Intrasexual Male Competition
Competition between males for access to females or the resources desired by females
Sexual selection leads to the development of traits and behaviors that improve reproductive success. Darwin recognized two types of sexual selection: intersexual and intrasexual selection. Intersexual selection occurs when one sex chooses which members of the opposite sex to mate with, while intrasexual selection occurs when members of the same sex compete for mates (Darwin 1871).
In many species, males compete through intrasexual competition to gain access to females. Critical work in Drosophila in the 1940s revealed that the mean number of offspring a male can produce increases with the number of mates he has, while the mean number of offspring a female can produce does not vary greatly and is not limited by access to mates (as long as there is at least one mate) (Bateman 1948). This finding also applies to mammals. For example, male red deer have more reproductive potential and greater reproductive skew than females. Some males will successfully fight for mates and produce ten times more offspring than a female will produce, while many males will sire no offspring at all (Lewin 1982).
This chapter will first provide an overview of the different forms of both direct and indirect male-male competition. It will go on to discuss how male-male competition relates to other forms of sexual selection, and will conclude with a description of the role that intrasexual male competition has been proposed to play in human mating behavior.
Direct Intrasexual Male Competition
Since females are usually the choosier sex, males often compete with one another directly for mating access. This competition has led to the evolution of a diverse array of weaponry among animals. For example, many deer and elk species in the family Cervidae have large antlers that can be used in direct competition during the mating season to gain access to females. Further, in many primate species, males have large canines that can be used as lethal weapons in contest competition. Perhaps unsurprisingly, in species where males have weapons, weaponry is often one of the most variable morphological traits within the population (Emlen 2008).
Direct male-male competition does not always have to end in a fight. Sometimes, species with the deadliest weapons have a reduced likelihood of sustaining injuries from contest competitions because weapon size is hypothesized to provide an honest signal of a male’s fighting ability (Smith and Price 1973). Before any conflict occurs, a male can gauge the strength of his competitor and decide whether he will be able to successfully compete against him. As a result, some of the most common fights that occur between males are those where male weaponry is equally matched.
In populations where individuals spend time together and have repeated interactions with others in their social group, dominance hierarchies may emerge as a product of male-male competition. Among different species, dominance structures can vary based on the strength, stability, and linearity of the dominance hierarchy. Often, males will compete directly with one another to achieve dominance. However, it remains unclear whether variation in strength and fighting ability between males or social constructs such as winner-and-loser effects more strongly influence how dominance is determined in a social group. While the former hypothesis is well supported in the literature, research on baboons indicates that winner-and-loser effects may also play a role in the construction of dominance hierarchies (Franz et al. 2015).
In species that fight for dominance, high-ranking social status may serve several functions. Dominant males often have priority of access to resources, including females and food, which increases their reproductive success and may improve their health, respectively. Dominance hierarchies also create a social order that prevents individuals from having to constantly fight potentially deadly battles with conspecifics.
Just as weapon size is thought to signal strength in many male mammals, in species where males fight for dominance, dominant males will sometimes display a badge of status. Gelada males compete to be one-male unit (OMU) holders, with each OMU containing multiple females. Males that have an OMU have the reddest chest patches, with patch redness reflecting the number of females in the OMU (Bergman et al. 2009). In male mandrills, red coloration displayed on their face and anogenital region indicates social rank, and more fights occur between males of similar colors than those that show a large difference in red coloration (Setchell and Wickings 2005). Therefore, dominance hierarchies and badges of status provide mechanisms by which males can compete for access to females while limiting the number of potentially deadly contests.
Indirect Intrasexual Male Competition
Sperm Competition and Mate Guarding
Intrasexual male competition can also act indirectly. One common form of indirect intrasexual competition occurs through postcopulatory sperm competition. In species where females mate with several different males, the males’ sperm compete to fertilize the female’s gametes. In polyandrous and polygynandrous mating systems, males will have larger testes than in monogamous or polygynous mating systems (Harcourt 2005). In primates, the size of the testes is often related to the mating system of the species: males in polygynous mating systems have smaller testes weights while males in polyandrous and polygynandrous mating systems have larger testes weights. In other taxa, such as insects, males in a polyandrous or polygynandrous mating system may have enlarged accessory glands to produce more seminal fluid (Mikheyev 2004). Finally, in species with a polyandrous or polygynandrous mating system, a male’s ejaculate may form a copulatory plug that is made up of a coagulation of seminal fluid and prevents the sperm of other males from reaching the ova (Dixson and Anderson 2002).
In addition to sperm competition, another postcopulatory indirect form of intrasexual male competition is mate guarding. This form of competition is most common in taxa where multiple males mate with one or more females during the mating period or in cases where sperm is a limited resource. After copulation, a male may prevent other males from mating with the female or may prevent a female from seeking out additional mates. Mate guarding can be accomplished by staying close to the female for an extended period, postcopulatory mounting, extending copulation, or coalitionary guarding, depending on the species (Edward et al. 2015). Sometimes, only certain males in the population will guard females. This may be due to the fact that mate guarding is energetically expensive and affects a male’s feeding time and energy balance (Girard-Buttoz et al. 2014).
