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When the sun sets, darkness guides the ways many animals live and die. The night helps wary mice hide from some predators, but it also helps hunters like keen-eyed owls score their next meal. Many species mate under cover of darkness, like calling crickets that find one another in the night. Animals from seals to dung beetles rely on the stars or moon in dark skies for navigation. The cycling of day and night creates a natural clock that enables a restorative night’s sleep for primates like us. When the days shorten, changes in light help ready hibernators for seasonal slumber or cue migrating monarch butterflies that it’s time to fly.
“There’s an entire evolutionary history of predictable patterns of light and dark,” says Travis Longcore, an environmental scientist at the University of California, Los Angeles, and co-editor of the book Ecological Consequences of Artificial Night Lighting.
But now that artificial lighting has exploded around the globe, it’s having enormous impacts. Purple martins are migrating earlier in the spring, as the birds’ seasonal clocks are confused by artificial light. Artificial light exposure lowers levels of melatonin, a hormone that helps regulate sleep cycles, in dozens of species, adding stress and depriving them of a good night’s rest. “Wherever you look you’re going to find influences, because we’ve just so profoundly changed what the natural rhythms are to artificial rhythms,” says Longcore.
Research into the many ways artificial light at night influences creatures has exploded in recent years. We searched across the animal kingdom to spotlight 11 examples of how street lights, porch lights, illuminated buildings and more dramatically affect animal behavior.
Spider brains shrink in the spotlight
When faced with artificial light exposure, Australian garden orb-weaving spiders suffer dwindling volume in a brain region linked to their vision, according to a September study in Biology Letters.
Spider brains aren’t centralized like ours. They consist of distinct structures linked together, which makes it easier to see changes to specific regions and, potentially, the functions they govern. The University of Melbourne’s Nikolas Willmott and colleagues recently used micro-CT scans to spot brain shrinkage when the spiders were exposed to light at night.
Willmott theorizes that the stresses of artificial light may lead them to reallocate resources into more important parts of the brain. Or, he suggests, the light might disrupt hormonal processes in that part of the brain, which in turn stunts its growth and development.
Willmott hopes future research can provide a better understanding of the mechanisms driving these artificial light effects on the arachnid brain, and whether light could even produce physical changes like eye size. “There is still a lot that we don’t know,” he says.
City lights lead birds astray
Hundreds of bird species migrate in the dark, when the air is cool and predators few, following the moon and stars along ancient routes. But dazzling arrays of unfamiliar lights have appeared along the way. Urban buildings and towers often disorient birds, and for unknown reasons even attract them, causing the birds to crash or circle confusedly until they succumb to exhaustion. Millions of birds die this way, a problem of such scale that Philadelphia, Chicago, San Francisco and other cities have adopted “Lights Out” initiatives during migration periods, when unnecessary lights are dimmed or simply turned off.
Mining, hydrocarbon extraction, fishing and other industries also produce lots of artificial light in places with few people. And skyglow from artificial light often causes migrating birds to linger where they normally wouldn’t, disrupting their annual trips. “We know now from the work that’s been done on birds we are literally changing the places birds migrate across continents,” Longcore says.
Light pollution also hurts birds by altering their biological clocks. Birds at overwintering sites may mistake artificial light for the longer periods of daylight that alert them to the coming of spring. This could spur early migrations, when ideal temperatures and food along their routes may not be available.
Illumination can ruin coral reproduction
The global atlas of artificial light in the sea reports that biologically significant levels of artificial light pollution penetrate to depths of at least three feet across 735,000 square miles of the ocean. That affected area is about the size of Mexico.
Light pollution has impacts offshore, where research shows that it may hinder coral reproduction. Many corals reproduce just once a year during a period of darkness in the days after a full moon. The synchronization has taken place over countless generations. Each individual coral releases eggs or sperm simultaneously, creating the best conditions for fertilization success, genetic diversity and protection from predators—which simply can’t consume all the eggs.
But research shows that artificial light, particularly LED lights rich in the same blue wavelengths as moonlight, can throw off this intricate dance and trigger corals in the same locations to spawn at different times. This may disrupt reproduction in an era when corals are in serious decline and face a plethora of other threats like rising water temperatures and destructive diseases.
Lights can send baby sea turtles down dangerous paths
Sea turtles spend years at sea and migrate thousands of miles, but when it’s time for females to lay eggs, they depend on nesting beaches and often return to the same location. These coastal areas are also desirable for humans, who have developed many of them and inundated areas with nighttime illumination.
Artificial lights hinder sea turtle nesting success in multiple ways. They discourage female turtles from returning to historical nesting beaches—making them choose less successful locations simply because they are dark. Worse, young turtles hatch at night, when predators are scarce. Light from the moon and stars in the night sky reflects on the ocean and guides the hatchlings to the water. But hatchlings are also attracted by bright artificial lights and may follow them inland, where they become exhausted and dehydrated, or fall prey to birds or other predators. Research also shows that even those hatchlings that reach the ocean can be attracted by artificial lights on the water, which can prevent them from successfully dispersing.
Artificial lights fragment bat habitat
A recent study of North American bat species tracked their reactions to the LED lights typically found in backyards and on garages. Little brown bats avoided the lights entirely to distances of at least 250 feet—where human eyes can no longer even notice the glow. This light aversion, shared by big brown bats, may eliminate many roosting locations and reduce or fragment the crucial habitat they use for feeding. Other species, like eastern red bats, did not seem to mind lights and may feed on insects drawn to the glow.
