The Things Animals Do for “Love”
Webster’s Dictionary defines courtship as engaging in activity that leads to mating. The ultimate purpose of courtship is the passing on of an organism’s genes to subsequent generations of the species. The reproductive strategies of accomplishing this objective are varied. Broadly speaking, the optimum methods for males and females are quite different. Because mammalian females produce far fewer gametes or sex cells than males, their reproductive potential is much smaller. Females of some species produce only a single egg at ovulation, whereas the male usually produces millions of sperm.
In general, the optimum breeding strategy for females is to select a mate whose genes will enable the offspring to achieve a high rate of survival and reproducibility. For the male, the best plan is to mate with as many females as possible. However, many different factors come into play that cause these strategies to be modified.
Mammalian courtship depends on the female’s ability to alert the male of her receptiveness to mating. Her signals fall basically into three categories: visual, acoustic, and olfactory. The posterior swelling of female chimpanzees and Old World monkeys is an example of a visual signal. Female elephants, on the other hand, advertise their sexual receptivity by emitting low frequency sound waves. Although inaudible to human ears, hulls have been known to find estrus females by following these sounds.
Olfactory signals are especially important to solitary mammals such as insectivores, many of the smaller marsupials, some carnivores, and some ungulates. The female often relies on scent to advertise her receptiveness to the advances of a male. She deposits these olfactory substances, called pheromones, on objects such as branches, leaves, and rocks. Frequently territorial, solitary mammals also use pheromones to avoid one another. Individuals mark their territories in order to minimize aggressive encounters, which sometimes can result in serious fighting. This strategy is particularly important with carnivores that are armed heavily with sharp teeth and claws.
Sometimes carnivores indicate sexual receptiveness through fighting. This type of fighting is playful in nature. With certain species, it is a mating ritual. For instance, with many feline species, the male will bite the nape of the female’s neck when copulating. This behavior is a ritualized form of the neck bite that cats often employ in dispatching prey.
In contrast to the solitary species, the social structure of a variety of mammalian species consists of a single male with a harem of many females. This type of grouping is seen in koalas, primates, pinnipeds such as elephant seals, and a variety of odd and even-toed, hoofed mammals. Male elephant seals engage in bloody battles to determine who will be the harem master, biting competitors in the face, neck, and shoulders. The injuries that the males sustain in these battles are less serious than they appear, since the areas of the body attacked are well-protected by fat.
The horns and antlers that most even-toed, hoofed mammals (artiodactyls) wear serve as weapons during ritualized contests between males. They can be used effectively to defend against predators, but this is not their primary purpose. Males may fight to gather a harem of females, as in the case of many deer such as the wapiti or American elk, or they may battle over possession of a territory into which females will enter. Wildebeests and gazelles fall into this latter category.
Another kind of African antelope, the kob, also establishes and defends a territory. These territories are located on a mating ground referred to as a lek. Females visit these territories and may mate with as many as ten different males while moving outside of their own territories. The males whose territories are at the center of the lek mate more frequently than those males with territories near the edge. The most successful males may be those that mate with the most females, and have the most effective display subsequent to copulation.
Unlike birds, monogamy in mammals is rather rare, seen in only about five percent of living species, including some primates such as the gibbons of southeastern Asia, and the owl or night monkey of South America. In these species, the sexes are virtually the same size, in contrast to the marked size difference between sexes one sees in polygamous species such as baboons and gorillas.
On your next visit to the Zoo, take a closer look at some of the mammals and their activities. With a little bit of luck and patience, you just might find yourself witnessing firsthand some examples of mammalian courtship.
The original article was published in Zoo View (Winter 1991) as “Courtship in Mammals,” written by Michael J. Crotty