Adult Play and Sexual Selection
|Garry Chick (2013), Scholarpedia, 8(3):30432.||doi:10.4249/scholarpedia.30432||revision #143444 [link to/cite this article]|
Play and playfulness are enigmatic from an evolutionary perspective as it is not obvious that they provide any survival or reproductive benefits. Indeed, animals engaged in play can be injured or even killed, waste energy that could presumably be better put to other uses, or open themselves to predation through reduced vigilance. Play fighting can escalate to real fighting if signals are misread or one player acts too aggressively. But, since play is so common, especially among mammals but also many birds, perhaps other vertebrates such as reptiles, or even certain invertebrates, such as the octopus (Burghardt, 2005), its selective advantages must somehow outweigh these costs. A variety of explanations for play, especially among juveniles where it is most common, have been proposed. In social species, it functions in the learning of social rules and by sharpening the physical, cognitive, and emotional skills required for success in society (Pellis & Pellis, 2009). Bekoff (2001) and Bekoff and Pierce (2010) note that, in play, social animals learn to “behave fairly” toward others, that is, they learn cooperation, fairness, and trust. Pellis, Pellis, and Bell (2010) report that cortical changes in the brains of young rats mediate how play affects social skill development such that “rats that play as juveniles are more socially competent as adults” (p. 278). Play may also serve to strengthen bones and muscles and improve endurance while practicing what one is to do as an adult (Martin & Caro, 1985). Juvenile predators, such as felines or canines, play at predatory-like activities, such as pouncing, wrestling, biting, and chasing while prey species, such as mice or deer, play at escape-like activities, such as kicking or running. Among humans, children commonly play at adult-like activities, often differentiated by sex. Girls often play at household maintenance and child-rearing type activities while boys engage in play that mimics aspects of adult male work (Bock, 2005; Bock & Johnson, 2004; Lancy, 1996). Although play as practice or rehearsal for adult roles is one of the most common explanations for play behavior (Schwartzman, 1978), one should not assume that the functions of play are the same across species, or across cultures, as adaptive benefits identified in some animals or some societies may not replicate in others (Burghardt, 2005; Pellis et al., 2010; Sharpe, 2005a, 2005b, 2005c; Sharpe & Cherry, 2003).
From an evolutionary perspective the presence of play among juvenile animals is most often explained by natural selection (e.g., Fagen, 1981). That is, youthful behaviors, including play, are thought to contribute to survival and reproductive success through enhanced learning, exercise, or other functions, are selected for and, to the extent that they are heritable, are passed to succeeding generations. Adult play and playfulness presents an additional paradox, however. Evolutionary explanations based on the acquisition of adult skills are not persuasive since adults presumably already possess and use those skills. Therefore, as might be expected, play is considerably less common among adult animals than juveniles. But there are exceptions. While adults of many species of mammals occasionally play with juveniles and, more rarely, among themselves or with objects, adult play is very common in a few species (Pellis et al., 2010). These include some domesticated animals, such as dogs and cats, but also humans. Possible explanations for this puzzle may be found in aspects of Darwin’s (1859) theory of evolution by natural selection.
Forms of Selection
In 1859, Charles Darwin used four simple facts to show that natural selection is the prime mover in biological evolution (Fagen, 1995). First, individual members of species vary among themselves. Second, some of that variation is heritable. Third, the number of individuals in any particular species increases faster than the resources available to them. Fourth, individuals may vary in ways that allows them to survive and reproduce more successfully than others of their species. Therefore, those that survive and reproduce more successfully than their conspecifics are naturally selected (Darwin, 1859). Offspring will resemble their parents and tend to be more successful at reproduction themselves, leading to the accumulation of useful variations that constitute evolutionary change.
Darwin (1859) began Origin of Species by explaining a form of natural selection he called artificial selection. This is where humans dictate evolutionary change by selectively breeding animals and plants that have characteristics that we find desirable. Darwin may have done so because the method and results of artificial selection were well known in his day. He noted “One of the most remarkable features in our domesticated races is that we see in them adaptation, not indeed to the animal’s or plant’s own good, but to man’s use or fancy” (Darwin, 1859, p. 21). Darwin described two forms of artificial selection. First, methodical selection is deliberate in that it is goal driven. For example, humans bred the wild grass, Teosinte and its descendants selectively over generations to produce modern maize with its comparatively huge ears of seeds. But artificial selection can also be unconscious where individual members of a species are bred preferentially but without a specific goal in mind. We may have, at least initially, selected for playfulness in dogs unconsciously. This is illustrated by the project begun in 1959 by Russian biologists led by Dmitri Belyaev wherein they selectively bred the silver fox (Vulpes vulpes) based on “flight distance,” that is, how close a human can approach the animal before it flees. Belyaev and his colleagues bred foxes with the shortest flight distance and, after only the tenth generation, 18 percent of the foxes were extremely tame, like domestic dogs, and remained playful into adulthood (Trut, 1999). It may be, therefore, that the wolf ancestors of dogs, were domesticated, not for their playfulness, but for tameness. Just as we have domesticated other species, it may be that we have also domesticated ourselves. Hare, Wobber, and Wrangham (2012) claim that the bonobo (Pan paniscus) is less aggressive than the chimpanzee (Pan troglodytes) due to self-domestication where females, in particular, select for non-aggressive males. It is possible, therefore, that adult playfulness is part of a larger suite of juvenile traits retained in adulthood by some species.
