Although the exact way by which light affects the reproductive cycle is still disputed, it undoubtedly varies from group to group. In birds, light passes either through the eyes or through the bony tissue of the skull and stimulates the development of certain cells in the forepart of the brain. These cells then secrete a substance that stimulates the anterior pituitary gland, which is located at the base of the brain, to produce an array of regulatory substances (hormones), called gonadotropins, that are carried by the blood to the gonads (ovaries and testes), where they directly stimulate the development of eggs and sperm. The gonads, in turn, produce the sex hormones—estrogen in the female and testosterone in the male—that directly control several overt aspects of reproductive behaviour.

Unlike the higher animals, the gonads of insects apparently do not themselves secrete hormones. Instead, stimulation by the corpus allatum, an organ in insects that corresponds in function to the pituitary gland, causes the secretion of liquid substances on the body surface. These substances are transmitted as liquids, or, even more significantly, as gases, to the recipient, in which they are usually detected by olfaction or taste. Such substances, which are called ectohormones, or pheromones, may serve as the major regulation and communication system for reproduction as well as other behaviour in insects.

In the absence of all other stimuli, many types of sexual behaviour can be induced simply by an injection of the appropriate gonadal hormone. Conversely, removal of the gonads usually inhibits most sexual behaviour. The apparent failure of complete hormonal control over reproductive behaviour has been a subject of much investigation and dispute. There is much evidence that many types of reproductive behaviour are or can be controlled solely by neural mechanisms, bypassing the hormonal system and any effect that it might exert on the nervous system to produce behaviour. Several types of reproductive behaviour controlled solely or almost solely by neural mechanisms are involved in or triggered by the processes that are initiated by courtship.

Visual clues

The appearance of many higher vertebrates changes with the onset of reproductive activity. The so-called prenuptial molt in many male birds results in the attainment of the nuptial plumage, which often differs radically from that possessed by the bird at other times of the year or from that possessed by a nonreproductive individual. The hindquarters of female baboons become bright red in colour, which indicates, or advertises, the fact that she is in estrus and sexually receptive. Such changes in appearance are less common in the lower animals but do occur in many fishes, crabs, and cephalopods (e.g., squids and octopuses).

Often associated with changes in appearance are changes in behaviour, particularly the increase in aggressive behaviour between males, often a prime feature in attracting females; such changes have interesting evolutionary implications. In certain grouse, for example, females are most attracted to males that engage in the greatest amount of fighting. No doubt, fighting in some groups of mammals also serves this function as well as others.

In many animals the rise in aggression takes the form of territoriality, in which an individual, usually a male, defends a particular location or territory by excluding from it all other males of his own kind. Occasionally, other species are also excluded when it is to the advantage of the defending individual to do so. Territorial behaviour involves many functions, not all of which are directly concerned with reproduction. For purposes of advertising, however, territoriality probably reduces the amount of interference between males and also makes it easier for females to find males at the proper time.

Auditory clues

The fact that sound signals can travel around barriers, whereas visual signals cannot, accounts for their widespread use in indicating sexual receptiveness, especially in frogs, insects, and birds. Like visual signals, a sound for advertising purposes usually encodes several pieces of information; for example, the signals usually reveal to the receiver the caller’s species, its sex, and, in some cases, whether or not it is mated. The vocalizations of one type of frog also reveal the number of other males located nearby. This information, a critical clue for females, is a measure of how good the habitat is for depositing eggs. The sounds produced by the wings of mosquitoes attract females and are species specific. Humans have taken advantage of this signal by using artificial sound generators to eradicate certain mosquitoes. Advertising signals also serve to repel other males; a classical example is the territorial song of many songbirds.

Olfactory clues

Researchers have now become aware of the enormous amount of information that is passed between animals by chemical means. Well known are the urine, feces, and scent markings employed by most mammals to delimit their breeding territories and to advertise their sexual state. Males of a number of mammals are capable of determining if a female will be sexually receptive simply by smelling her urine markings. A substance in the urine of male mice, on the other hand, actually induces and accelerates the estrous cycle of females. A female gypsy moth is able to attract males thousands of metres downwind of it simply by releasing minute quantities of its sex pheromone each second. It has been calculated that one female silkworm moth carries only about 1.5 micrograms (1.5 × 10^-6 gram) of its sex attractant, called bombykol, at any given moment; theoretically, this is enough to activate more than 1,000,000,000 males. The sex attractant of barnacles, which are otherwise rather sessile (sedentary) organisms, causes individuals to aggregate during the breeding period.

Another possible channel of communication occurs in a few fishes, namely electric discharge. Evidence suggests that weak electric fields and discharges in the Mormyridae of Africa and Gymnotidae of South America represent the major mode of social interaction in these families.