Animal Courtship and Mating

V

Types of Mating

In many aquatic animals, the union of sperm and egg takes place outside the body of the female, a process called external fertilization. Sponges, oysters, certain fish, and other aquatic organisms that rely on external fertilization often release great quantities of sperm and eggs, sometimes in the millions, an adaptation that ensures fertilization even if most of the gametes are dispersed by currents. Other aquatic organisms—frogs, for example—depend on close physical proximity and nearly simultaneous release of gametes to guarantee fertilization. Frogs carry out a well-timed courtship dance, in which the male climbs on the back of the female and clasps her in his arms. This stimulates the female to lay eggs, which the male sprays with his sperm.

In nearly all land-living organisms, fertilization is internal. Most birds transfer sperm by aligning their cloacas, an opening in the lower abdomen designed for reproduction as well as excretion of wastes. Some birds, such as bald eagles and swifts, accomplish the alignment of cloacas in mid-flight. The males of aquatic reptiles, such as turtles, and crocodiles, and aquatic birds, including ducks and geese, have a penis within the cloaca that is inserted into the female’s cloaca during sexual activity. These animals frequently copulate in water, and the penis helps transfer sperm before it is carried away by the currents Ostriches and rheas also contain a penis within the cloaca.

Virtually all snakes and lizards display yet another variation of internal fertilization. Located in the tail are two penises, called hemepenes, which are covered by spines and knobs that help lock it into place during mating. The male uses one hemepene at a time for copulation. Male salamanders secrete large quantities of sperm in a packet called a spermatophore. The female picks up the spermatophore with the lips of her vent, the external opening that leads to the cloaca. In insects and virtually all mammals and marsupials, sperm is transferred when the male’s penis is inserted into the vagina of the female.

The time and energy required for courtship and mating, as well as reproduction, are very important in determining courtship arrangements and behaviors. These time and energy expenditures—the so-called parental investment—typically differ between males and females. In general, the parental investment of females is much greater than males. More energy is required to produce eggs than sperm, for example, since eggs can be thousands of times larger than sperm. An egg typically represents a considerable energy investment, and can contain stored food that is up to 20 percent the weight of the female. Males often expend significantly more energy in the courtship process, while females typically spend more time and energy laying eggs or giving birth and nursing the young.

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The way in which energy is spent to mate in turn influences social arrangements—whether species are monogamous, polygamous, or promiscuous. In monogamy, one individual mates with only one other individual, either for the duration of the breeding season or for life. In polygamy, an individual mates with more than one individual of the opposite sex in a single breeding season. When a male mates with more than one female, it is termed polygyny; when a female mates with more than one male, it is called polyandry. In promiscuity, both the male and female mate with more than one member of the opposite sex during the breeding season.

The evolution of these reproductive strategies has been influenced by a variety of factors: the abundance and distribution of the food supply; stage of development of the newly hatched or newly born young; and the presence of predators, among other variables. Most birds, for example, eat berries, seeds, insects, and other small foods that are widely distributed in the landscape, and it takes two parents to gather enough food for the helpless young. The female has the best chance of passing her genes to the next generation if she selects for the fertilization of her relatively few valuable eggs a strong, healthy male that can successfully defend a food-rich territory. The male bird has the best chance of passing his genes to the next generation if he establishes a monogamous relationship with the female, sticks around, and helps feed his helpless offspring. Some birds, however, such as sage grouse and turkeys, produce young that are well developed and need relatively little parental care. In this case, the male has the best chance of passing his genes to the next generation if he is polygynous, mating with as many females as possible, so that his sperm will fertilize the widest variety of eggs.

The type of mating strategy in turn dictates what an animal will be looking for as it chooses a mate. If newborns require parental help for feeding and protection, behavioral traits such as nest building may be important, which may be the reason some birds build dummy nests—nests designed for display rather than for incubating eggs or raising chicks—as part of their mating ritual. If the chooser, typically the female, is only looking for good genes and not a helpmate, her choice may be based on other outward features that signify something about the mate’s genetic makeup. Big antlers may signify the ability to find food despite extra weight; energetic dancing may signify stamina; shiny plumage may signify freedom from parasites; and deep coloration may signify surplus energy, enough to produce the extra pigment.

Mate choice for most animals is not necessarily a conscious decision. It more closely resembles instinct, an inborn pattern of behavior that is part of an animal’s genetic makeup, developed over generations as natural selection favors the behaviors that work best to perpetuate the species. Peahens are attracted to energetic peacocks with long tails and colorful plumage, for instance, perhaps because these traits signal endurance, vigor, or superior hunting abilities. Females who do not select males with long tails or shiny plumage pay a price—their sons will not be as attractive to females of the next generation, and their genes will not be passed on.

VI

Contemporary Studies

Courtship and mating behavior traditionally has been studied in wild populations through direct observation. Controlled experiments have also been conducted in field and lab settings. In one laboratory study of pheasants, for example, researchers investigated the criteria females use to select males. Male pheasants of many species sport two or three spurs on each leg, formidable weapons for fighting off competitors for their harems and predators of their young. Long spurs not only indicate good fighting ability, but better genetic makeup overall, as males with longer spurs outlive males with short spurs. The researchers attached shortened or lengthened artificial spurs to pheasant males, and found that females selected males with the longest spurs. This experiment, among others, suggests that certain physical characteristics signal key information about physiology, an important factor in mate selection.

More recently, investigations have been conducted at the molecular level. Studies show that genes linked to a set of immune-system proteins called the major histocompatibility complex (MHC) play a role in the mating behavior of mice. The different types of MHC are identified by specific urine odors, enabling potential mates to choose each other on the basis of a particular MHC. Other investigations have helped researchers pinpoint what they believe is the gene that codes for the proteins involved in courtship and mating behaviors of the male fruit fly. From observation of behavior in natural settings to investigation of genes, scientists continue to unravel the complexities of courtship and mating, the seasonal drama that generates life itself.

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