Scientists first started observing how streetlights disturbed commuting bats 15 years ago and found out that some species seemed hard-wired to avoid the lights. Such research illuminated a problem: Evaluating ecosystems during the day misses how important factors like artificial light affect animals when most humans are indoors or asleep. “If you don’t think of conservation planning from the nighttime view as well, you’ve haven’t really done it,” Longcore says. Light pollution’s impacts also vary by environment. For example, when well-lit areas also feature trees, bats do better, German researchers learned.
Human lights silence fireflies flashing “language”
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Artificial light creates a unique set of problems for creatures like fireflies that produce their own light.
Fireflies light up a field or backyard as a form of communication. Among their 2,000-odd, species the language-like function of the flashing can vary dramatically. Some light up when threatened, or when trying to lure prey. Flashes often send mating messages, projecting key information like sex and species to attract suitable partners.
Artificial light disrupts, or even shuts down flashing, including the wooing of mates—females stop illuminating, and discouraged males gradually give up, too.
A British study documented similar impacts on flightless fireflies known as glowworms. In fact, male glowworms under artificial light often simply tucked in their heads to protect themselves from the light and stopped moving or looking for a mate at all.
Illumination stunts toad growth
Artificial lights at night affect the ubiquitous American toad during various phases of its life cycle. An Ohio study examined tadpoles of the species from 40 artificial ponds; half exposed to small amounts of artificial light at night and the others left naturally in the dark. All the tadpoles became toads, but those from light-exposed ponds were 15 percent smaller. Researchers theorize that toads exposed to artificial lights at night burned energy by being active while the others rested.
Studies of Europe’s common toad also show that light can shut down activity—and confound mating efforts. French scientists exposed dozens of breeding males to various levels of nighttime light that mimicked natural moonlight, the glow of a streetlight or the floodlit landscape of an urban park. Toads exposed to streetlight conditions were 56 percent less active, while those in well-lit park conditions saw normal activity reduced by 73 percent. These toads may reallocate energy from activity to dealing with the stress of light exposure. The males exposed to artificial light also took longer to find mates, and those exposed to the streetlight intensity were 25 percent less likely to successfully fertilize a female.
Wallaby moms may delay births due to lights
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Tammar wallabies—small, kangaroo-like mammals from Australia—mate in October, part of the Southern Hemisphere’s spring. Females hold their embryos dormant until after December’s summer solstice then, as each day gets a bit shorter, deliver babies in late January.
But researchers near Perth compared populations living in natural darkness with those around a well-lit naval base. Moms living near the base’s lights apparently missed the cues of shortening days and delivered their babies a full month later. These moms produced far less melatonin, a hormone that helps regulate their cycles of sleep and wakefulness, and this may have helped delay the timing of births.
Researchers can’t say yet how this change might influence the wallabies’ survival, but serious problems are possible when species’ reproductive timelines get out of sync with seasonal weather conditions and food availabilities. “If you’re a species that depends on a seasonal resource,” Longcore explains, “you have to have signals that get you to that resource at the time when you need it.”
Spawning grunion shun the spotlight
In spring and summer, during the few days following a full moon, teeming grunion pack the shorelines during dramatic spawning runs. The fish beach themselves to lay, fertilize and bury eggs. After a week or so, the young hatch at high tide and head to sea.
But research reveals that on shorelines where artificial light creates a glow equivalent to that of a full moon or more, the fish are far less likely to run.
“You can basically predict where you’re going to have a good grunion run, based on the level of light,” Longcore says. Why do the fish avoid the spotlight? “I think it turns them into a buffet for the predators,” he says. “There are plenty of records of birds just sitting there and picking off the grunion.”
Artificial light aids cannibal crabs
The eternal dance between predators and prey plays out during the cycles of light and dark. Darkness can help some species avoid being eaten. So night lights can help predators more easily spot prey—including their own kind.
Consider the South American intertidal burrowing crab, a key species in the tidal flats and salt marsh grassland ecosystems of coastal Brazil and Argentina. Scientists ran an experimental study to see how artificial lights affected the ability of juvenile crabs to survive. Juvenile survival among crabs living near low-power LED lights was 61 percent lower than those living under natural dark conditions. One reason why? Light increased the incidence of cannibalism. Crabs living under natural conditions had a 30 percent better chance of avoiding being eaten by adult males of their own species, which were five times more abundant in lit areas.
Moths drawn to artificial lights succumb to exhaustion
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Why are insects irresistibly drawn to a light? Some scientists suggest heat is the attractant, while others believe the light is misinterpreted as a hole or passage.
But a study earlier this year may have answered the question: It suggests that many insects orbit around a light because they mistakenly believe they are orienting themselves by sky light. The authors observed that moths, dragonflies and other flying insects kept their backs continuously tilted toward the light as they moved, apparently believing it to be bright sky above them, contrasted with the dark ground below. “Flying animals need a reliable way to determine their orientation, especially relative to the direction of gravity,” lead author Samuel Fabian, a bioengineer at Imperial College London, explained in a press release.
Unfortunately, this behavior locks bugs into awkward orbits around artificial lights that eventually lead to exhaustion or crash landings. Turning off unneeded lights is a good way to reduce this carnage. But more necessary artificial lights that are left on will still have impacts, including many that are yet to be discovered. “A key next step for this research is to work out how distance changes the effect of lights at night,” Fabian said. “We know what’s happening at one meter from a light, but what’s happening at 100 meters?”