Bats are known as one of the most fascinating animals on this planet. From echolocating dolphins to giant pandas, these mammals all share an intriguing common trait — they hang upside down from ceilings.
But what happens when you bring together hundreds upon thousands of bats? You get a nightly migration that is so spectacularly beautiful (and loud), you’ll want to snap photos of these creatures in flight. If you live near any kind of migratory bat colony, you might even catch sight of some of these winged wonders firsthand each night.
It’s hard not to marvel at how many different species of bats exist around the world, including more than 400 varieties found. But while we know quite a bit about the habits of individual types of bats such as vampire bats (“Desmodus”), leaf-nosed bats, hoary bats, northern long-eared bats, and others, we don’t really understand much about their collective behavior.
That is until recently, when researchers began studying the mass exodus of over 500 million Mexican free-tailed bats who leave behind billions of seeds every year during the annual Southwestern seed dispersal event.
Now, new research published in PLOS One shows that these large groups of bats fly southward under the cover of darkness because they’re afraid of light. In other words, according to scientists, daylight doesn’t scare off these creatures — rather, it scares them right back into their caves.
Are bats attracted to light? Here are 4 Facts that you need to know:
1) Bats are drawn by the glow of a lighthouse
“We’ve been working with several different species of bats,” said study coauthor Dr. Brian Quinn, director of biological sciences at San Diego Zoo Global Institute, “and we were interested in knowing where they went after sunset.”
The team studied three kinds of Mexican free-tailed bats native to California, Arizona, New Mexico, and Texas: Pleidelsdorf’s big footed palm mouse (Mesophonys flagellum); Tlacochahuaya mammothsquare (Chionomops gundi); and Conejo soft-furred otter opossums (Diprmmys insularis). These species are part of a group of small mammals called insectivores, which feed almost exclusively on insects.
After setting motion-sensitive cameras throughout the area, the researchers watched the movements of the bats through infrared technology. After recording data for 24 hours per camera location, the team discovered that only 11 percent of recorded individuals flew outside between twilight and midnight, compared to nearly 40 percent doing so immediately before sunrise. Even when feeding activity was high, fewer than 10 percent of bats left their roosting sites during daylight hours. This trend held true regardless of whether the animals were male or female.
Dr. Quinn says the findings suggest that the bats’ aversion to sunlight could stem from natural selection pressures that resulted in adaptations to protect against predation. For example, he notes that diurnal raptors like owls prefer hunting at night, and crepuscular hunters like foxes usually hunt during early morning and evening.
Since bats tend to spend more time hanging out inside tunnels and underground burrows, experts believe they developed this habit to escape detection by those same predators. Or maybe the bats simply dislike being active during daylight since they often find themselves struggling to stay cool in hot weather. Whatever the case, the results show that daylight tends to spook these guys.
2) The moon lights up their path
As if having millions of tiny eyes isn’t enough, bats also rely heavily on echolocation. Echolocation refers to the ability to produce sounds using echoes reflected back from objects. Unlike humans, bats aren’t able to see well without making sounds. While our vision relies on a direct line of sight, bats navigate via sound waves bouncing off nearby surfaces. To create these beams of sound, they use vocal sac membranes located atop their mouths. When excited, the membrane vibrates rapidly creating audible clicks.
By measuring intervals between these clicks, bats can estimate distances based on travel speed. However, if bats hear another animal approaching, they must quickly increase the frequency of their calls to signal imminent danger. A potential predator may then hone in on the noise source and attack accordingly.
In the new paper, published Monday, June 23, 2019, in the journal PLOS ONE, lead author Dr. Nathan Rufino described how his team used this method to track the movement patterns of individual bats. Specifically, the researchers placed microphones underneath a cave entrance and attached solar powered tripods above these openings.
Then, they waited for bats to emerge from their homes and record the number of times they clicked. Most of the recordings took place at two locations: one site in the Santa Catalinas Mountains and one in the Valle Escondido region of Oaxaca, Mexico.
During the first half of the summer season, the team captured 1,955 echolocation events across both areas. More than 75 percent occurred between 9 p.m. and 6 a.m., suggesting that nighttime is generally preferred by these animals. During peak reproductive periods, females produced click sequences roughly four times longer than males, indicating greater urgency regarding finding mates. Males, however, tended to call more frequently than females. Overall, though, the pattern remained consistent in terms of the overall volume of nighttime calling.
“Our initial hypothesis was that bats would move away from the microphone towards dark places once they exited the tunnel,” wrote the authors in the paper. “Instead, we observed that bats moved toward light sources once they emerged from the tunnel… We hypothesize that this response is due to the increased visual sensitivity of the bat after emerging from its shelter.”
While the exact mechanism remains unclear, it seems clear that bats respond to the presence of light differently depending on their age, sex and size. Future studies will need to investigate exactly why certain bats seem less affected by sunlight than others.
3) Bats flying at dusk, and there’s no dawn for them yet
When migrating, bats typically choose to make landfall either close to dawn or shortly thereafter. Why? Because bats depend heavily on the sun’s positioning relative to Earth’s axis. As noted earlier, bats fly mostly at night and thus require a period of restful sleep following the day’s longest stretch of sunshine.
Otherwise, their bodies won’t absorb sufficient amounts of energy required to sustain them. Without that break, bats risk getting too weak to keep flying. So, instead of taking advantage of the full length of daylight, they wait for sunrise to arrive. Once it does, they begin searching for food and refilling their strength reserves.
This strategy makes sense considering that many species of bats are adapted to tropical climates. With average temperatures exceeding 90 degrees Fahrenheit (32 degrees Celsius), there’s little chance of overheating. Also, unlike temperate zone dwellers, tropical bats don’t experience frosty nights. On warmer days, they can extend their range farther north thanks to thermals rising off the surface of water vapor.
Another factor influencing these decisions could be related to reproduction. Many species of fruit-eating bats reproduce every few years whereas nectar-feeding ones breed regularly. Nectar bats need to replenish their stores of sugar daily in order to survive, so mating activities take priority at specific points during each calendar month.
Once winter rolls around, however, things change. Southern hemisphere colonies of Brazilian wandering spiders, social wasps and honey bees become inactive from October onwards. At this point, adult members of these populations return home to hibernate. Some hibernation strategies involve entering deep torpor states that reduce metabolic rates dramatically. Others attempt to conserve body heat by completely shutting down their hearts. Still others go rogue, choosing to wake up occasionally to eat and drink. In general, however, bats follow similar seasonal patterns although the precise details vary among localities.
4) Bat eyes can’t help but be dazzled by bright light
One of the main advantages of living beneath trees is that leaves provide excellent protection from harsh sunlight. Yet, despite this benefit, certain tree dwelling organisms still struggle to cope with excessive exposure to ultraviolet rays.
Bats fall victim to this problem as well. UV radiation produces damaging forms of oxygen molecules called reactive oxygen species (ROS), which damage DNA, proteins and lipids. Hence, many species of bats possess special pigments called melanin granules that act as filters protecting sensitive tissues from harmful effects of sunlight.
For instance, the hairy-faced fruit bat lives feeds primarily on fruits. Its skin contains lots of melanin granules giving it a brown color. Melanin helps filter out shortwave blue wavelengths associated with intense sunlight, keeping these cells safe from harm. Conversely, melanin absorbs redder, shorter wavelength photons emitted by stars, helping the animal better identify celestial landmarks.