to fly), compared to more than 60 percent in the burned plot.
This warped environment, he wrote at the time, has decoupled “the cues that individuals use to assess habitat quality from the true quality of the environment.” The flycatchers follow a time-tested decision-making procedure, one that steered their ancestors straight — only now, it betrays them.
Read More: How Do Animals Evolve to Be So Colorful?
4. Good Intentions: Be'er Sheva Fringe-Fingered Lizard
Perhaps the most disturbing fact in Robertson’s 2013 review is that the third largest source of evolutionary traps (after invasive species and agriculture) is ecological restoration. The Be'er Sheva fringe-fingered lizard fell victim to one such project.
Around the turn of the century, Israel’s forestry department converted part of the Negev Desert into a savanna, planting trees in a mosaic of natural and altered patches to boost species diversity. Promising as the effort seemed, it had an unintended side effect: The lizards, which were already critically endangered, disappeared from the area within a decade.
In 2010, Dror Hawlena, then an ecologist at Ben-Gurion University, argued that the trees could explain their extirpation — so many new perches for avian predators. The lizards, being evolutionarily accustomed to barren scrubland, probably wouldn’t recognize this kind of hazard. In other words, they couldn’t tell the difference between a safe environment and a treacherous one.
Discouraging as it may be that we imperil animals, even when we mean to do the opposite, evolutionary traps aren’t necessarily cause for hopelessness. They can often be reversed: By presenting the evidence from their research, Hawlena, and his colleagues convinced land-management agencies to halt “savannization” in Be'er Sheva fringe-fingered lizard habitat.
Read More: Island Lizards Shrink and Horses Get Bigger Because of These Evolutionary Patterns
5. Disarming the Trap: Sea Turtles and Aquatic Insects
At the end of the day, Robertson is optimistic. “Most of these problems,” he says, “seem to be tractable.” Case in point: In recent years, many Florida beach communities have taken steps to reduce light pollution, a source of deadly confusion for the region’s sea turtles.
When they hatch, turtles head straight for water. To guide them, they instinctively look for the brightest patch of sky, which is generally over the ocean. But these days that intuition sends them in exactly the wrong direction; thousands of hatchlings end up stranded on the sand, where they die of dehydration or get snatched by gulls.
In this scenario the solution was literally as easy as flipping a switch (or using less disruptive long-wavelength lights). And many traps can be undone without much trouble, even when they involve widespread human infrastructure.
For instance, aquatic insects are attracted to solar panels because they polarize light in the same way water surfaces do. Females sometimes choose them as egg-laying sites, and the eggs, of course, fail to hatch. But as it turns out, there’s a simple fix: In 2010, Robertson and a team of Hungarian scientists showed that insects lose all interest if you just put a grid of white lines around the black cells.
Read More: Living Fossils Revealed: The Hidden Evolution of These 4 Ancient Species
6. Traps as Tools: Goats in the Galapagos
Used wisely, evolutionary traps can even do good work, mainly by eradicating invasive species (themselves, a major cause of traps) via deception. Aptly enough, the best example comes from the very place that inspired Charles Darwin’s theory of evolution: the Galapagos Islands.
Centuries ago, during long ocean voyages, sailors would stock remote islands with livestock to ensure a food supply for the journey home. Predictably, those herds got bigger and bigger, out-competing native species and nudging many fragile ecosystems toward disaster.
By the late 1990s, several islands in the Galapagos were overwhelmed by some 200,000 feral goats, and rare tortoise populations were plummeting. So, conservationists launched Project Isabela. They killed 90 percent of the goats through aerial hunting — sniping them from helicopters — but as the herds thinned, the survivors became harder to detect. The final 10 percent demanded more cunning.
Enter the Mata Hari goats: Sterilized females with hormone implants to induce long-term estrus (also known as Trojan females). They emitted irresistible pheromones wherever they went, and the males came flocking, only to be picked off. Each one was “like a little vacuum cleaner,” Robertson says, “yanking in goats from around the island.”
As of 2006, the Galapagos were goat-free. Putting aside the ethics of culling 200,000 animals who wound up on the archipelago through no fault of their own, the scheme succeeded. For all the harm evolutionary traps continue to cause, Robertson says, “You can actually use [them] as tools to solve problems.”
Read More: How Did Animals Get Their Spots and Stripes?
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
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Cody Cottier is a contributing writer at Discover who loves exploring big questions about the universe and our home planet, the nature of consciousness, the ethical implications of science and more. He holds a bachelor's degree in journalism and media production from Washington State University.
