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Both. Many primates bite their nails. Some do not. It's not species specific. Chimpanzees, for example, usually bite their nails; some prefer to leave them alone. They do grow continuously, and nails not kept trimmed probably break off (looking at the thickness of those nails, I get the impression that this might be rather painful if they break too short.) ...


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Why some human beings bite their nails? Because, they are primates - it is inherited way to treat our nails, called onychophagia. While normal in some primates (and this is the way they treat their nails), it is considered abnormal in human.


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A comment by @ddiez made a good point at identifying some ambiguity in the question: sight vs concentration. Sight has been answered well by @Ilan. You can't test to see if someone is squinting to concentrate. I stick by my original comment: Isn't it a social tool to show others you're thinking? It would be difficult to come up with a scientific ...


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Short answer The number of eyes are not always even-numbered. In fact, many present-day lower vertebrates feature an uneven number of eyes. Background Eyes can be generally defined as photosensitive organs. Besides the regular paired eyes, lower vertebrates such as fish, amphibians and reptiles feature an unpaired (single) brain structure called the pineal ...


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The phenomenon in question is probably related to people with uncorrected/undercorrected myopia. This "ancient" phenomenon is called stenopeic slit effect (a case of pinhole effect): when the person squints the visual acuity becomes better because of smaller blur circles: the "slit" between the eyelids is the key point improving blurring. One can check if ...


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Short answer Echolocating bats have relatively large sensory epithelia in their inner ear, that may correlate with their high upper frequency limit of up to 200 kHz. The basilar membrane is thinner and stiffer, possibly allowing it to decode higher frequencies. Background In terms of the place theory of hearing, the cochlea acts as a frequency transformer, ...



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