Males can also compete indirectly prior to copulation. When reproductive skew is low in a polygynandrous group, several males may mate with several females over an extended period and reduce the amount of time they spend on other activities. In these scenarios, the males who spend the longest amount of time actively mating with females are the ones who sire the most offspring. For example, rhesus macaque males experience increases in their fat reserves prior to the mating season but during the mating season, males lose up to 10% of their body weight by focusing on mating and mate guarding rather than activities such as feeding (Bercovitch 1997).
Endurance rivalry is not limited to species with moderate reproductive skew. Elephant seals are one of the most sexually size dimorphic mammals with some males weighing more than three times as much as females. Over the 3-month reproductive season, a few of the largest males fight off smaller males to achieve high rank, defend large groups of females, and sire almost all of the offspring. While males compete directly, they also engage in endurance rivalry since females stay on land for the duration of the mating season while feeding occurs in the water. Reproductive success is greatest for males with the largest fat reserves and can invest in mating and mate guarding rather than feeding (Andersson 1994).
Alternative Reproductive Tactics
In populations where males experience a shallow reproductive skew, selection may favor the development of alternative reproductive tactics. These tactics include any behavior that is distinct from the most commonly used mating strategies and may be maintained by frequency-dependent selection, heterozygote advantage, or condition-dependent selection (Neff and Svensson 2013). For example, males may employ sneaking behaviors, morphologically resemble females, or mate guard their mates. These mating strategies enable males to avoid intrasexual conflict while gaining access to females.
Side-blotched lizards provide a notable example of alternative reproductive tactics. In populations where a majority of males occupy large territories and monopolize females, sneaker males secretively mate with females without having to compete with other males for territory thus increasing their reproductive success compared to males who cannot acquire territory and do not use this tactic (Zamudio and Sinervo 2000). In primate species such as Japanese macaques and Hanuman langurs, where males often live in multi-male multi-female groups, some males may choose to live alone or in all-male groups (Setchell 2008). These males will mate with females opportunistically during the mating season thus avoiding competition with dominant males in the social group.
Intrasexual Male Competition in the Context of Total Sexual Selection
Although intrasexual male competition and female mate choice are often studied in isolation, it is important to acknowledge the role that female choice plays on intrasexual male competition. In populations where mate choice occurs, the preferences of the female should influence the type of competition that occurs between males. If females require a home territory, males should compete for the resources that the female needs, such as land, rather than directly competing for females. In contrast, if a population is structured into one-male, multi-female groups, males should engage in contest competition with other males to gain access to these one-male units.
Intrasexual male competition may also influence mate choice. Some argue that, over time, weapons that were originally used for male-male competition may have become intersexual signals used by females to determine preferred mating partners. Further, in species that exercise female choice, direct intrasexual male competition may provide females with information about the strongest male and influence their mating preferences. This will reinforce selection on traits used in competition (Hunt et al. 2009).
Intrasexual male competition and female mate choice rarely act independently. To better understand the selection pressures on a male sexual trait, it is essential to determine total sexual selection on the trait (Hunt et al. 2009). Total sexual selection encompasses the strength of selection imposed on both sexes by competition and mate choice. When male-male competition and female mate choice interact on the same trait, the effects of these processes can be reinforcing, such that total sexual selection is similar to the strength of each selection pressure independently or opposing, with the resulting total sexual selection differing from either of the selection pressures on its own (Hunt et al. 2009). Intrasexual male competition and female mate choice can also act on different temporal scales. When male-male competition occurs prior to female mate choice, male-male competition will eliminate some of the phenotypes from which females can choose. When the two processes overlap temporally, both processes select male traits from the same pool of phenotypes and some males may gain access to females by obtaining exclusive mating opportunities while other males gain access to females by displaying attractive traits (Hunt et al. 2009).
Intrasexual Male Competition in Humans
Over our evolutionary history, intrasexual male competition has been an important component of human behavior. One way to evaluate mating and social systems of the past and make assumptions about intrasexual competition is through studies of sexual size dimorphism. Since body size is heritable and helps males win contests, the larger the size dimorphism between male and female primates, the more frequently males compete directly for females (Andersson 1994). Our hominin ancestors, such as Australopithecus and early Homo, likely displayed a significant sexual size dimorphism. Over the course of hominization, however, this body mass dimorphism was reduced and modern human populations show a modest body mass dimorphism of about 15% (Smith and Jungers 1997). Humans have also lost the canine size dimorphism that is present in many primate taxa. However, since human females store more body fat than males, some have argued that fat-free body mass should be used to measure sexual size dimorphism. Using this method, the ratio of fat-free mass in males compared to females is 1.41, indicative of a significant, moderate size dimorphism (Lassek and Gaulin 2009). Thus, humans experience some degree of body size dimorphism that, while reduced compared to our hominin ancestors, highly suggests that males compete both directly and indirectly for access to females and resources desired by females (Triveres 1972).
