Animal mating

As one observes the diverse and elaborate movements and behaviors seen in courtship displayed by all types of animals, it is easy to lose sight of what courtship is meant to accomplish. Whether relatively simple or highly complex, the ultimate goal of courtship is to bring together two animals of different sexes of the same species to bring about successful mating and reproduction. Although they may not always be as obvious, mating patterns among animals also are quite diverse.

Reproduction, one of nature’s most fundamental and essential functions, is the process by which all living organisms produce offspring. The need to propagate is a cardinal necessity for the preservation of the species. Each living organism has its own unique way of accomplishing this requirement. The higher the animal, the more intricate the process. In one-celled organisms, reproduction is usually asexual, where only one living entity is required for procreation. The one-celled animal simply splits in two, losing its original identity and thereby creating two new organisms that are exact duplicates of the parent. The process is known as fission. Some single-celled entities reproduce sexually, where two similar organisms fuse, exchange nuclear materials, and then break apart, after which each organism reproduces by fission. This form of unicellular reproduction is known as conjugation. Sometimes, after conjugation, the participating organisms do not reproduce. It appears that the process is merely to revitalize the organisms. This is the most primitive method of sexual reproduction. The new organisms that are produced are from two distinct parents, having definite genetic characteristics of their own.

The procreation process in most multicellular animals involves a more complex form of sexual reproduction. Here, unique and differentiated male and female reproductive cells called gametes unite to form a single cell known as the zygote. The zygote undergoes successive divisions to form a new multicellular organism, where half the genes in the zygote come from one parent and half from the other, creating a singularly different living creature.

Since most animal species have sexual reproduction, it must confer some advantages. Sexual reproduction results in maintaining high levels of genetic variability within a population. Genetic variability produces variability in behavior, structure, and physiology and provides a species with greater flexibility in meeting changing environmental conditions. There must also be some long-term evolutionary advantages.

A dictionary definition of mating encompasses the idea of individuals coming together to form a pair, with the implication of doing so to produce offspring. If an emphasis is placed on the latter part of the definition, then it is seen that mating may involve all degrees and types of relationships and interactions among animals. In animals that have sexual reproduction, the union of male and female sperm and eggs, also known as gametes or sex cells, may be accomplished by either external or internal fertilization. Simple or elaborate courtship rituals ready animals for mating and fertilization. Several factors may influence reproduction. Most animals have a distinct time of the year, the breeding season, when reproduction is possible. Depending on the species, the breeding season may be in the spring, summer, fall, or winter. In other species, including humans and some primates, breeding may occur throughout the year. The breeding season starts with the onset of courtship and finishes when the last offspring are weaned. The same species may have different breeding seasons depending on their location. In temperate regions of the world, there may be much variation in breeding in different years. Any factor that affects the health of animals may modify their breeding season, including food availability, temperature, light, and population density. In general, the breeding season is usually coordinated to maximize the likelihood of survival of the young.

Mating Systems

The means by which males and females are brought together in courtship and, ultimately, copulation is achieved by some type of mating system or pattern. Systems vary widely throughout the animal kingdom, but there are several general groupings. (A reading of the literature in cultural anthropology shows that virtually all these mating systems can be found in different human cultures, as well.) Mating systems are typically classified under three headings: promiscuity, monogamy, and polygamy. It also is useful to delineate subcategories within some of these.

Promiscuity, as it defines a mating system, means that no pair bond is formed between individuals. A technical term for this system is polybranchygamy, which literally means “many brief matings.” In this system, any male may mate with any female, and no one individual has exclusive rights over any individuals of the opposite sex. Males and females may copulate with one or many of the opposite sex. Promiscuity is found more often in males than in females, and this observation is probably related to the fact that in such species, males have a far lesser investment in offspring than females. The system is found in a small percentage of bird species and many species of mammals. Promiscuity is demonstrated well in those species that form leks. A lek is an area or territory used for communal courtship displays and mating by certain species. Leks have been observed among grouse, some African antelope, a species of bat, and some insects. Leks are used solely for mating. Females are attracted to the leks by elaborate courtship displays by the males, and mate with one or a small number of the males. Usually, only a select few of the males perform most of the matings. In one study of grouse, less than 10 percent of the males carried out more than 75 percent of all copulations. The African antelope, the Uganda kob, is an example of a mammal that utilizes a lek. Here, also, a small percentage of males breed most of the adult females. The system is highly effective, as nearly all females produce offspring.

A second major type of mating system is monogamy. It is a system in which a pair-bond is formed between one female and one male. The pair-bond may exist for only one breeding season (annual monogamy), or it may persist for one or more breeding seasons (perennial monogamy). Monogamy is very common in birds, with a large majority of species showing it. Swans and eagles are examples of species showing perennial monogamy, and sparrows and warblers are examples of species showing annual monogamy. Although it has been thought that many birds practice true monogamy, evidence has accumulated that in monogamous birds, there are frequent matings of the males with females outside of the primary pair-bond relationship. These matings are known as extra-pair-bond copulations and offer advantages to the males if they result in the successful fertilization of additional eggs. Monogamy seems to occur when both the male and female have nearly identical roles in the rearing of the young.

