Female frogs literally can't hear half the things that male frogs say

China's concave-eared frogs are one of two amphibians that use ultrasonic frequencies to communicate. There's only one small problem: the males and the females have evolved along such vastly separate lines that females are completely deaf to the males' ultrasonic cries.

Various mammals, including rodents, bats, and marine animals like dolphins and whales, use ultrasonic frequencies to sidestep their noisy surroundings. Ultrasonic frequencies are just beyond are own hearing range, but they can be very useful for animals that live in perpetually noisy environment. We now know that two frog species - the concave-eared frogs of China and the awesomely named hole-in-the-head frogs of Borneo - have developed the ability to call out in ultrasonic frequencies.

But this particular adaptation might just be too completely out of sync with the rest of the concave-eared frogs' evolutionary history to be of much use. Researchers at the Chinese Academy of Science took female members of the species and placed them in a darkened room. When they played recordings of males making calls in regular sonic frequencies, the females responded immediately. However, when ultrasonic chirps were played, the females remained completely motionless.

What's more, analysis of their brain activity showed no response whatsoever to the chirps, so it's not that the females were simply ignoring them. They were actually completely deaf to the calls. It's particularly weird because the females are still able to call out in ultrasonic frequencies - they just can't hear what they're saying. As you might imagine, that's extreme unusual.

So what's going on? The answer probably lies in the high levels of sexual dimorphism in the species. These frogs are called concave-eared because they have very thin eardrums that are sunken into their ears. This arrangement allows them to hear ultrasonic frequencies. But the females have very different eardrums, which are much thicker and closer to the surface of the skin, which is probably what deafens them to the ultrasonic sounds.

The researchers believe that no other species possesses this particular sexual dimorphism. The frogs probably evolved their different ears due to their particular surroundings. The males live around a noisy stream, which is what forced them to develop ultrasonic communication in the first place. But the females live in much quieter areas, like caves or trees, and it's only when it's time to reproduce that they leave in search of males.

During the rest of the females' life, there's no need for ultrasonic communication, which is apparently why they haven't developed the ability to hear those frequencies. It's a fascinating lesson in the complexities of evolution - while the evolutionary advantage for ultrasonic communication in males was strong enough that females now possess the ability to make those calls, apparently the ability to hear it didn't matter as much. If you feel like reading all this as some sort of analogy for human gender relations...well, I don't think there's any possible way that I could stop you.

Nature Communications via Discoblog.