Reproductive strategies in animals

The term “reproductive strategies” is a slight misnomer regarding animal reproduction. The word strategy implies that an organism has had conscious forethought in determining how to proceed with its reproductive events and that some planning has occurred. This is virtually impossible except for humans, who plan aspects of their parenthood. Having a reproductive strategy implies only that an organism has evolved a pattern that maximizes its success in producing offspring.

The concept of reproductive strategies is closely related to that of natural selection. Natural selection results in the more fit individuals within a population, under a given set of environmental circumstances, being more likely to pass on their genes to future generations. By this process, the gene pool (genetic makeup) of the population is altered over time. An organism’s fitness can be assessed by evaluating two key characteristics—survival and reproductive success. The organism’s reproductive strategy, then, is that blend of traits enabling it to have the highest overall reproductive success. The application of the term “reproductive strategy” has also been extended to describe patterns beyond individual organisms: the population, species—even entire groups of similar species, such as carnivorous mammals.

Examination of reproductive strategies is part of the larger study of life-history evolution, which attempts to understand why a given set of basic traits has evolved. These traits include not only those pertaining to reproduction but also those such as body size and longevity. To consider a reproductive strategy appropriately, one must view it within the context of the organism’s overall life history precisely because these traits (particularly body size) often affect reproductive traits. One should also evaluate the role that the organism’s ancestry plays in these processes. A species’ evolutionary history can have a profound effect on its current attributes.

Traits and Behaviors

A reproductive strategy consists of a collection of basic reproductive traits, including litter, or “clutch,” size (the number of offspring produced per birth), the number of litters per year, the number of litters in a lifetime, and the time between litters, gestation, or pregnancy length. The age of the mother’s first pregnancy is also a consideration. Another trait is the degree of development of the young at birth. In different species, mothers put varying levels of time and energy into the production of either relatively immature or altricial offspring or offspring that are well-developed or precocial.

Reproductive strategies also consist of behavioral elements, such as the mating system and the amount of parental care. Mating systems include monogamy (in which one male is mated to one female) and polygamy (in which an individual of one sex is mated to more than one from the other). The type of polygamy when one male mates with several females is called polygyny; the reverse is known as polyandry.

Finally, physiological events such as those involved in ovulation (what happens when the egg or eggs are shed from the ovary) may also be used to characterize a reproductive strategy. Most mammals are spontaneous ovulators, including humans, horses, monkeys, sheep, pigs, cows, and many rodents. These females shed their eggs during a relatively predictable, intervaled reproductive cycle without any physical stimulation as a result of hormones. Other mammalian species are induced ovulators—a female ovulates only after being physically stimulated by visual, auditory, olfactory, or mechanical stimulation by a male during copulation. Induced or reflex ovulatory animals include ferrets, minks, raccoons, cats, rabbits, and bears. Camelids like llamas and camels begin ovulating after sexual contact with a male’s seminal fluid, which contains an ovulation-inducing factor known as the β-nerve growth factor (β-NGF). These patterns, induced and spontaneous ovulation, may be regarded as alternative reproductive strategies, each enabling a type of species to reproduce successfully under certain conditions.

The overall effectiveness of a reproductive strategy is important to consider with respect to the relative success of the offspring (even those in future generations) in leaving their own descendants. A sound reproductive strategy results in increased fitness. An organism’s fitness as it affects the population’s gene pool may not be adequately assessed until several generations have passed.

The r and K Selection Model

The model of r and K selection is the most widely cited description of how certain reproductive traits are most effective under certain environmental conditions. To appreciate this model, an understanding of elementary population dynamics is needed. At the early stages of a population’s growth, the rate of addition of new individuals (designated r) tends to be slow. After a sufficient number of individuals is reached, the growth rate can increase sharply, resulting in a boom phase. In most environments, however, unrestrained growth cannot continue indefinitely. Critical resources—food, water, and protective cover—become more scarce as the environment’s carrying capacity (K) is approached. Carrying capacity is the maximum population size an area can support. When the population approaches this level, the growth rate slows as individuals have fewer resources to convert into the production of new offspring.

