This story originally appeared on Atlas Obscura.
In September 1865, a young Charles Darwin first set foot on the Galapagos Islands and started taking notes. These writings, later published as The Voyage of the Beagle, featured long accounts of the island's geology and wildlife. They contained the kernels of what would become his "Theory of Evolution by Natural Selection," and brought him great acclaim as a keenly observant naturalist.
But one of the things that fascinated Darwin most about these islands was that the birds of the Galapagos weren't afraid of him.
"All of them are often approached sufficiently near to be killed with a switch," he wrote. "A gun is here almost superfluous; for with the muzzle I pushed a hawk off the branch of a tree […] It would appear that the birds of this archipelago, not having as yet learnt that man is a more dangerous animal than the tortoise or the [marine iguana], disregard him."
Darwin called these birds "tame," and spoke of their gradual change into gun-fearing skittishness as increased "wildness." In actuality, of course, the opposite was true—the less contact birds had with humans, the more wild and less afraid they were—but his slip-up is telling: we expect small birds to be naturally scared. Indeed, we expect it of all prey animals, from quivering rabbits to wide-eyed deer to trigger-happy herds of antelope. But is this fair? Do most prey animals spend their lives in constant fear? And if not, why not—how do they figure out when to turn their wariness off and on again?
Two centuries after Darwin's encounters with the unfazed birds, Joel Berger found himself asking the same set of questions. A conservation biologist specializing in large hoofed mammals, Berger was fueled not just by the existentialism of many predator-prey musings, but by a sense of true urgency. As various human interventions rattle the food chain, and previously fixed roles shift, how and why animals get scared determines their survival, Berger explains in The Better To Eat You With, a book-length account of his decades of prey-chasing travel and research. Human decisions (like the extinction and reintroduction of wolves in Yellowstone) and accidents (like the arrival of Burmese Pythons in the Everglades) put animals in new kinds of danger, and whether or not they recognize that is often a matter of life and death.
Like Darwin, Berger is fascinated by "naive animals"—prey who have never met their predators. He sees in them an opportunity to test where threat recognition comes from: whether it's baked into prey DNA, passed down via what Berger calls a "culture of fear," or, as usual with this type of nature vs. nurture question, a mix of the two. If, say, elk are hard-wired to be terrified of wolves, it won't matter whether a whole herd has never met a wolf before—they'll run at the first whiff of one. But if new elk generations need to be taught this fear, and "predator-naive individuals are less fearful than their predator-savvy counterparts," those blissfully unaware elk are in trouble, Berger explains.
Finding answers is a multilayered process. First of all, Berger writes, trying to infer an animal's state of mind involves learning to speak its behavioral language, a complex lexicon of movements, noises, and reactions that run the gamut from subtle to obvious. Anxious birds may sing out warning calls, while scared fish school up and flee. A moose fearing danger might retract its ears, or sniff the air slightly faster, while whole groups of frightened antelopes will jump up and down, a behavior called "stotting" or "pronking." "Even bison, despite their massive bodies, sometimes stot when alarmed," sending the whole landscape into nervous tremors, writes Berger.
Once you can tell whether or not they're scared, the next step is to figure out exactly what scares them. Over the course of long studies of different moose populations, Berger tested all sorts of potential triggers. He brought speakers out to moose territory, played them the calls of wolves, (moose killers), ravens (often found near moose kills) and howler monkeys (across the ocean from moose kills), and compared the moose's reactions. He snuck up on them and tallied how close he could get before they ran away. He goaded them with snowballs mixed with bear poop and wolf urine, to test whether smells alone could spur fear. Berger found that moose that lived near wolves were more afraid of their sounds and scents, "ceasing to feed and preparing to flee." Moose without sharp-toothed neighbors displayed signs of wariness, but when confronted with evidence of the enemy, they browsed on, indifferently. They knew how to be afraid, but not what to be afraid of. If a real wolf had been there, he'd have had an easy meal.
Other animals show similar forgetfulness. Alaskan ground squirrels, whose ancestors lived with snakes three million years ago, are only somewhat fearful of snakes today—they run away from and kick sand at them, but not as quickly or effectively as their snake-experienced cousins do. Conversely, elephants and rhinos in Kenya have learned to fear the sound of Maasai cowbells, and baboons hide at the sight of guns. After careful study, Berger thinks mother moose who survive encounters with wolves may teach their new fear to their calves, placing the next generation pretty high on the learning curve.
Even the animals of the Galapagos are figuring this out. The same marine iguanas that once let Darwin throw them into the sea still allow people to come very close to them—but studies show that when humans approach, some iguanas have begun releasing stress hormones and attempting to flee, "albeit inefficiently and largely unsuccessfully." 250 years after Darwin effectively opened the chain of islands to evolution-loving tourists, said tourists are now participating in evolution, slowly instilling fear in the population, drop by drop. In a rapidly changing world, that might be the healthiest thing for them.
All in Your Head is a series that takes a scientific look at all things spooky and scary. Follow along here. We're also swapping stories with Atlas Obscura. Check them out for more spooky stories here.