All forms of life must selectively detect and take in chemicals, and so chemical signaling occurs at many levels in all cells. Hormones operate within an organism, pheromones are signals between conspecifics, and allomones are intended for interspecies communication. In this section we will focus on pheromones.
Two different systems of reception are employed for chemical communication. Airborne and waterborne chemicals received at a distance from their source are detected by olfactory reception, or smell. Other chemicals require contact reception, direct contact of the receiver with the source of the pheromone. Pheromones are usually produced by glands located on the skin, or by ducts within the body that have ducts leading to the body's exterior; the second variety of glands are known as exocrine glands. Mammals produce pheromones in the sudiferous (fluid-producing) gland and the sebaceous (waxy-substance producing) gland. Some mammals have other specialized glands. Insects produce a large variety of pheromones, most notably from their mandibular glands, thoracic glands, and stingers.
A good example of a multipurpose pheromone is the Queen substance employed by some eusocial bees. This pheromone motivates and attracts workers, releases swarming, and is a sex pheromone. Absence of queen substance indicates the colony has grown too large (the queen is too far away to smell her) and so workers will build queen cells to rear new queens. An abrupt absence of Queen substance results in emergency queen rearing, since that absence is probably an indication of the queen's death.
Other examples of chemical signaling include alarm pheromones, such as bee stings. Isopentyl acetate, the chemical injected into a sting wound not only serves to wound an enemy, but to alert other bees to danger, and in some case cause swarming. The reason "killer bees" are so deadly is not because they have a more venomous sting, but rather because these bees have a lower threshold for alarm pheromone and so an entire swarm will react and sting the enemy to death. Ants and snakes employ trail pheromones to mark the path to a food source. These chemicals are laid out along a trail, and the next ant will follow the trail by means of contact reception. Many animals, from moths to cats, use pheromones to attract mates.
Visual signals are limited because they require a direct line of sight and lighted conditions, and they only last as long as the sender is signaling. However, studies of communication have overemphasized visual communication, most likely because humans and primates are much more dependent on this type of communication than non-primate animals. We will not spend much time on visual signals because we are already familiar with them from our daily lives. The sender can send a signal by performing a display or by assuming a specific body posture. The receiver views the signal by means of eyes, which the brain translates into a visual image. Visual images are received in real-time, and so are generally dynamic signals.
Visual signals allow for a certain amount of cheating; that is, deceptive signals can lure receivers into responding to the benefit of the sender and the detriment of the receiver. Photuris fireflies are the only predatory species of firefly. By mimicking the female response of the prey species the "femme fatale" Photuris female lures in males, and then preys upon them. Wary males are careful in responding to female displays of their own species for fear of being preyed upon by the Photuris females. In this way, the prey males experience conflicting pressures from natural selection, which demands both individual survival and mating for species survival. This example reveals another problem with visual signals--they are not receiver specific. Any animal can potentially react to the visual signal of any species.
Most displays reveal information about the signaler, whether it be fitness, disposition, or location. Representational information imparts information about the environment external to the sender. This is a more complicated form of communication, as it requires first assimilating information about the environment, and then divulging that information to others. The honeybee dance language is an example of representational information, imparting both the distance and the direction from the hive to food. A forager will return from a food source and, by performing a directed series of movements, can inform a second wave of foragers as to the location of the same food source.
Acoustic signals are energetically costly, but can travel great distances, degrading with increasing distance. Many animals produce sounds to impart information, however only humans have a well-developed language. There is some evidence that Vervet monkeys have a language consisting of three distinct words: snake, eagle, and leopard. As it turns out, these alarm calls actually represent the type of threat, rather than the specific type of predator. The snake call warns conspecifics of the presence of a slow predator on the ground. Vervets respond to this call by standing up and looking around. The eagle call indicates a fast-flying predator. Vervets will run for cover and look up. The leopard call alerts the monkeys to a fast-running predator, and they respond by running up a tree. We will learn more about acoustical signals, namely song, in the section on Bird Song.
Physical contact is limited in its ability to communicate because it is extremely short-range. Many invertebrates use antennae as the first line of contact with objects and organisms. The honeybee waggle dance used to explain the location of a food source is often performed in a dark hive, and so the foragers receive their information by interpreting the dance with their antennae. The most common use of tactile communication occurs during copulation. Tactile stimulation by males will often let a female know when to adopt a sexually receptive posture, as in rodents. In primates, grooming is an extremely important social activity. It functions to remove parasites, but also to secure social bonds. This is also true of humans, for whom touch is an initimate form of communication.
Sharks and some fish have electroreceptors that are used to detect objects and to socially communicate. Electrolocation is a form of autocommunication; signalers send and receive their own signals. The difference between the emitted and received signals yield information about the environment through which the signal has passed. Species that use electrical signals for social communication are nocturnal or inhabit murky waters where visual communication is limited. Electrical signals are useful because they are extremely precise; they are limited to use in aquatic environments, though, because air is ineffective as an electrical insulator or conductor.
As we have seen, a wide variety of signals are used in animal communication. Of course, each has its advantages and disadvantages, and are more useful in certain situations than in others. Otherwise, evolution would have only produced one type. In , we can see a comparison of visual, acoustic, chemical, and tactile signals. Acoustic and chemical signals are useful when obstacles stand between the signaler and the receiver, whole tactile and visual signals are not useful unless there is a clear path. Chemical signals can persist for long periods of time, while other signal types occur in real time, and so are only fleeting messages.