When Animals Take the Night Shift

Several years ago, Kaitlyn Gaynor and her colleagues noticed an intriguing pattern. It started with data from Tanzania, where motion-detecting cameras captured a trend: Antelope that had once roamed primarily in the day were now roving more at night. As Gaynor, a doctoral candidate at UC Berkeley, and her fellow researchers discussed the change, they realized that a similar nocturnal shift had occurred in many other mammals, too. In Mozambique, elephants had begun traveling on roads in the dark, when they were relatively free of humans, and staying in the forest by day; in Nepal, tigers were moving about more often by moonlight, while people slept; in Poland, boars living in a national park split their days evenly between waking and sleeping, while urban-dwelling boars were awake almost exclusively at night. Once the nocturnal phenomenon “was on our radar,” Gaynor told me recently, “we started seeing it, really, everywhere.” Everywhere is no exaggeration: In a paper published in June, Gaynor and her co-authors offered evidence of nocturnal shifts in dozens of species that come into regular contact with humans, on every continent but Antarctica. Gaynor suspects that these behavioral changes are bringing with them rapid evolutionary change as well.

The study of what scientists call “human-induced evolution” took off almost 20 years ago, when the marine ecologist Stephen R. Palumbi, writing in the journal Science, said that humans “may be the world’s dominant evolutionary force.” Of course, humans have been deliberately manipulating plants and animals to bring out preferable traits for ages (Exhibit A: beagles, mastiffs, and Chihuahuas). But we also induce evolution in less intentional ways. For instance, Chris Darimont, a conservation scientist at the University of Victoria, in British Columbia, points to a simple mechanism he calls “selective killing”: Keeping all the big salmon and letting the little ones slip through the nets gives the smaller salmon a survival advantage, bringing down the species’ overall size.

Humans’ geographic reshuffling of species has ignited evolutionary transformations, too. When traders from Europe introduced periwinkle snails to North America in the late 19th century, hermit crabs in New England quickly grew larger bodies and claws in order to fit into the snails’ relatively roomy shells.

And look at climate change: In 2014, unusually low temperatures in southern Texas, brought about by a countrywide polar vortex, killed green anole lizards that lacked genes for “cold hardiness,” explains Shane Campbell-Staton, the UCLA biologist who documented the die-off. The elimination of these lizards genetically reconfigured the Texas green-anole population, which Campbell-Staton suspects will help the lizards better withstand future temperature drops.

The Darwinian effects of increased nocturnality may be even more far-reaching. Researchers have long known that animals avoid sharing the same physical spaces as humans, driven away from us by the overwhelming threat they sense from our mere presence. (In fact, some evidence indicates that nonlethal human activities such as hiking or picnicking induce nearly as much terror in wild animals as does hunting them.) Gaynor believes that as nocturnal behavior becomes more entrenched, more radical adaptations may follow: Formerly day-dwelling mammals could acquire traits suited to navigating in the dark, such as larger corneas, more sensitive ears, and a stronger sense of smell. To attract mates, animals might have to develop nonvisual reproductive rituals; as they begin relying more on sound for communication, vocal pathways might morph, changing the noises the animals make.

Changes in one species, Gaynor notes, will likely give rise to changes in others. Red brocket deer in Argentina have learned to avoid hunters by foraging at night, reducing competition with a related deer species. Research already shows that, as some California coyotes have taken to the night, they’ve begun preying more on nocturnal rodents they used to eat less of. And some nocturnal prey species, Gaynor and her colleagues have written, will likely become more active in the daytime, using humans as “temporal shields” against predators.

To glimpse how human-induced evolution could reorganize the planet, we need only look at the way apex predators of the past forced other animals to become nocturnal. For some 180 million years, as tyrannosaurs stalked the Earth and pterodactyls ruled the skies, tiny mammals sought refuge in the dark. When the dinosaurs went extinct, mammals slowly reclaimed the day—until one species of ape attained global dominance, scaring other mammals back into the night. For now.

This article appears in the October 2018 print edition with the headline “Up All Night.”