Perception

Every organism needs to adapt itself to the changing conditions in its environment, and therefore to receive informations from it, through senses. Sense organs encompass a wide range of signals: touch is a simple perception of close mechanical stimuli, while hearing is a specialized form of touch that perceives the vibrations on long distance; olfaction and taste perceive the chemical makeup of near compounds; sight is the perception of electromagnetic waves (light), while electroception and magnetoception perceive directly electric and magnetic fields, respectively.

Most organisms have one or two highly developed senses more prominent than others. Many small organisms that need to know only their immediate surroundings deal well with "contact senses" such as touch and taste, while larger animals that live in open environment will benefit more from "long-range senses" such as sight and hearing; many mammals have a well-developed olfaction, while birds (and humans) prefer sight; bats and cetaceans have come up with echolocation, a derived form of hearing; and so on.

Touch
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Tactile senses (perception of mechanical stress) are the most simple and primitive, found in virtually every organism on Earth. They include the perception of pressure, vibrations (see hearing, below) and tension of body tissues. For example, a complex network of nerves allows a fly to adjust the shape of the wings and the frequency of flapping to counter irregular wind, while earthworms and many insects gain most of their sensorial informations from vibrations in the ground. Particularly, social insects such as ants and honeybees communicate mainly through touch.

Fish have two specialized tactile organs: the Weber organ, a group of vertebrae appendages that detect water pressure through changes of shape in the swim bladder, and the lateral line (also present in some amphibians), a system of hair cells than runs along the fish' side perceiving water currents and vortices. The venus flytrap, a carnivorous plant, has extraordinarily sensitive hairs that cause the leaves to close when they detect an insect.

While useful and versatile, touch detects only objects in direct proximity, and it's useless at greater range; besides unicellular and plant-like organisms, it would be developed mostly in dark, noisy and turbulent environments and probably by slow-moving or static organisms, especially in a very dense medium and without large predators. Burrowing organisms would be suited to evolve a good sense of touch, though it wouldn't likely be the main sense. A particular method of tactile communication, found in Xenology, could be found in organisms that can alter their skin texture in shifting corrugations and papillae, as octopi do.

Hearing
Hearing is the perception of sound waves, that is, the vibrations trasmitted through matter (ground, water or air). Like other waves, they can be classified by intensity (the amount of energy carried by the wave) and frequency (the number of cyclical variations that occur per unit of time). They're far slower than light, but they can spread behind corners and through all solid matter (though not in the vacuum). Their speed increases with the medium density (343 m/s through air at 20°C and 1 atm, 1482 m/s through water at 20°C, 5960 m/s through steel).

Frequency is measured in hertz (Hz), where one Hz means one cycle per second. Human beings can hear sounds between 12 and 20 000 Hz; sounds with a lower frequency are called infrasounds, those with a higher frequency ultrasounds. Dogs, mice, dolphins, bats, etc. can hear ultrasounds; as a rule of thumb, the smaller an animal is, the higher are the sounds it can hear (or produce).

Soundwaves are collected by the tympanic membrane or eardrum. In mammals, vibrations are also trasmitted through three bones (the auditory ossicles: malleus, incus and stapes) towards the cochlea, where they activate ciliate cells connected to the auditory nerve. Snakes lack an eardrum, but they can perceive sounds with the vibration of quadrate bone, while elephants can communicate by receiving with their feet low-frequency sounds transmitted in the ground.

Mammals also have a pinna or auricula, a funnel-shaped structures that collects and directs sounds; many can move their pinnae with specialized muscles to direct them towards the source of a sound. Owls have horn-like feathers and a dish-shaped face with the same function. The presence of two (or more) ears allows to detect the direction a sound came from by measuring the difference between the time when it hit the eardrums; an owl can perceive a difference of 0.0002 seconds.