This pattern is defined as density-dependent population growth—the density or number of individuals per area that influences its growth. This description of population dynamics is also referred to as logistic growth and was conceived by the Belgian mathematician Pierre-François Verhulst in the early nineteenth century. It has successfully described population growth in many species.

The r and K selection model was presented by Robert H. MacArthur and Edward O. Wilson in their influential book, The Theory of Island Biogeography (1967). They argued that in the early phase of a population’s growth, individuals should evolve traits associated with high reproductive output. This enables them to take advantage of the relatively plentiful supply of food. The evolution of such traits is called r selection after the high population growth rates occurring during this phase. They also suggested that, as the carrying capacity was approached, individuals would be selected that could adjust their lives to the now-reduced circumstances. This process is called K selection. Such individuals should be more efficient in the conversion of food into offspring, producing fewer young than those living in the population’s early phase. In a sense, a shift from productive to efficient individuals occurs as the population grows.

Other biologists, most notably Eric Pianka, have extended this concept of r and K selection to entire species rather than only to individuals at different stages of a population’s growth. Highly variable or unpredictable climates commonly create situations in which population size is first diminished but then grows rapidly. Species commonly occurring in such environments are referred to as r strategists. Those living in more constant, relatively predictable climates are less likely to go through such an explosive growth phase. These species are considered to be K strategists. According to this scheme, an r strategist is characterized by small body size, rapid development, a high rate of population increase, early age of first reproduction, a single or few reproductive events, and many small offspring. The K strategist has the opposite qualities—large size, slower development, delayed age of first reproduction, repeated reproduction, and fewer, larger offspring.

Various combinations of r and K traits may occur in a species, and few are entirely r- or K-selected. Populations of the same species commonly occupy different habitats during their lives or across their geographic ranges. An organism might thus shift strategies in response to environmental changes—it may, however, be constrained by its phylogeny or ancestry in the degree to which its strategies are flexible.

Criticism of the Model

Because the r and K model of reproductive strategies seems to explain patterns observed in nature, it has become widely accepted. It has also met with considerable criticism. Charges against it include arguments that the logistic population-growth model (on which the r and K strategies model is based) is too simplistic. Another is that cases of r and K selection have not been adequately tested. Mark Boyce, an ecologist, has persuasively argued that for the r and K model to be most useful, it must be viewed as a model of how population density affects life-history traits. Within this framework, also called density-dependent natural selection, the concept of r and K selection remains true to the one that MacArthur and Wilson originally proposed. Boyce suggests that the ability of r and K selection to explain reproductive strategies will have the best chance of being realized when approached in this fashion.

In addition to the r and K model, there are many other ways of describing reproductive strategies. For example, some species, such as the Chinook salmon, are semelparous: They reproduce only once before dying. The alternative is to be iteroparous—having two or more reproductive events over the organism’s life. If juvenile death rates are high, an individual might be better off reproducing on several occasions rather than only once. (This reproductive strategy is referred to as “bet-hedging.”) Finally, it has also been useful to evaluate reproductive strategies based on the proportion of energy that goes into reproduction relative to that devoted to all other body functions. This mode of analysis addresses such considerations as reproductive effort and resource allocation.

The concept of r and K selection is not the primary framework used in twenty-first-century studies of animal reproduction history and adaptations, with some scientists opting for a life-history paradigm or demographic theory. However, it is still used in some cancer evolution studies and in research evaluating microorganisms involved in polluted biological wastewater. It is also sometimes applied in specific aspects of animal reproduction, such as the amount of nutrient energy a mother bird expends to create an egg.

