Intrasexual selection is competition among members of the same sex for access to mates, while intersexual selection is the choosiness one sex exercises in picking a partner. Think of antlers clashing versus a peahen evaluating a peacock’s tail. Both fall under the umbrella of sexual selection, and both shape traits that have nothing to do with day-to-day survival, but the pressures they create are distinct enough to push evolution in different directions. The reality, though, is messier than that clean split suggests, because the two forces often act on the same animals, sometimes on the same body parts, and occasionally flip which sex does which.
What Intrasexual Selection Actually Looks Like
Intrasexual selection is, at its core, about beating the competition before a mate even enters the picture. In many species this means literal combat: male elk locking antlers, elephant seals slamming into each other on a beach, stag beetles grappling with oversized mandibles. These contests determine who gets to mate and who walks away empty-handed. The weapons involved tend to be exaggerated, metabolically expensive, and often limited to one sex. A study across 148 chameleon species found that the total number of weaponry structures in males was strongly linked to body size, while female weapon counts showed no such pattern.1Evolution. Macroevolution of sexually selected weapons: weapon evolution in chameleons Males are, broadly speaking, the sex that arms up.
But weapons are not the whole story. Intrasexual competition also includes territorial defense, where a male stakes out a patch of habitat and drives off rivals, and dominance hierarchies, where rank in a social group translates into mating access. Some species have evolved more creative workarounds. Alternative mating tactics, such as “sneaker” males that mimic females to slip past territorial males, are overwhelmingly documented in males rather than females, likely because the opportunity for sexual selection is typically higher among males.2PubMed Central. Polyandry and alternative mating tactics
Weapon size itself is not always static. In some species, sexually dimorphic weapons fluctuate with the seasons, shrinking or becoming less pronounced outside the breeding period when the cost of maintaining them outweighs any competitive benefit.3PubMed Central. It takes two: Seasonal variation in sexually dimorphic weaponry results from divergent changes in males and females The economics of intrasexual selection are always in play: a weapon or body size advantage pays off only when there are mates to fight over.
What Intersexual Selection Actually Looks Like
Intersexual selection works through preference rather than force. One sex, usually but not always the female, evaluates potential mates and preferentially breeds with individuals displaying certain traits. The result is that traits favored by choosy individuals get amplified across generations, even if they carry a survival cost. The peacock’s tail is the textbook example: unwieldy for flying, conspicuous to predators, yet perpetuated because peahens consistently prefer males with bigger, more elaborate fans.
The preferences themselves can arise for different reasons. In some cases, females gain a tangible payoff from their choice. Desert fruit flies, for instance, prefer males that provide greater resistance to desiccation stress, a direct benefit that improves their own survival or that of their offspring.4PubMed Central. Conflict between direct and indirect benefits of female choice in desert Drosophila Other times, the benefits are indirect: a female chooses a flashy male because his display signals genetic quality, and her offspring inherit those “good genes.”
A third pathway has nothing to do with male quality at all. Sensory exploitation occurs when males evolve traits that tap into pre-existing sensory biases in females, biases that originally evolved for foraging, predator detection, or other non-mating functions. Research and theoretical models suggest that this route has been widespread across animal groups.5PubMed Central. Sensory exploitation and sexual conflict In these cases, the male’s ornament is less an honest signal of fitness and more a hack that exploits how the female brain is wired.
When One Trait Serves Both Purposes
The clean textbook separation between “weapons for fighting” and “ornaments for attracting” breaks down regularly in nature. Many secondary sexual traits do double duty. A literature review proposed that traits functioning in both male-male competition and female choice commonly originate through intrasexual selection and then serve as honest signals of fighting ability or dominance that females can also evaluate.6Biological Journal of the Linnean Society. Armaments and ornaments: an evolutionary explanation of traits of dual utility A set of large antlers, for example, helps win fights and simultaneously advertises to watching females that the bearer is a formidable competitor.
