Pandas are not the most motivated animals when it comes to sex. Their apparent disinterest in self-preservation has vexed even the most dogged conservationists for more than 50 years. Why, scientists have asked, won’t they just mate? Researchers think they may have found one reason: noise pollution. They published their findings in the journal Global Ecology and Conservation.

Sound is an essential element of panda sex. Unlike other bears, female pandas are spontaneous ovulators; that is, it happens when it happens, instead of being triggered by an external factor like sex or seasonal changes. But giant pandas are loners, which means it's unlikely that a male panda and a female panda will be in the same place when she starts ovulating. To find each other, they start yelling, and continue to do so even once they’re together. Each outburst could potentially contain a lot of important information about the noise-maker, including their location and their intention—but only if other pandas can hear and understand them.

While we know a lot about bears, we know very little about their hearing. There’s only one species whose hearing abilities have been quantified: polar bears. So conservation biologists at the San Diego Zoo decided to give their five pandas a hearing test.

“… The ability to discriminate between fine-scale differences in vocalizations is important for successful reproduction,” lead author Megan Owen said in a press statement, “and so, a thorough understanding of acoustic ecology is merited in order to estimate the potential for disturbance.”

For obvious reasons, this was a slightly complicated endeavor. First, the researchers had to construct a soundproof testing booth, which in this case meant gluing acoustic foam and plywood onto a transport crate. They outfitted the booth with microphones, speakers, a camera, colored lights, a camera, and a target.

The pandas were somewhat used to taking tests, so it wasn’t difficult for the zoo staff to train them in the booth. The test format played to their natural strength: not moving. The bears were taught to stay still until they heard a noise, at which point they would turn and put their noses against the target.

Once a bear was in the booth, the researchers played a series of noises that pandas might hear in the wild, including cubs crying and squawking and adults growling. As in hearing tests for humans, the sounds varied in pitch and volume.

The researchers found that the pandas had surprisingly sensitive hearing and were able to detect noises between 0.10 and 70.0 kHz—well into ultrasonic range. Their ears were as sharp, and, at some pitches, sharper, than polar bears’.

Wouldn't acute hearing be a good thing for a species that relies on sound to get busy? Yes and no. It’s great if pandas are the only noisemakers around. But as climate change reduces their habitat, even pandas on nature reserves are being pushed closer to people and all of our noisy business. The researchers say anthropogenic, or human-made noise, may drown out the sounds the pandas most need to hear.