Monogamy is far less common in mammals than in birds, but there are some notable examples. If monogamy is likely to occur where there is equal parental investment by both males and females, it might be predicted that there is less need for male mammals to practice monogamy since only the females nourish the young with milk. Among mammals, monogamy is observed in gibbons, foxes, wolves, beavers, red foxes, and some small rodents. Why should there be monogamy among any mammals? It would be advantageous for males to be monogamous if, by doing so, they increased the likelihood of survival of their offspring. Males could do so if they helped to feed the young and helped to defend the territory against predators. Monogamy can also be looked at from the perspective of the female, a so-called female-enforced monogamy, which may take place if females can gain sole benefit from the male’s efforts without having to share them with other females.

The third major type of mating system is polygamy. In polygamy, an individual of one sex forms a pair bond with several members of the opposite sex. The two major subtypes of polygamy are polyandry, where one female mates with more than one male, and polygyny, where one male mates with more than one female. Within both polygyny and polyandry, there may be either a serial type or a simultaneous type. In the former case, one male or one female bonds with several members of the opposite sex but only one at a time. In the latter case, one male or one female bonds with several individuals of the opposite sex at the same time. Simultaneous polygyny is often referred to as harem polygyny. Although it appears to resemble promiscuity, it differs from promiscuity in that a pair-bond is formed even though it may be temporary. In altricial birds, in which the young are born in a helpless state, it is often observed that polygyny is present in habitats where food is unevenly distributed, and one male can provide food for more than one female. Polygyny may also result in cases where there is a lack of availability of suitable territories for breeding and where there may be the pressure of heavy predation.

Hermaphroditism

Although most animal species are dioecious, meaning they have two separate sexes, male and female, some individuals and even whole species are monoecious and have both sexes in one individual. Monoecious animals are also called hermaphrodites. Although hermaphroditism is unusual in humans and most mammals, in some species, it is the rule. There are many invertebrates in which the same individual produces both eggs and sperm. Some examples of hermaphroditic species include garden snails, free-living flatworms, frogs, the common earthworm, and some fish. It might seem that the hermaphroditic condition is beneficial and efficient, since any single individual is capable of producing and delivering both eggs and sperm. However, there is at least one major drawback: the possibility of self-fertilization. Self-fertilization does not produce as much genetic variability in offspring as would be expected under cross-fertilization. In species in which hermaphroditism is the rule, there are a number of processes at work that make self-fertilization unlikely, if not impossible.

Some animals produce eggs at one time and sperm at a different time, making it impossible to self-fertilize. In animals in which the sex organs mature at different times, a condition known as protandry (male to female; such as clownfish) or protogyny (female to male; some wrasses), the animal alternates between being different sexes, functioning as a female or male first and then becoming the other sex at a later time. In some species, including earthworms, two individuals engage in mutual copulation. The two worms are held together by a secretion produced by both. The togetherness allows sufficient time for sperm from one worm to travel to the other worm and fertilize that worm’s eggs, and vice versa. Although copulation is typically brief in most animals, it may last three hours in the earthworm.

It would be remiss not to mention another example of parthenogenesis. There are a few species of fish and lizards in which only females are known and in which the offspring are produced from eggs without fertilization by sperm. Parthenogenesis also occurs in invertebrates, including velvet worms. The Amazon molly, a fish, goes through the acts of courtship and mating with a male, but it is with males of other species, and sperm do not fertilize any of her eggs. The eggs develop parthenogenetically and produce another generation, apparently only of females. This can result in polyembryonically produced siblings that are identical to one another but genetically unrelated to their mother. Parthenogenesis is common in the insect order Hymenoptera, which includes bees and wasps.

Although uncommon, hermaphroditism may result from mutations in some species. Scientists have observed hermaphroditism in frog and fish species that are typically gonochoric after exposure to certain herbicides that reach their habitats through polluted runoff. In sheep, the condition may result from a mutation in the SRY (sex-determining region Y) gene. Similarly, as French bulldogs increased in popularity in the early twenty-first century, the occurrence of hermaphroditism also increased with irresponsible breeding practices.

Even a brief discussion of mating in animals reveals the complex and diverse methods used to produce another generation of the rich diversity of animal life on Earth.

Principal Terms

Asexual Reproduction: Reproduction without the union of male and female sex cells

Dioecious: Having two separate sexes, namely male and female

Estrus: The period of the sexual cycle during which a female is sexually receptive

Gamete: A sex cell, either male or female

Hermaphrodite: An animal with both male and female sex organs

Monogamy: A mating system in which one male and one female comprise the main breeding unit

Polygamy: A mating system in which a single adult of one sex mates with several members of the opposite sex

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