Although these days most of our life is spent in front of devices that cram the world into a convenient, two-dimensional space, the moment when we peel our eyes off the screen brings the realization that we still live in a universe that has a pesky third dimension. Sights, sounds, and smells come to us from all directions, and our sense organs use a number of sophisticated methods to locate their sources. We may not know it, but every second that we are awake our brains are hard at work, performing complicated calculations, triangulating, comparing arrival time differentials, and measuring particle velocity in signals that reach our eyes, ears, and nostrils. This usually helps us avoid walking into a lamppost, jump away from a barking dog, and know that the smelt-ee is not always the delt-ee. But in order to capture the distance and directionality of a signal, we need at least two sensors separated from each other, and this is the reason why virtually all animals have at least two eyes, two nose holes (or a forked tongue), and two ears. Virtually all animals, but not all – some have only one ear.
It is quite reasonable to ask what good would a single ear do: sure, you will hear the noise of a roaring lion at night, but you will not be able to tell where the animal is. Hearing the noise getting louder, but not having the information about its location, you are as likely to run towards it as away from it, and natural selection has made sure that such confused individuals don’t get to pass their genes onto the next generation. And yet one animal, the wonderful praying mantis, somehow manages to go through life with a single, cyclopean ear.
For the longest time nobody realized that mantids can hear at all. Scientists had poked and prodded their bodies for hundreds of years, and nobody found anything that resembled the typical, paired organs that are used by animals to detect sound waves. It wasn’t until 1986, when David Yager and Ronald Hoy jammed a metal probe directly into the nervous system of a praying mantis and discovered, by measuring nerve cell activity, that these animals were amazingly good at perceiving certain sound frequencies. Only then did they figure out that the mysterious hole between the last pair of a mantis’ legs is an ear – a single one.
Subsequent experiments confirmed the long-standing principle of hearing – despite its sensitivity, the mantids’ single ear was no good for detecting the direction of sound. Why have it then? The clue comes from the range of frequencies their ear is attuned to. They are the same frequencies, ultrasounds between 25 and 100 kHz, that bats use to locate their flying prey. Mantids, it turns out, can hear bats hunting in the air. Many mantids are good fliers, and males will often fly at night, following pheromonal trails emitted by the females. But mantids are not nearly as fast and skillful in the air as are bats, and in trying to fly away from a bat they would invariably end up in becoming its meal. From the survival point of view, it makes far more sense to simply dive bomb to the ground the moment a hunting bat is detected, and disappear into the grass or bushes. And for this to do a mantis does not need to know where the bat is, all it needs to know is that the bat is out there.
Additional confirmation of the role of the mantids’ single ear comes from the sexual dimorphism in the ear development. In many mantids it is only the male who flies, and in these species females have strongly reduced ears, or none at all. Ears are also absent in wingless and flightless species of mantids.