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Introduction

The mating strategies of animals are diverse and variable both across and within species. Animal sexual behavior and mate choice directly affect social structure and relationships in many different mating systems, whether monogamous, polygamous, polyandrous, or polygynous. Though males and females in a given population typically employ a predominant reproductive strategy based on the overarching mating system, there is still significant variation in behavior among individuals of the same sex. Mating strategies used by males or females that differ from the prevailing strategy of the sex are known as alternative mating strategies. Alternative strategies provide animals of certain phenotypes with a different means for obtaining mates. The study of alternative mating strategies is critical to correctly characterizing the diverse sexual behavior practiced by animals in a population and understanding the strength of selection on these individuals.

Strategies & Selection

Alternative mating strategies have been observed among both male and female animals. Most typically, alternative strategies will be adopted in the face of competition within a sex, especially in species that mate multiply. In these scenarios, some individuals will adopt very different mating strategies to achieve reproductive success. The result over time will be a variety of strategies and phenotypes, consisting of both unsuccessful and successful conventional individuals and unconventional individuals who mate through alternative means. Successful strategies will be maintained through sexual selection.

In many cases, the coexistence of alternative and traditional mating strategies will both maximize the average fitness of the sex in question and be evolutionarily stable for a population. However, the utilization of alternative mating strategies may oscillate as a result of varying reproductive conditions, such as the availability of potential mates. Under changing circumstances, the existence of a variety of strategies allows individuals to choose the conditional behavior that will currently maximize their fitness.

Selection

Conventional and alternative mating behaviors arise through the pressures of sexual selection. More specifically, varying levels of reproductive success will select for phenotypes and strategies that maximize an animal's chance of obtaining a mate. As a result, certain animals will successfully use a conventional mating strategy while others employing this strategy will not obtain mates. Over time, phenotypic variance both between and within the sexes will result, with males exhibiting greater diversity in phenotype. The resulting variance in male fitness will create a niche in which alternative strategies may develop, such as sneaking to obtain a mate. The alternative behaviors will persist as part of this polymorphism, or variety of phenotypes, because the average fitness of unconventional males will equal the average reproductive success of conventional males.

Alternative behaviors will be maintained through frequency-dependent selection because of their equal fitness benefits and functional equivalence. Under frequency-dependent selection, the fitness of a given phenotype is determined by its frequency relative to other phenotypes within a population. Similarly, negative frequency-dependent selection describes a scenario in which rarer phenotypes experience greater fitness. Given that the utilization of alternative mating strategies has been shown to fluctuate over time, it has been suggested that frequency or negative frequency-dependent selection is the mechanism through which alternative mating strategies are maintained in animal populations.

Figure 1: This graph describes the fitness payoffs of varying mating strategies in relation to an animal's status. At the intermediate point s, the fitness benefits of A and B are equal, and either phenotype may be expressed. Individuals of higher status above point s will receive more fitness benefits more from exhibiting phenotype A, while those of lower status will achieve higher fitness with phenotype B.

A second proposed model for the maintenance of alternative mating behaviors is status-dependent selection. This describes a conditional strategy in which the fitnesses of alternative phenotypes depend on the status, or competitive ability, of an individual. Status includes environmental and genetic factors as well as age and size, and determines the level of fitness that may be obtained from a given phenotype. As shown in Figure 1, the fitness benefits of a given phenotype vary based on whether an individual is of high or low status. In a case where two phenotypes and strategies are possible, such as mate guarding or sneaking, there will be an intermediate point of intersection where the fitness gained from these alternative behaviors will be equivalent. At this point (s), the fitness gained from these strategies will be equal, and the particular strategy employed at a given time will depend on an individual’s status. A low status individual below the switch point will obtain higher fitness with phenotype B, while an individual of high status above the switch point will benefit from higher fitness with phenotype A. Such a model shows how individuals of lesser status or competitive ability may maximize their fitness by exhibiting an alternative phenotype. In this manner, these selective forces will maintain the phenotypic diversity observed among animals with respect to mating behavior, though strategies utilized will depend on a variety of circumstances.

What is a strategy?

Most of the organisms in question do not have the cognitive capacity to “strategize” in the human sense of the word, so what is a strategy? Here, a strategy is an underlying rule for making decisions about a certain behavior. A strategy provides an organism with a set of tactics that are adaptive in various circumstances. A tactic is an action taken to achieve a specific goal. For example, a wolf encounters a fallen tree and its strategy is defined by two tactics that may allow the wolf to pass the obstacle: jump over it or crawl under it. Considering the current environmental conditions, the surroundings, and the size of the tree, the wolf will decide between the tactics dictated by its strategy. In the context of a mating system, this means that individuals in a given population have strategies that allow them to obtain mates in different ways to maximize their reproductive success given their phenotypic, environmental, or social circumstances.

