An alternative mating strategy is a strategy used by male or female animals, often with distinct phenotypes, that differs from the prevailing mating strategy of their sex. Such strategies are diverse and variable both across and within species. Animal sexual behaviour 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, individuals of the same sex often use different mating strategies. Among some reptiles, frogs and fish, large males defend females, while small males may use sneaking tactics to mate without being noticed.
Strategies and 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 evolutionarily stable strategies and phenotypes, consisting of both conventional individuals and unconventional individuals who mate through alternative means. Successful strategies are 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 behaviour that will currently maximize their fitness.
Selection.
Conventional and alternative mating behaviours arise through 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 successfully use a conventional mating strategy while others employing this strategy fail to obtain mates. Over time, phenotypic variance arises both between and within the sexes, with males exhibiting greater diversity in phenotype. The resulting variance in male fitness creates a niche in which alternative strategies may develop, such as sneaking to obtain a mate. The alternative behaviours persist as part of this polymorphism, or variety of phenotypes, because the average fitness of unconventional males equals the average reproductive success of conventional males.
Alternative behaviours are 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.
A second proposed model for the maintenance of alternative mating behaviours is status-dependent selection. This describes a conditional strategy in which the fitness 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 behaviours 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 behaviour, though strategies utilized will depend on a variety of circumstances.
Strategy.
Most of the organisms in question do not have the cognitive capacity to “strategize” in a logical sense. Here, a strategy is an underlying rule for making decisions about a certain behaviour. 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 behaviour 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.
Female assessment of males.
While the majority of the research into the interactions that lead to alternative mating strategies has a focus on male to male competition, the interaction between males and females also plays a significant role in the mating strategy used (see Sexual Selection). Female assessment of the males (see Female Mate Choice) plays a role in the number of males opting to use an alternative mating technique. Females are likely to not choose to mate with males of a lower quality, so these males will have to adopt alternative mating techniques in order to mate. The ability of the female to assess possible mates also plays a role in the frequency of alternative mating strategies. If a female is unable to assess and choose mates accurately, for example due to time constraints or assessment costs, then males of a lower quality are more likely to be chosen. While if the females have much time and resources available to them, allowing them to accurately choose males, then the lower-quality males are unlikely to be chosen and so will have to adopt alternative mating techniques.
The number of mates available to the female will also change the frequency of males adopting alternative mating techniques. If the female has a small selection of males to mate with then males of a lower quality are more likely to be chosen by the females as they have fewer options. This means that males that would normally have to adopt an alternative mating strategy in a larger population can now mate using the primary mating 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 behaviours as they relate to fitness. Given that a strategy describes a set of pre-programmed rules that specify particular behaviours, 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 behaviour. In contrast, a mixed strategy describes a scenario involving the probabilistic expression of behaviours 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 behaviours. Instead, a conditional strategy involving alternative behaviours may best characterize alternative mating strategies.
Condition-dependent behaviour in the context of mating may result from changes in resource availability and intrasexual competition for mates. When competition decreases, the expression of alternative behaviours also decreases. Changes in mating behaviours, 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 behaviours 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 behaviour that maximizes fitness under certain circumstances evolves.
Alternative mating 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 behaviour in males.
"Sneaking" is any strategy that allows a male to access a female partner, avoiding more dominant males, for example those guarding a harem, as in the red deer and elephant seal. The behaviour occurs in many different animal groups, including birds (for example, the ruff), reptiles (for example, the marine iguana), fish (for example, the cichlid "Herichthys minckleyi"), and invertebrates (for example, "Phanaeus" beetles).
Horned beetles ("Onthophagus acuminatus").
Horned beetles ("Onthophagus acuminatus") 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