Direct Intrasexual Male Competition
One method of intrasexual competition in human males is violence. Beginning early in life, males are more aggressive than females and more frequently engage in rough-and-tumble play (Ellis et al. 2008). Further, males generally have a higher pain tolerance than females (Ellis et al. 2008). In the tribal Yanomamo society, males engage in contests that involve shouting matches and fights with clubs, axes, machetes, and arrows (Chagnon 1988). In a rural population in Senegal, Sereer wrestling is a traditional fight that occurs regularly in villages and winners often increase their access to mates (Llaurens et al. 2009). According to the Global Study on Homicide, 95% of the perpetrators and 80% of the victims of homicide are male (Caman et al. 2017).
As in primates, human males have also been proposed to compete directly through displays of dominance. Men of high status may procure more opportunities to mate and high status has been shown to be a necessity for women in a mate-screening process (Li et al. 2002). One suggested mechanism by which males display dominance is voice pitch. Studies indicate that when voice pitch is artificially lowered, men perceive these voices as more dominant than higher pitched voices (Puts et al. 2006). Further, men who believe they are dominant to another man will lower their voice pitch when speaking, while men who believe they are less dominant will raise their voice pitch (Puts et al. 2006). Among the Hazda, lower voice pitch has been suggested to increase reproductive success in males but not in females (Apicella et al. 2007). However, the relationship between voice pitch and dominance has recently come into question, with some arguing that it lacks evidence or is simply the product of an innate assumption that larger objects and organisms emit a lower pitch than smaller ones (Feinberg et al. 2019).
In addition to voice pitch, facial characteristics may also serve as a dominance display. Across two distinct cultures, men were considered more aggressive and of higher social status if they had a beard than if they were clean shaven and this sensitivity was stronger in men than in women. This may occur because beards increase the perception of the size of the jaw and length of the face (Dixson and Vasey 2012). Further, computer-manipulated masculinized faces are perceived by men as more dominant than feminized faces and a negative correlation exists between one’s self-rated dominance and perceived dominance in masculine faces (Watkins et al. 2010a, b).
Indirect Intrasexual Male Competition
Men have been proposed to utilize two distinct alternative mating tactics: a “quality” strategy or a “quantity” strategy (Hirsch and Paul 1996). The former strategy usually involves long-term relationships, while the latter occurs over a much shorter time frame of a few months to a few hours. Through the “quantity” strategy, men attempt to gain access to a large number of females while minimizing costs and the potential for commitment using tactics that often involve threats and psychological pressure. Using the “quality” strategy, men attempt to ensure high partner commitment, paternity certainty, and female reproductive value using tactics such as resource expenditure and honest advertisement (Buss and Schmitt 1993; Hirsch and Paul 1996).
Men have also been proposed to compete indirectly for access to mates though mate guarding. The Mate Retention Inventory catalogs 19 tactics that males use to guard women such as calling at unexpected times and preventing a woman from going to events without her male partner (Buss 1988). These mate-guarding tactics are more commonly used when the male’s partner is young and attractive and when confronted by males who are more economically successful (Buss 2002). Failure to mate guard can result in reproductive costs such as providing parental investment to another man’s offspring and losing the female to another male.
Similar to many mammalian species, humans also employ sneaky copulation tactics and may engage in sperm competition. In a survey of Australians, 28% of men and 22% of women reported engaging in extrapair copulations (Simmons et al. 2004). The rate of extrapair paternity can vary greatly, ranging from 0.03% to 31.8% in a modest number of studies, with Western populations generally having lower rates than pastoral and tribal populations (Scelza 2011; Simmons et al. 2004). Consistent with the statistics above, male relative testes size is greater than expected for a monogamous species suggesting that sperm competition may occur (Simmons et al. 2004). Men have a higher percentage of motile sperm in their ejaculates when viewing a sexually explicit image depicting two men and one woman than an image depicting three women, which has been proposed to suggest that evocation of sperm competition alters sperm composition (Kilgallon and Simmons 2005).
Intrasexual male competition occurs when males compete with one another for access to females or resources desired by females. This competition can occur directly, such as through contests, or indirectly through several methods including sperm competition, mate guarding, endurance rivalry, and sneaky copulations. Importantly, intrasexual male competition does not occur in isolation and intersexual competition often operates conjointly with intrasexual competition. Finally, over our evolutionary history, human males have had to compete for access to mates and modern males experience intrasexual competition that occurs both directly and indirectly.
- Andersson, M. B. (1994). Sexual selection. Princeton: Princeton University Press.Google Scholar
- Girard-Buttoz, C., Heistermann, M., Rahmi, E., Agil, M., Fauzan, P. A., & Engelhardt, A. (2014). Costs of and Investment in Mate-Guarding in wild long-tailed macaques (Macaca fascicularis): Influences of female characteristics and male–female social bonds. International Journal of Primatology, 35, 701–724.CrossRefGoogle Scholar
- Triveres, R. L. (1972). Parental investment and sexual selection. In B. Campbell (Ed.), Sexual selection and the descent of man (pp. 136–179). Chicago: Aldine.Google Scholar