It is important to recognize that organisms within a population may not always have the same strategy, and different strategies may offer individuals either a range of tactical options or just one tactic. Furthermore, given strategy may be considered Mendelian, developmental, conditional, or a combination of the above. A Mendelian strategy depends on a genetically determined phenotypic difference, such as body size. This is the case in marine isopods, described below. Developmentally driven strategies are associated with phenotypic differences caused by varying conditions during the course of development that affect body size or overall adult health. Individuals may also have a conditional behavior strategy that depends not on the genetic or developmental impact on one's life circumstance, but on external factors. These may include the number of available mates, or the number of nearby competitors and their employed tactics. Additionally, some mating strategies will be impacted by the interaction of multiple factors, so these categorizations of Mendelian, developmental, and conditional are not mutually exclusive. They simply offer ways to think about alternative mating strategies and their root causes.

In any case, the mating strategies employed by organisms in various situations will ultimately depend on the strength of selection acting to maintain or eliminate certain reproductive strategies. If sexual selection strongly favors one mating strategy over a potential alternative, individuals not conforming to the successful strategy will fail to reproduce, thus preventing future generations from inheriting the unsuccessful strategy.

Evolutionarily Stable Strategy

The diversity of mating strategies within animal populations may be understood through evolutionary game theory concepts that assess the costs and benefits of reproductive decision-making. The Evolutionarily Stable Strategy (ESS) concept provides a particularly useful framework for considering alternative behaviors as they relate to fitness. Given that a strategy describes a set of pre-programmed rules that specify particular behaviors, an evolutionarily stable strategy is one that persists in a population due to its benefits to fitness. An ESS will be maintained in a population if it accords higher average fitness than other strategies, or a level of average individual fitness equivalent to all other strategies within the population.

Within an evolutionarily stable strategy, several scenarios are possible, including pure and mixed strategies. A pure strategy is one not affected by chance, in which an individual only expresses one strategic behavior. In contrast, a mixed strategy describes a scenario involving the probabilistic expression of behaviors among individuals. For example, an individual under a mixed strategy could express one mating tactic, such as sneaking, with a certain frequency and another tactic, such as mate guarding, at all other times. Though a mixed strategy is theoretically possible, it has not been documented in the context of alternative mating behaviors. Instead, a conditional strategy involving alternative behaviors may best characterize alternative mating strategies.

Condition-dependent behavior in the context of mating may result from changes in resource availability and intrasexual competition for mates. When competition decreases, the expression of alternative behaviors also decreases. Changes in mating behaviors, especially among alternative males, have been documented in insects, fish, and amphibians upon removal of dominant males. Additionally, the availability of mates and resources also affects the expression of alternative strategies within a sex. The gain or loss of territory has been shown to affect mating approaches among insect species, while the receptivity and spatial distribution of mates impacts tactics used among insects, fish, and mammals. Mating behaviors are also affected by an individual’s size and age, as smaller or younger individuals are more likely to attempt reproduction through alternative means, including mimicry or sneak tactics. As a result, the ability to choose a behavior that maximizes fitness under certain circumstances evolves.

Alternative Male Strategies

It has long been known that males in a wide variety of animal populations practice alternative mating strategies in order to maximize their reproductive fitness. This is especially common when there is male-male competition for access to mates. In cases where such alternative strategies are as successful at obtaining mates as the predominant strategy, a coexistence of different mating strategies will evolve. Below are a few common examples of male alternative mating strategies.

Sneaking behavior in males

Sneaking behavior may refer to a strategy that allows a male to more stealthily access a female partner, often avoiding altercations with other more dominant males.

Horned beetles (Onthophagus acuminatus)

Horned beetles demonstrate alternative mating strategies due to different nutritious conditions during development that affect adult body size. In this species, males who receive high levels of nutrition during development will surpass a size threshold above which they develop large horns. Males who do not pass the threshold will develop either small or nonexistent horns. These varying phenotypes will lead individual males to adopt different mating strategies. Those who develop long horns will practice mate guarding, protecting the entrance to the tunnel in which a female is resting or feeding. These males will fight any male that attempts to enter. This is a common strategy observed in populations in which females are dispersed and have synchronized periods of fertility, as well as those in which females are found in clusters that can be guarded to maintain access to more than one female.