Crested auklets, a seabird nesting in the North Pacific, provide one of the clearest experimental demonstrations of this overlap. Researchers found that crest length predicted dominance within each sex: longer-crested birds won more aggressive encounters against shorter-crested birds. But in a separate experiment that controlled for intrasexual competition, both males and females directed more sexual displays toward opposite-sex models with longer crests. The crest is simultaneously a weapon badge and a mate-attraction ornament, favored by both intrasexual and intersexual selection at once.7PubMed. Experimental evidence for mutual inter- and intrasexual selection favouring a crested auklet ornament
Work on pied flycatchers illustrates how the same species can have some traits shaped by one force and other traits shaped by the other, or both. In that system, body size was pushed upward by intrasexual competition for territories but was simultaneously under stabilizing selection during the mate-choice phase. Meanwhile, a brown wing patch was shaped mainly by female preference, and a white wing patch was under directional selection during both competitive and choosy phases.8PubMed. The Evolution of Sexual Dimorphism by Sexual Selection: The Separate Effects of Intrasexual Selection and Intersexual Selection The upshot is that the two selection pressures can push the same trait in the same direction, push different traits in different directions, or push the same trait in opposing directions depending on what stage of mating is underway.
The Hidden Arena After Mating
Sexual selection does not necessarily end when mating begins. Post-copulatory selection extends both intrasexual and intersexual dynamics inside the female’s reproductive tract. Sperm competition is the post-copulatory equivalent of male-male combat: when a female mates with multiple males, their sperm compete for fertilization, and traits like sperm count, swimming speed, and seminal fluid chemistry can determine paternity. Cryptic female choice is the post-copulatory equivalent of mate choice: females can bias sperm use and influence which male’s sperm actually fertilizes their eggs, even after mating has occurred.9PubMed Central. Postcopulatory sexual selection: Darwin’s omission and its consequences
Cryptic female choice is particularly fascinating because it is invisible from the outside. It encompasses a range of mechanisms that bias paternity share after mating, and researchers have spent decades cataloging the ways females exert control at this stage.10PubMed Central. Postmating Female Control: 20 Years of Cryptic Female Choice One consequence with broad evolutionary significance is that cryptic female choice can result in assortative fertilization, where females bias fertilization toward males of their own species during sperm competition, a process known as conspecific sperm precedence that may help maintain reproductive barriers between closely related species.11PubMed. Cryptic female choice can maintain reproductive isolation
Post-copulatory selection matters for understanding the difference between intrasexual and intersexual processes because it demonstrates that the same conceptual distinction, competition among same-sex individuals versus choice by the other sex, applies at every stage of reproduction, not just the visible courtship phase.
When the Roles Reverse
The reason females are typically the choosy sex and males are typically the competitive sex traces back to a basic asymmetry in reproductive investment. Eggs are large and energetically expensive; sperm are small and cheap. This initial difference tends to snowball: the sex that invests more per offspring becomes a limiting resource, and the other sex competes for access. Modeling work confirms that small initial differences in parental investment tend to increase through positive evolutionary feedback, formally supporting what had long been argued in words but never rigorously demonstrated.12PubMed Central. Coevolution of parental investment and sexually selected traits drives sex-role divergence
But this is a tendency, not a law. The key insight, articulated clearly in the early 1970s, is that whichever sex invests more should be choosy, and whichever invests less should compete. In species where male parental investment strongly exceeds that of the female, females compete among themselves for males and males become selective.13Biological Journal of the Linnean Society. The 150th anniversary of The Descent of Man: Darwin and the impact of sex-role reversal on sexual selection research These sex-role-reversed species exist in nature: jacanas, pipefish, and giant water bugs are classic examples where males do most of the parenting and females fight for access to them. In species where males limit the reproductive rate of females, the female takes on the competitive role.14PubMed. How do we decide that a species is sex-role reversed?
Sex-role reversal reminds us that “intrasexual” and “intersexual” describe types of selective pressure, not which sex does what. In a reversed species, intrasexual selection still means same-sex competition, it is just females doing the competing. Intersexual selection still means mate choice, it is just males doing the choosing.
