Signals serve many different purposes. Some are big announcements to everyone, and others are subtle whispers to a particular listener. Of course, like coded messages in a war, if one animal can 'tap in' to the communication system of another then it can exploit the situation and turn it to its own advantage. There is a constant battle between species, between sexes and between individuals.

With all this signalling going on, animals have to be able to filter out what is relevant to them. The first set of filters are an animal's sense organs. Animals can only see colours over a certain range of frequencies. For example, humans have receptors for red, green and blue but cannot see infrared and ultraviolet. Other animals have different ranges over which they can see colours, and so can often see signals that we can't. Hearing is very similar. Some animals can hear a very wide range of sounds, but others are attuned only to a very specific frequency, such as the call of their mate, or the echo from their own echolocation sound.

A second layer of filtering occurs in the brain - how the signal is interpreted. Many animals have been shown to have a 'sensory bias'. Although their senses may be able to detect a wide range of colours, smells, vibrations or sounds, they respond differently to them, ignoring some signals and perceiving others very strongly.

Although these filters help an animal detect the signals that are important to it, and respond quickly and accurately to them, they also make exploitation of their senses easier. Sometimes that 'exploitation' can be mutually beneficial. For example, poisonous animals need to let predators know that they are poisonous before it's too late. They don't want the predators to make any mistakes. So they try to exploit the detection and learning systems of their predators to make mistakes as unlikely as possible. They use colours that their predators can see, and repetitive patterns, which they can recognise from any direction and that are easily remembered. Many insects have bold red and black markings that birds learn to avoid quickly. But moths also have night-time predators such as bats, which cannot see these signs, so have evolved the use of sound for a similar warning effect. For example tiger moths make a loud noise, perfectly within the bats' range of hearing and timed to be in between the bats' echolocation calls so that they are as unmistakeable as possible.

Not all such 'exploitation' is so two-sided though. Some predators have learnt to exploit their prey. For example, one bat species from South America has 'cracked the frog code'. Their main prey, frogs, call to each other to attract mates. The bat, though, has tuned its hearing to hear these calls, and can even distinguish the different species of frog, capturing those that are edible and avoiding those that are poisonous.

The same is true of signals between males and females. Males often have displays designed to attract the attention of choosy females who spend some time deliberating over who to mate with. However, if females have a sensory bias, to help them find food say, the males can exploit that to attract their attention. For example, the cassowary (a large flightless bird) feeds on bright blue and red fruit. It seems that the males have taken advantage of the females' searching for these by having bright blue and red wattles hanging from their throats.

So, the world is full of complex communication between animals and plants, each adapted to get the message to a targeted audience as clearly as possible. However, these systems are ripe for exploitation by 'cheats' and predators who can use the established system for their own purposes. Eventually these may force a system to change.