Smaller males with little or no horns have little chance of beating larger males in altercations and will thus adopt an alternative sneaking strategy, digging a new tunnel that will allow them to intercept the female’s tunnel without being noticed by the guarding male. Both of these strategies have proven, thus far, to be reproductively effective for the males practicing them, and adoption of these alternative mating strategies has contributed to the maintenance of a dimorphic male population.

High-backed pygmy swordtail (Xiphophorus multilineatus)

Pygmy swordtail males offer another example of sneaking as an alternative mating strategy. However, in this case, male phenotype is not a contributing factor. Rather, it is the size of the potential female mate that determines a male’s strategy. Larger females will prefer males who court them, while smaller females will exhibit a weaker preference. Some males will adopt the more common courting strategy, but others will practice the alternative strategy of sneaking in order to engage in a mating event with females of all sizes. Though this behavior is not preferred by the female, it is reproductively successful enough to be maintained in the population as an alternative male mating strategy.

Female mimicry by males

Males practicing female mimicry may do so in order to gain access to mates in areas of where only females congregate.

Paracerceis sculpta illustration

In this species, there are three genetically distinct male morphs. Alpha males, which represent the largest and most common male morph, tend to defend harems in order to monopolize access to a large number of females. This is the predominant mating strategy in this species. Beta males are about the same size as female isopods, and they take advantage of that fact by mimicking female behavior in order to enter harems and gain access to fertile females. Gamma males are the smallest morph. These individuals adopt a sneaking strategy and rely on their small body size to enter harems undetected and remain in them while they seek mating opportunities.

These very distinct strategies, all determined by a single genetic locus, give equivalent lifetime mating success to each of the three morphs, indicating that natural selection is not acting on one morph more strongly than another. All three alleles expressed in the marine isopod population will continue to contribute to male morphology as long as the reproductive success granted by each one continues to be as beneficial as the others.

Alternative Female Strategies

Historically, while male alternative strategies have been well documented, alternative female strategies have not been studied extensively. This large discrepancy in information is mostly due to two factors. First, male mating behavior is typically driven by competition for mates, such as physical competition, territoriality, or parental care investment. Thus, male alternative behaviors arise as a direct result of these various forms of competition. However, females typically do not compete directly for these resources or mates. Instead, females indirectly compete through differences in premating, mating and post-mating behavior. The subtle nature of female competition makes alternative behaviors very difficult to study relative to males. Second, males are more likely to experience sexual selection than females. Due to this increased selection, it is statistically more likely for alternative strategies to evolve in males than females. However, though subtle and slightly more rare, females are still major participants in the mating process and can also experience limitations in access to males and male parental care. Thus, alternative female strategies have evolved to circumvent these limitations. Below are some examples of alternative female strategies seen in nature.

Copying mate choice

In the guppy, Poecilia reticulata, females will copy another female’s mate choice if given the opportunity to watch the other female choose. While older females do not copy younger females, younger females will copy older females. This copying behavior arises from a difference in ability to assess males. Since this behavior only arises when in the presence of another female, it is a behavioral alternative to the norm of just choosing a male mate based on personal assessment.

Sneaking behavior in females

In the damselfish, Chromis multilineata, females can often become infected with the parasite Anilocra chromis. In the event of infection, males do not allow infected females into the nest and do not mate with them. Thus, to bypass this limitation to mating, infected females will often sneak into male nests. Although the female is often immediately chased out, this behavior serves as evidence that sneaking is not just an alternative male strategy. In fact, sneaking is just a common strategy for any sex that is denied mating to a certain class of animals. The strategy of these infected females is therefore another behavioral alternative strategy.

Male mimicry by females

In damselflies, Ischnura, females are frequently harassed by males that wish to mate. There is significant variation in the females’ physical abilities to tolerate male mating harassment. In this species, there is a physical dimorphism: one type is cryptic (heteromorphic) and the other type looks like a male (andromorph). In many cases the andromorph even behaves like a male when among other males. Studies have found that the andromorph only mates half as often as the heteromorph. While a decrease in mating would be devastating for males, it is often an advantage in females. For females, excessive mating is a waste of time and energy and it increases exposure to predators. Thus, the ability to ward off extra mating gives the andromorphs a frequency dependent selective advantage. This is example of a traditionally male characterized Mendelian alternative strategy that has now been observed in females.