How Both Forces Apply to Humans
Applying sexual selection to humans is contentious, but the evolutionary evidence points to both mechanisms operating across our history. A review in evolutionary psychology noted that contest competition, the use of force or the threat of force to exclude same-sex competitors, has probably been particularly important among men, while female mating competition mainly took the form of mate choice.15Current Opinion in Psychology. Human sexual selection
A more detailed functional analysis argues that men’s physical traits are better designed for contest competition than for attracting mates. Size, muscularity, strength, aggression, and deep voices all plausibly helped ancestral males win direct contests, while facial hair signals dominance more effectively than it increases attractiveness. The argument is that male monopolization of females was imperfect, though, so female mate choice, sperm competition, and other mechanisms also shaped male traits. For women, the analysis suggests that male mate choice was probably central to female competition, because ancestral females could not constrain the choices of larger and more aggressive males through force, and more attractive women could secure greater male investment.16Evolution and Human Behavior. Beauty and the beast: Mechanisms of sexual selection in humans
What makes humans unusual is that both sexes invest heavily in offspring, which muddies the prediction that one sex should be purely competitive and the other purely choosy. In practice, both men and women show evidence of mate choice and both engage in intrasexual competition, though the forms differ. Men’s competition has historically been more physical; women’s competition tends toward social competition, reputation management, and self-enhancement. Mate preferences in humans are also shaped by social learning, which can spread preferences for certain physical traits within a population and generate cultural differences in what people find attractive.17PubMed Central. Social learning and human mate preferences: a potential mechanism for generating and maintaining between-population diversity in attraction This cultural layer sits on top of whatever evolved preferences exist, making human sexual selection a tangled mix of biology and social context.
Why Environmental Conditions Change the Balance
The relative strength of intrasexual versus intersexual selection is not fixed even within a species. Environmental changes can shift the balance. When food availability drops or predator pressure rises, animals may invest less time and energy in courtship displays or mate searching, weakening intersexual selection. Alternatively, environmental disruption can alter population density, sex ratios, or the distribution of resources that mates compete over, reshaping intrasexual competition in the process.18Oxford Academic. Editorial: Sexual selection and environmental change: what do we know and what comes next?
Consider a population of birds where females normally choose males based on elaborate song. If habitat degradation increases background noise, females may not be able to hear the differences between males, effectively neutralizing intersexual selection on song complexity. Meanwhile, if the same habitat change concentrates males around fewer nesting sites, intrasexual competition for territory could intensify. The selective landscape is dynamic, and which type of sexual selection dominates can shift within a few generations depending on ecological context.
The Speciation Question
One of the bigger implications of the intrasexual-intersexual distinction concerns how new species form. If females in one population develop a preference for, say, red coloring while females in another population prefer blue, intersexual selection could drive the two populations apart reproductively. This idea, that mate choice accelerates speciation, has received considerable theoretical attention. But a systematic review pointed out that most research on sexual selection’s role in speciation has focused on mate choice while largely neglecting mate competition.19Journal of Evolutionary Biology. The role of mate competition in speciation and divergence: a systematic review
That blind spot matters. If male-male competition drives divergence in body size or weaponry between populations, it could create barriers to interbreeding just as mate choice can. Imagine two populations of beetles where males in one population evolve larger horns and males in the other evolve wider bodies for grappling. If females then evolve preferences matched to their own population’s fighting style, intrasexual and intersexual selection reinforce each other and reproductive isolation deepens. The limited attention given to competition’s role in speciation means we likely underestimate its contribution.
A Controversy That Shaped the Field
The distinction between intrasexual and intersexual selection has a complicated intellectual history. Darwin proposed both mechanisms in 1871, but his co-discoverer of natural selection, Alfred Russel Wallace, fiercely opposed the idea of female choice. Wallace considered it unnecessary and anthropomorphic, arguing that the traits Darwin attributed to female preference could be explained by natural selection alone. The result of this controversy was that most evolutionary biologists avoided sexual selection as a research topic for roughly 80 years, with Ronald Fisher being a major exception.20PubMed. Sexual selection: Another Darwinian process
The modern approach inherits from both men. It treats sexual selection as a special aspect of natural selection, following Wallace’s instinct to unify, but fully accepts mate choice as a real and powerful evolutionary force, vindicating Darwin. That decades-long suppression of the field helps explain why some aspects of sexual selection, particularly post-copulatory processes and the interaction between the two types, are still catching up empirically. Researchers are still mapping territory that might have been charted a century earlier had the Darwin-Wallace disagreement not chilled the field.