BIOLOGY Volume 2 - A Guide to General Biology - 2004

17. ANIMAL COORDINATION AND REGULATION

17.8. Innate Behavior

17.8.7. Courtship and Mating

Courtship and mating involve a multitude of complex, ritualized, species-specific behaviors. In birds, mammals, and certain fish, these two processes typically follow immediately after a male establishes his territory. Courtship is a complex of behavioral reactions designed to stimulate a partner's sexual activity. If both parents participate in rearing offspring (as, for example, in thrushes), courtship helps form breeding pairs, whereas in gregarious animals (such as baboons), it leads to The formation of harems. Most such species exhibit seasonal sexual activity linked to biological rhythms (Section 17.8.5).

Courtship is regulated primarily by motivational and releasing signals, culminating in mating (copulation) as its final stage. Behavioral responses during mating are initiated by releasing signals and terminated by releasing-inhibiting signals associated with the ejaculation of sperm by the male.

Motivational signals for courtship in most species are external factors (such as photoperiod) that elevate sex hormone levels and induce gonad maturation. In many cases, this leads to dramatic changes in secondary sexual characteristics and behavioral patterns, including color changes (such as the appearance of a red belly in male three-spined sticklebacks), the enlargement of specific body parts (the growth of specialized ornamental feathers in male birds-of-paradise, etc.), mating calls (in nightingales, etc.), display postures (as in the great crested grebe — see Fig. 17.63), and The Use of chemical sex attractants (as in butterflies).

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Fig. 17.63. An element of the courtship ritual in the great crested grebe. The male and female display nesting material to each other. (From Huxley J. S., (1914). Proc. Zool. Soc. Lond., 1914(2), 491-562.)

Among the numerous signals used during courtship to attract members of the opposite sex, visual, auditory, and olfactory signals play the most critical role. For example, the male fiddler crab (Uca) attracts a female by waving its enlarged claw and performing fiddling-like movements (a visual signal). As the female approaches, the intensity of these movements increases. Many insects, amphibians, birds, and mammals use acoustic signals during courtship. For instance, females of certain mosquitoes attract males with a buzzing sound produced by wing vibration, while grasshoppers, crickets, and cicadas chirp during courtship. This chirping sound is generated either by rubbing the hind legs against each other or against the tegmina (hard forewings), or by rubbing the tegmina against one another. In either case, the acoustic "pattern" of the chirping is species-specific, eliciting a response only from individuals of the same species.

Some spiders attract partners using mechanical Methods. The male approaches a female sitting at the center of her web, plucking the silk threads at a frequency characteristic of that species. These web vibrations pacify the female and reduce her innate aggressiveness, allowing the male to approach and fertilize her. If the male mistakenly approaches a female of a different species or "strikes a false note," he will be immediately attacked and killed!

For courtship and mating (as well as for regulating social behavior, as described below), many animals secrete small quantities of specific chemicals that trigger a targeted physiological or behavioral response in other individuals of the same species. These substances, known as pheromones, are typically highly volatile, low-molecular-weight compounds. Many act as natural sex attractants. The first of these to be identified were civetone and muscone, secreted by musk deer; civetone is secreted by the anal glands, and muscone by an abdominal gland. Both substances have a strong musky scent and are used in perfumery. Mares, cows, and bitches release pheromones during estrus. While the human olfactory epithelium does not detect these odors, males of the respective species respond to them strongly. Bombykol, a pheromone secreted by eversible glands at the end of the abdomen of an unfertilized female silkworm moth, can attract males of the same species from considerable distances. Olfactory receptors on the antennae of male moths detect pheromones present in extremely low concentrations in the air; this prompts the males to exhibit rheotaxis, flying upwind until they reach the female. Pheromones are increasingly utilized as biological pest control agents. For example, to control the spongy moth, the pheromone gyplure is placed in traps. The scent lures males to the pheromone source, where they can be captured and eliminated. This not only immediately reduces the population density but also leaves many females unfertilized, thereby decreasing the size of the next pest generation as well.

Sometimes animals use pheromones to induce mating. For instance, the male queen butterfly (Danaus gilippus) applies his secretion to the female's antennae using specialized brush-like appendages that evert from the tip of his abdomen, thereby inducing her to assume the copulatory posture. The Sequential Stages of courtship and mating in this species are shown in Fig. 17.64.

Fig. 17.64. Courtship and copulation in the queen butterfly (Danaus gilippus). Arrows point from signals to the responses they elicit. (From Brower L. P., Brower J. V. Z., Cranston, F. P. (1965). Zoologica, 50, 18.)

In some animals, courtship is accompanied by conflict behavior from one or both sexes. If the individuals are normally solitary, this shift in their attitude toward conspecifics is likely driven by elevated Blood levels of Sex Hormones. However, it is also possible that these mating conflicts serve to strengthen bonds within forming pairs and synchronize gonad maturation in both partners by the time of mating. In certain spiders, such as tarantulas, the conflict between male and female subsides only by the moment of copulation, which nonetheless ends with the female killing the male.