California is known for its giant, multi-laned highways bustling under bright blue skies. And while this infrastructure is useful to humans, the Golden State’s many highways represent an almost impenetrable barrier for wildlife.

Soon, habitats on either side of the 10-laned 101 Freeway in Agoura Hills, California will be reconnected thanks to the world’s largest wildlife crossing, according to the Los Angeles Times. The Wallis Annenberg Wildlife Crossing is scheduled for completion on December 2.

Wildlife crossings are structures that help animals safely cross roads. In the United States, collisions between motorists and large animals take place more than a million times a year. Experts estimate that this results in 400 human deaths every year. But the problem isn’t just road accidents. Habitat fragmentation is also a concern, especially for species that require a large range. Inbreeding can occur if a small enough group of animals becomes isolated. 

In the Los Angeles area, mountain lions (Puma concolor), also known as pumas or cougars, are currently the species most vulnerable to genetic isolation caused by roads and development, according to the project’s website. And the 101 Freeway appears to be the most important obstacle, out of all the roads in the area, to the region’s ecological health. 

As such, the Wallis Annenberg Wildlife Crossing aims to reconnect local plants and animals in the ecosystem of the Santa Monica Mountains.  It’s about 210 feet long and about 170 to 174 feet wide. A thick, sturdy-looking vegetation-covered bridge already straddles the busy freeway—but the weather might throw a wrench in the project’s schedule. 

“We’re just hoping El Niño‘s on our side,” Beth Pratt, president of the Wildlife Crossing Fund, tells the Los Angeles Times, in reference to the climate pattern that typically brings wetter winter weather along the United States’ southern border. Flooding would put work on pause, according to the outlet. “I keep saying, ‘Mother Nature, we’re trying to help you. Let’s not throw any more complications our way.’” 

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A single bat colony contains hundreds or even thousands of the winged mammals. But with less-than-ideal eyesight and so many neighbors, how do mothers identify their pups amid the extremely crowded conditions? Writing in a new study published today in the journal Frontiers in Ethology, researchers say it often comes down to the subtle, unique vocalizations emitted by their individual offspring. To assess their responses, the team relied on an even more delicate biological signal—the mother bat’s own heartbeat.

The investigation began when biologists at Japan’s Doshisha University noticed a surprising—but consistent—behavior among the Japanese house bats (Alionoctula abramus) in their laboratory colony. Whenever a pup wandered too far from its roosting area, only its mother raced to the rescue.

“We knew that pups emit isolation calls—acoustic distress signals—when separated. But seeing this strict behavioral selectivity made us wonder: what is happening inside the mother when she hears these calls?” recalled study co-author and Doshisha University biologist Kazuki Yoshino-Hashizawa.

To learn more, Yoshino-Hashizawa and his colleagues monitored four mother bats who each recently birthed a litter of three pups. Some of the litters only contained females, so they decided to exclude the young males from the study to avoid sex-based variations. At two days old, the team recorded clips of the female pups’ tiny isolation calls. These function like the cries from a human baby, and each vocalization has its own unique tones.

However, Japanese house bats grow quickly. The new generation started making echolocation calls at about eight days old, and they ceased isolation calls after around 12 days. After roughly one month, they began flying like adults and set off on their own within the colony. The researchers then recorded their adult echolocation calls, as well.

Around 40 days after the pups’ birth, the team isolated each mother and played them the various audio clips while monitoring their heart rates using an ECG machine. As a control, they also played the recording of a random adult male bat. The study’s authors quickly saw that a mother bat’s heart rate sped up when they heard cries of their own pups, but not the pups that belong to other bat moms. Surprisingly, the mother bats’ heart rates boosted the most from recordings of the older, independent pups. Researchers theorize this might relate to the bats’ memory of more recent calls, but that idea requires more study to confirm.

Still, don’t get overly sentimental. The team also recorded strong heart rate reactions when the mothers heard calls from the random adult male. It’s possible the bats interpreted the sounds as a threat, but the team stopped short of categorizing the reactions as good or bad.

“‘[Heart rate measurement] cannot distinguish between positive emotions, such as maternal motivation, and negative ones, such as stress or alarm,” Yoshino-Hashizawa added.

Instead, the study authors suggest heart rates may be a solid starting point when trying to analyze bat behaviors and responses to their everyday environment. At the same time, a mother bat’s heart certainly seems to patter a bit more for her pups.

“[This] suggests that these vocalizations instantly physiologically prime the mother to engage in maternal behavior,” explained Yoshino-Hashizawa.

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