Animal reproduction

The ability to reproduce is central to the existence of any organism. Simple one-celled organisms reproduce asexually by duplicating their genetic material and dividing in half. For reproduction in sexual species, the participation of two individuals is essential. Mating partners have a common interest: production of successful offspring. They also have conflicting interests. Appreciating the diversity of reproductive behaviors that occur in different organisms requires an understanding of the reproductive conflicts between mates even more than their mutual reproductive interests.

Early sexual species most likely consisted of individuals that produced gametes of similar size. Except that one set of individuals produced smaller gametes that would be called sperm, and another set produced larger gametes called eggs, males and females did not exist. Yet, reproduction was sexual. Through time it is thought that distinct sexes evolved because some individuals obtained reproductive advantages by producing smaller than average gametes whose greater motility increased their likelihood of fertilizing other gametes, while other individuals obtained reproductive advantages by producing larger than average gametes whose greater stores of nutrients increased the survival prospects of their young. Such evolution of gamete dimorphism would then lead to the many specializations in appearance and behavior of present-day males and females.

Reproductive Success

For species whose parents do not provide care for their young, the maximum number of offspring that an individual female can produce is determined by the number of eggs that she can manufacture. For species in which parents do provide care, the number of young a female can produce might be limited more by the number of young she can raise than by the number of eggs she can make. For both these types of species, females that copulate with more than one male produce roughly the same number of young each season as females that mate with only one male. Thus, the quality of an individual mate, or the resources of paternal care that he can provide, should affect the reproductive success of females more than the number of mates they can obtain. As a result, females of most species are expected to be selective about which male fertilizes their eggs.

In contrast, the reproductive success of males of most species is not limited by the number of sperm they can produce. In species lacking parental care, males that mate with the most females usually leave the most offspring. In parental species, the reproductive consequences of mating with more than one female are slightly more complex. If one parent can provide sufficient care to raise young to independence, males that mate with multiple females, usually leave more offspring than males that mate with only one female. If both parents are necessary to raise young, males that mate with only one female should leave more offspring than males with multiple mates. Given the reproductive advantages of mating with multiple females and a tendency for breeding males to outnumber breeding females, competition for mating opportunities is more pronounced in males than in females for many species. Furthermore, males should usually be less selective in mate choice than females.

Mate Competition and Mate Choice

Charles Darwin realized that the struggle to obtain mates could be an important process affecting the evolution of organisms. He deemed this process sexual selection to distinguish it from the struggle for existence, or natural selection, and suggested two components of sexual selection: mate competition and mate choice. Biologists rarely question the importance of mate competition because it often involves frequent and conspicuous aggressive interactions and distinctive weapons, such as antlers of various ungulate species, horns of bovids, and enlarged canine teeth of primates.

Mate choice is an important evolutionary mechanism that ensures reproductive success, the continuation of the species, and the best quality offspring possible. It is sometimes an exceedingly subtle behavior that is difficult to document, but in other animals, their mate choice behaviors are well documented, such as birds and fishlike guppies. The environment can strongly influence mate choice. When the risk of predation is low, animals are choosier, as well as in areas with abundant options for potential mates. Additionally, the individual state of an animal, such as their age, parasite load, body size, and whether they have reproduced before, influences their mate choice choosiness. To avoid inbreeding, some mice species rely on a particular protein in one another's urine to indicate if they are related, but in most species, offspring leave the family group in adolescence, making choosing a relative as a mate unlikely. Notably, some criticism remains in the literature regarding the underlying mechanisms of mate choice.

Fighting for Mates

Similarities and differences in the details of mate competition and mate choice of different species can be exemplified by comparing three North American vertebrates: bullfrogs, sage grouse, and elephant seals. Male bullfrogs fight each other for small areas in ponds that females use as egg deposition sites. The wrestling matches that occur between males do not involve weapons as such, but being larger than an opponent almost always confers success. During nighttime choruses, females move among the territorial males, apparently assessing the features of the male and his territory. Pairing begins when a female approaches a particular calling male and touches him. The male clasps the female, and within an hour, the female releases up to twenty thousand eggs, which the male fertilizes externally. Neither the male nor the female provides parental care for their young. Each year, roughly half of the males in a population obtain mates, and the most successful male may mate with six or seven different females.

Male elephant seals can weigh as much as three thousand kilograms and are highly aggressive. Rather than fight for territories, they fight directly for groups of females, called harems, that haul out on land to give birth. Males that monopolize large groups of females might mate with ninety or more females each year. One study revealed that less than 10 percent of the males sire all the pups in a breeding colony. Success in male competition not only involves being large but also involves having formidable canine teeth to use as weapons. During a fight, males rear up to half their length, slam themselves against the opponent, and bite him on the neck. The skin on the chests of males is highly cornified; these “shields” provide some protection against such onslaughts, but the injury is still common. Mate choice by females is limited to vocalizing before and during copulation. If the male attempting to mate with her is a subordinate individual, the dominant male quickly responds to the call and attacks the copulating male.

Male sage grouse often congregate, or “lek,” in traditional areas where they display and fight to control small territories. The territories function as courtship sites and places to copulate. Males provide neither resources nor parental care for their young, and females initiate mating and appear to be highly selective in mate choice. Near unanimity in preferred mates by the females in the population results in only a few males obtaining all the matings.

