- The senses of taste and smell
- Chemoreception in different organisms
- Behaviour and chemoreception
The specificity of pheromones depends on the specificity of perception as well as production. Little is known of the physiology of individual receptor cells outside the insects, which have receptor cells that are highly specific, at least for the major pheromone components. In many cases, when an attractant pheromone has two major components, the recipient has large numbers of cells specific to each of the compounds, often in the same sensillum. Very often the cells are extremely sensitive, enabling the animal to respond to very low concentrations of compounds.
Primer pheromones are important in aspects of social physiology in a range of animals. In mammals they are influential in coordinating reproductive physiology, and compounds excreted in the urine are especially important (see below Behaviour and chemoreception: Mammals). For example, the physiology of female mice is affected by the odour of urine produced by males and other females. Dominant males have the greatest effect, causing the release of luteinizing hormone in the female, which leads, together with contact with the male, to ovulation. In contrast, the urine of other females tends to delay ovulation. In the presence of a male, a female increases the rate at which she produces urine, and this causes the release of testosterone in the male.
Comparable pheromones are produced by locusts. A mature male desert locust produces a maturation pheromone from glands scattered throughout the epidermis. The pheromone can act via the olfactory system of the recipient or, if the insects come into contact, via the contact chemoreceptor system, although this is not known with certainty. The pheromone speeds up sexual maturation by affecting the endocrine system in individuals of both sexes, with the result that in a swarm of locusts sexual maturation tends to be synchronized.
Primer pheromones are especially important in the maintenance of colony structure in social insects. Queen honeybees secrete “queen substance” from their mandibular glands. When an unfertilized queen leaves the colony, queen substance acts as an olfactory attractant for males. The same compound within the colony modifies the behaviour of workers, preventing them from rearing more queens, and also affects their physiology, disrupting the development of their ovaries.
Movement toward an odour source
Attraction to the source of an odour poses problems for all animals using the sense of smell. It had been supposed that animals simply moved up a concentration gradient, from an area of low odour concentration to an area of high odour concentration, ending near the source of an odour. However, consideration of the movement of odour molecules in air or water showed that, in general, such gradients do not exist under natural conditions. Wind flow varies in both direction and strength. In addition, during the day, when the ground is heated, rising and falling air movements contribute to turbulence. As a result, odour molecules, even when continuously released at the source, become dispersed as a series of wisps, similar to the way that smoke from a chimney becomes dispersed. As a consequence, a stationary animal or an animal moving toward an odour source in a straight line will encounter bursts of odour with relatively long intervals between bursts. This is true whatever the distance from the source, although at short distances bursts contain more peaks with high concentrations of odour molecules. Only by averaging the concentration over a period of time and distance is it possible to follow a gradient of odour. Some animals may do this, but insects (and probably many other organisms) use a different strategy. In these organisms an odour has the effect of switching on a behavioral program that uses some signal other than odour to locate the source. In many cases the other signal is wind direction, and the animal moves upwind, ultimately arriving at the source of an odour. This mechanism is called odour-modulate