Purgatorius, our earliest primate grandmother.
Purgatorius fossils were first found by scientists in the 1960s on Montana's Purgatory Hill. Her teeth were specialized for both crunchy bugs and squishy fruits. Unlike her earlier ancestors, Purgatorius had hinged, rotating ankles, which were especially good for climbing trees and skittering along branches. Life in the trees also selected for differences in sensory organs, compared to life on the ground. Gradually, our sound sensors evolved. Interestingly, they evolved differently in male and female primates.
Today's primates are able to hear much lower frequencies than many other mammals. Consider the dog whistle. Or hearing range is low enough that we can't hear it. The best theory for this difference is our move to the canopy. According to Bohannon, "It's actually a physics problem: when you're at ground level, you can bounce your sound waves off the earth, doubling your signal strength. When you're in the trees, the ground is too far away to amplify your vocalizations." Additionally, the leaves, fruits, etc., dampen sounds. "Animals generally adapt to a soundscape in one of two ways: they tweak their pitch range or they boost their volume. Primates did both: they evolved to both hear and produce lower pitches, and they found ways to get louder. By lowering the pitch, they automatically gave themselves more distance, since the lower the pitch of a sound, the longer the sound wave, and the longer the sound wave, the farther it travels." This is why you can hear the bass of a car stereo from further away than the lyrics of the song. Because male and female primates have different needs, male and female hearing evolved slightly differently. Generally speaking, men's ears are better tuned to lower pitches while women's ears are more sensitive to higher pitches -- usually those above 2 kHz. "That just so happens to correspond to the standard pitch of a baby's cries."
"Now, if you're a female primate, there are obvious evolutionary advantages to being able to hear your baby well. So, while the entire primate line might have shifted the bottom end of their hearing downward--presumably to correspond to long-distance, low-band communication through the forest canopy--being the primary caretakers, females would particularly need to retain their ability to hear their higher pitched offspring. Via pathways that are still mysterious, female-typical hearing became tuned to these higher pitches. Most women can hear them better than men even in noisy places. And while typical masculine ears tend to lose their higher range as they age, women's ears are better at hanging on to those pitches. Importantly, our better ability to hear the very upper end of the human register is also tied to hardwired emotional response: baby cries alarm women more than men. It's not that men can't hear the kid crying, but that for many adult men, their ears snip off the upper end."
This difference in ranges of hearing has consequences beyond response time to baby's cries. In the UK, sonic anti-loitering devices, called Mosquitos, began to be installed about 15 years ago. These devices produce a high pitched sound that can't be heard by adult men, but is meant to drive away teenagers. These devices can often also be heard by women, and as well as children and babies. The high pitched buzz is painful, resulting in headaches, vertigo, and other symptoms. Some cities have gone so far as to install them in subway stations. That's a problem. There are also numerous accounts online of private citizens installing them to torment the neighbors' "noisy" children.
There have been many calls to ban the devices, but thus far, no country has. I just learned that my city, Philadelphia, has even installed them in parks and turns them on each evening. Apparently, 7-11s also have them. No wonder I have a headache every time we leave the city.
The City of Spokane has banned the devices. They have also been banned in ScotRail stations in Scotland.
These differences are also an issue in design of products and public spaces not intended to to be painful.
In addition to our higher range of hearing, women's ears also tend to amplify sounds more than man's ears, by way of movements of hair cells that respond to sounds in the environment and boost the signal, creating otoacoustic emissions. Women's otoacoustic emissions tend to be both stronger and more frequent that men's. So we hear more things, we hear them more loudly, and we continue to hear them as we age.
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