Hear, hear! (The sense of hearing 1)

There have been many attempts to design interesting sensory systems for alien animals. Some tried to equip animals with radio or radar, which poses the difficulty that such radiation passes easily through biological tissues, making them hard to detect. There have been attempts to do away with vision altogether and replace it with something else, such as echolocation. The most famous example is probably Barlowe's Expedition. Although his artistic prowess made the result look spectacular, life without vision on a planet with light seems very unlikely. The echolocation was explored in four posts: one, two, three and four. 

I cannot remember a fictional world in which the total absence of hearing is presented as an interesting twist. That absence suggests that hearing is accepted as a simple given. Are there environments in which hearing cannot work? Yes, there are. If we define hearing as the ability to pick up mechanical vibrations in a surrounding medium, close to the Wikipedia definition, then the absence of that medium will abolish hearing. The resulting environment coins the nice phrase 'In space no-one can hear you scream'. True, but you cannot breathe there either, and that is definitely a bigger problem than not being able to scream. I intend to write two posts about hearing in a terrestrial environment with an Earth-like atmospheres. The present one will have a look at evolution of hearing on Earth, to see if that helps designing alien hearing. 

Picking up mechanical vibrations from the air is not fundamentally different from doing so in another medium, such as water. The medium could be expanded to include touching an object, such as the ground. Picking up ground vibrations is so close to hearing that you could arguably include that in the 'hearing' concept. The ability to register vibrations lies at the heart of our kind of 'air hearing'. Where it is easy to evolve 'air hearing' for animals that once came out of water will depend on their starting point. Here is a very nice paper comparing vertebrate and insect hearing. The basic problem has to do with being surrounded by air. Bodies are mostly water, so vibrations in water will readily pass into watery bodies, where they can be picked up. But picking up air vibrations in a watery body requires a signal transformation. The main way to do that, in vertebrates and insects, is to use a taut membrane to pick up air vibrations. 

Click to enlarge; source and copyright: https://www.frontiersin.org/articles/10.3389/fevo.2021.667218/full

 Vertebrates 

Vertebrates apparently evolved their eardrum, or tympanic membrane, from skin. Making an eardrum work means the drum must be open to the air on one end, but there also must be air on the other side of the drum, inside the animal. Having bubbles of air in the body is not a standard part of animal anatomy though, so the middle ear had to evolve from scratch too. The illustration above shows that eardrums were present at the start of the Mesozoic, not earlier. As if that was not difficult enough, the system also needed pressure transducers, for which rearranged jaw bones were press-ganged. 

It seems that hearing was not really an easy process for Earth vertebrates, so perhaps 'air hearing' is not an automatic given. The paper suggests that vertebrates, before they evolved proper 'air hearing', may have picked up vibrations through the ground, perhaps to sense predators. That makes me wonder whether predators at the time would have been under evolutionary pressure to walk softly. 

Insects 

The paper also goes into insect hearing and makes the point that insects had it much easier: forming an eardrum was a matter of thinning the exoskeleton, and the insects' exoskeleton, right at the air-body interface, already carried lots of mechanoreceptors that could easily be given a new job. Insects have air-filled tracheae (breathing tubes) everywhere, so forming the equivalent of a middle ear was not difficult either. Apparently, hearing evolved in insects independently at least 19 times, resulting in a great variety of ear designs, found all over insect bodies. 

Click to enlarge; source here

Another paper provided details of such insect ears. A few examples are shown above the arrows indicate ears. The authors write that about the only place where you will not find insect ears is on the sides of their heads. Instead, insect ears are most often found on the body itself, but they can also be situated on the legs, mouth parts and even on wings. As for the number of legs, many insects have just two ears, but some grasshoppers have no less than 6 paired 'functional auditory organs in their abdomen', giving them 12 ears. Some insects have two ears per leg, giving them four ears. Mantids are perhaps the oddest, with just one ear in a groove on the midline of the thorax. Although there is a tympanic membrane on each side of a narrow grove, these are so close together that they function as one ear. Mantids are 'the only known terrestrial animal with a single ear'. The figure above shows an exception: some mantids do have two ears, but both are in the midline, and they are sensitive to different frequencies. 

click to enlarge; source here

 Spiders 

There are no tympanic membranes to be found in spiders at all, so for a long time it was thought that they were deaf. However, they are apparently able to pick up sounds with hairs on the tip of their legs. This was only published in 2016, so there will be probably more discoveries in the field of spider hearing. 

 

The investigators made a very nice video of their somewhat accidental findings, that you can find on their website (they allow downloading, and the quality there is better than shown here; I just enclosed it for ease of use). If you need more, the authors went on to study hearing in 'ogre-faced spiders' later, also with a nice video). 

What does it all mean for speculative biology? 

As usual, there are various solutions to the problem (and we haven't even touched upon frequency ranges and other intricacies!). People who design vertebrate analogues might do well to think about how Earth vertebrates and insects developed eardrum-based hearing: you need some suitable tissue with easy access to outside air, so that tissue can be thinned to form a membrane. You also need a way to have air on the inside of the membrane too. That alone would suggest a place near the mouth. It will also help if that place is already equipped with the ability to sense movement, of jaws, hairs, scales, etc. But the spider example shows, as usual, that there are other ways too, and hairs seem to excellent starting material to evolve hearing. 

The next post will be about sound localisation. Sound does not travel in neat straight lines, which is just one of the things that complicate that particular problem.