In all three of these species, many males compete for mates by fighting for territories; however, some males employ different tactics to obtain mates. Small, young bullfrog males remain silent near large, calling males and attempt to intercept any female attracted by the calling male. Small, young elephant seal males lurk about on the surf and grab females as they leave the land to feed in the ocean. These males force females to copulate. Some sage grouse males attempt copulation with females away from the display arena. In another lekking bird species, the ruff, two types of males exist: territorial males and satellite males. The latter males are nonaggressive and appear to capitalize on the ability of territorial males to attract females. Unlike the case of bullfrog males, however, genetic differences of male ruffs produce striking differences in plumage and behavior rather than their ages.

Mate choice by females is well-developed in bullfrogs and sage grouse. The benefit that female bullfrogs obtain by choosing particular males is relatively straightforward: Chosen males tend to control superior egg deposition sites that increase offspring survival. The benefits that female sage grouse obtain from mate choice are unknown. Mate choice in such lekking species continues to pose a significant question for biologists.

Competition and Reproductive Success

The differential success of males in mate competition and mate attraction may translate to large differences in reproductive success. In contrast, variation in reproductive success among females is usually low because most females mate and produce at least some offspring. The relative amount of variation in reproductive success within each sex can influence the evolution of sexual traits. When only a few adults produce most of the offspring in a population, genes affecting the traits that underscore their success will be passed on to their offspring and quickly become the predominant characteristics of future generations. In contrast, if the most successful individual produces only slightly more offspring than other individuals, genes from all these parents will be present in roughly similar numbers in subsequent generations.

A consequence of greater variation in the reproductive success of males relative to females is the evolution of elaborate sexual characteristics that are expressed only in males. These traits can be morphological, physiological, or behavioral. The extent of sexual dimorphism is predicted to be related to the relative variation in reproductive success in the sexes. Thus, species in which one or a few males sire most of the offspring produced in the population would tend to be species with considerable phenotypic differences between the sexes.

Field Research and Laboratory Studies

Studies of the reproductive behavior of organisms usually involve observation and experimentation of male and female interactions in nature or the laboratory. Early studies were mostly observational and cataloged the most typical behavior patterns observed in each sex. These studies ignored differences in behaviors among individuals. Because such differences can have significant consequences in terms of reproductive success, more recent studies usually involve marking males and females for individual recognition and recording various features of their morphology, behavior, and reproductive success.

Quantifying reproductive success in nature is a difficult task, and various methods have been used for different organisms. For all studies, the identity of each individual must be known. For some species, researchers can assess only the number of copulations that individuals obtain; for other species, they can count the number of young born; in others, they can determine the number of young that survive to independence or even sexual maturation.

Laboratory and field experimentation has been used to study a plethora of questions concerning the acquisition, function, and evolutionary significance of a variety of reproductive behaviors. Early studies of bird song investigated not only the role of song in attracting mates but also how individuals acquired their species-typical song. Choice experiments on female frogs using either naturally calling males or playbacks of recorded male calls revealed the call characteristics that females use in species and mate recognition. Crossing different species that varied in reproductive behaviors and noting the characteristics of their hybrid offspring provided some insights into the genetic basis of various behaviors. Staging aggressive interactions between males that differ physically in some regard demonstrated the significance of various male characteristics.

Researchers investigate general trends in reproductive behaviors by using the comparative method. This method usually works best when fairly closely related taxa (for example, species within the same genera or family) are considered. Using the comparative method, researchers can look for the relationships between the degree of sexual dimorphism in some characteristic and sex-specific differences in reproductive success variation, or the ecological and social conditions that affect male behaviors, such as territoriality, or female behaviors, such as the amount of maternal care provided to young.

A thorough understanding of reproductive behaviors requires the creative use of all three methods of investigation: observations of individuals in nature to document normal behavioral patterns, precise experimentation on behaviors under controlled conditions to understand mechanisms of behavior and the stimuli that produce them, and comparison of trends in closely related species to gain insights into the evolutionary history of behavioral traits.

Reproduction is one of the defining attributes of life itself, and for most organisms, reproduction is sexual. Despite its universality, the behavior patterns associated with sexual reproduction are exceedingly diverse. For solitary organisms, sexual reproduction may be the only form of social behavior. For highly social species, individual interactions can be much more complex but still usually influenced by sex in some manner. Biologists seek to find some order to the variety of reproductive behaviors observed in different organisms. A unifying theme for this diversity is an evolutionary one. How do the sexes differ in maximizing the number of offspring they can produce? More specifically, how do males or females maximize the number of offspring they produce, given the behavior patterns of the other sex and the various ecological factors that affect them? Thus, research on reproductive behaviors has gone well beyond merely describing what animals do to reproduce to determining why they do what they do.

Principal Terms

Gamete: A sexual reproductive cell that must fuse with another cell to produce offspring: a sperm or egg

Mate Choice: The tendency of members of one sex to mate with particular members of the other sex

Mate Competition: Competition among members of one sex for mating opportunities with members of the opposite sex

Natural Selection: The process that occurs when inherited physical or behavioral differences among individuals cause some individuals to leave more offspring than others

Reproductive Success: The number of offspring produced by one individual relative to other individuals in the same population

Sexual Dimorphism: An observable difference between males and females in morphology, physiology, and behavior

Sexual Selection: The process that occurs when inherited physical or behavioral differences among individuals cause some individuals to obtain more matings than others

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