Studying Reproductive Strategies

Initially, one who studies the reproductive strategy of an organism should attempt to characterize its reproduction fully. The sample examined must be representative of the population under consideration—it should account for the variability of the traits being measured. Studies can involve any of several approaches. Short-term laboratory studies can uncover some hard-to-observe features, but there is no substitute for long-term field research. By studying an organism’s reproduction in nature, a biologist has the best chance of determining how its reproduction is shaped by an environment. If the research is performed over several seasons or years, patterns of variability can be better understood. This is important in determining how the physical environment influences reproductive traits.

After data have been systematically collected, it might then be possible to characterize a reproductive strategy. Imagine that a mouse population becomes established in a previously uninhabited area and that the population has a high reproductive rate (it produces large litters). The young develop quickly and produce many young themselves. Because of this combination of circumstances, one might consider the reproductive strategy to be r-selected since the population has a high reproductive output in an unexploited area. Though the concept of r and K strategies is problematic, it is still common to typify a strategy as r- or K-selected based upon this approach.

Because a reproductive strategy needs to be seen as part of an organism’s overall life history, however, other things should be measured to understand it fully. These may include the life span and population attributes such as survival patterns. Values should be taken for different age groups to characterize the population’s strategy. Correlational analysis is a statistical procedure that is used to evaluate reproductive strategies. Through such a methodology, one assesses the degree of association between two variables or factors. This may involve relationships between two reproductive variables or between a reproductive and an environmental variable—for example, to determine whether there is a significant correlation between litter size and decreasing body size in mammals. If one were found to occur, the conclusion that smaller species typically have larger litters might be drawn, which is, in fact, true. Such an analysis enables the characterization of a change in reproductive strategy based on body size. Simply establishing a correlation does not prove that causation has occurred—it does not automatically mean that one factor is responsible for the expression of the other.

Multivariate statistical procedures are also used to analyze reproductive strategies. These allow the determination of how groups of reproductive traits are associated and how they can be explained by several factors. One might determine that a certain bird species produces its greatest number of young and that the young grow most rapidly at northern locations having high snow levels. Such an approach is often needed in dealing with reproductive strategies—a combination of traits typically requires explanation.

Reproduction and Survival

The characterization of an organism’s reproductive strategy involves more than an understanding of reproductive traits. There is a successful process by which offspring are produced, and reproductive success is one of the two principal measures of fitness—the other is survival. Because a successful reproductive strategy ultimately results in high fitness, any discussion of these strategies bears directly on issues of natural selection and evolution.

An organism’s reproductive strategy represents perhaps the most significant way in which the organism adapts to its environment. A successful reproductive strategy represents a successful mode of passing genes on to the next generation, so traits associated with a reproductive strategy are under intense natural selection pressure. If environmental conditions change, the original strategy may no longer be as successful. To the extent that an organism can shift its reproductive strategy as circumstances change, its genes will persist.

The study of reproductive strategies has helped scientists understand why certain modes of reproduction occur based on observations of a species itself and its environment. An understanding of reproductive strategies may also have practical use. An organism’s reproduction directly influences its population dynamics.

If an animal has small litters and is at an early age at first reproduction, its population should grow at a concomitantly high rate. These and other components of reproduction may strongly affect a species’ population growth. Knowing how reproduction influences population dynamics is important in wildlife management activities, ranging from strict preservation efforts to overseeing trophy hunting.

Principal Terms

Bet-hedging: A reproductive strategy in which an organism reproduces on several occasions rather than focusing efforts on a single or few reproductive events

K Strategy: A reproductive strategy typified by low reproductive output common in species living in areas having limited critical resources

Litter Size: The number of offspring produced per birth; also referred to as “clutch size”

Population Density: Number of individuals per unit of area, especially when it pertains to the group’s reproductive potential

R Strategy: A reproductive strategy involving high reproductive output; found often in unstable or previously unoccupied areas

Reproductive Strategy: A set of traits that characterizes the successful reproductive habits of a group of organisms

Bibliography

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