Darwin also faced a somewhat different question. The exaggerated ears of modern maize are clearly the result of artificial selection and the playfulness of adult dogs appears to be so, as well. However, why have males, and sometimes females, of many species developed seemingly maladaptive traits such as extreme ornamentation, as among many birds, or exaggerated weaponry, as with the moose or stag beetle? In response, Darwin (1859) proposed sexual selection.
Darwin (1859) proposed sexual selection early The Origin of Species but analyzed it in much greater detail in The Descent of Man, and Selection in Relation to Sex in 1871. In The Origin, he wrote:
Inasmuch as peculiarities often appear under domestication in one sex and become hereditarily attached to that sex, the same fact probably occurs under nature, and if so, natural selection will be able to modify one sex in its functional relations to the other sex, or in relation to wholly different habits of life in the two sexes, as is sometimes the case with insects. And this leads me to say a few words on what I call Sexual Selection. This depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring. (1859, p. 87-88)
Sexual selection allowed Darwin to explain what appear to be strange and even maladaptive physical or behavioral characteristics of animals including the ornate plumage of the peacock, the enormous antlers of the male red deer, and the elaborate mating display of the sage grouse. The bright red plumage of the male American cardinal, for example, seemingly makes him an easier target for predators, such as hawks, than the much better camouflaged female. But, as Darwin indicated, sexual selection is not so much about survival as about reproduction.
Darwin (1859, 1871) claimed that sexual selection can operate in two ways. First, in what is known as “intersexual selection” or “female choice,” one sex, typically males, competes in terms of physical or behavioral attractiveness to the other sex, typically females, who, in turn, choose to mate with the one or those they find most desirable. Darwin wrote:
I can see no good reason to doubt that female birds, by selecting, during thousands of generations, the most melodious or beautiful males, according to their standard of beauty, might produce a marked effect. (1859, p. 88)
Second, in “intrasexual selection,” or “male competition,” one sex, typically males, engages in contests for access to the other sex, typically females. These contests range from displays and bluffs to aggressive physical combat that can result in death for one, and, rarely, both (as when male deer get their antlers locked together) combatants. Another type of competition occurs when males compete for possession of a resource critical to females, typically food or breeding locations. According to Darwin:
This form of selection depends, not on a struggle for existence in relation to other organic beings or to external conditions, but on a struggle between the individuals of one sex, generally the males, for the possession of the other sex. (1859, p. 67)
Thus it is, as I believe, that when the males and females of any animal have the same general habits of life, but differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection. (1859, p. 66)
Sexual Selection and Play
In a 1998 presentation, Chick proposed that adult human play and playfulness could be explained by sexual selection. In particular, he claimed that males seek playful females for long-term mates because playfulness signals youth, health, and, therefore, fecundity. Females, on the other hand, seek playful males because playfulness is a signal of non-aggressiveness. Chick (2001) therefore referred to this as a signal theory of play.
Play and Playfulness in Males
Ghiselin (1974, 1982) proposed that play, as well as art, science, religion, and morals, may be the result of artificial selection. In particular, he suggested that playful people are less likely to harm each other: “We seek their company and cherish them, and this may reasonably be expected to increase their Darwinian fitness” (1982, p. 165). The problem is that the situation is asymmetric: males are much more likely to harm females and their offspring than females are to harm males in nearly all species where play is common. Male lions, for example, typically kill suckling cubs sired by the previous dominant male when they assume control of a pride. This eliminates the progeny of another male and brings females into estrus more quickly. Packer and Pusey (1983) estimated that about one-fourth of all lion cubs are killed by males other than their sire. Similarly, infanticide has been observed among apes as well as a variety of monkeys. While infanticide among modern humans is rare, when it does occur, it is much more likely to be perpetrated by stepfathers than biological fathers (Buss, 1989; Daly & Wilson, 1998; Harris et al. 2007).
According to Wrangham and Peterson (1996), “Most female orangutans are raped regularly” and “Battering females is relatively common among chimpanzees” (p. 151). Among humans, Kyriacou et al. (1999) claimed “Domestic violence is the most common cause of nonfatal injury to women in the United States” and “one third of the homicides of women in the United States are committee by a spouse or partner” (p. 1892). García-Moreno et al. (2005) found that between 35 and 76 percent of women in 13 of the 15 sites in ten countries studied around the world had been either physically or sexually assaulted by the age of 15 or older. Overwhelmingly, a current or previous partner perpetrated the assaults.
Hence, males, especially those with which females have close or intimate relationships, can be dangerous to females and their offspring in many species. But there are exceptions. Female spotted hyenas, for example, are exposed to high levels of androgenic hormones in utero (Licht et al., 1992). As adults, they exhibit highly masculinized behavior, are larger and more aggressive than males (Frank, Glickman, & Powch, 1990: Frank, Glickman, & Zabel, 1989), socially dominant, and have an erectile pseudopenis that closely mimics male genitalia (Kruuk, 1972). Nevertheless, males of most species are generally much more aggressive toward females and their offspring than vice versa. This asymmetric relationship seems to be more in accord with sexual selection, as proposed by Darwin (1859, 1871) than artificial selection, including self-domestication. Therefore, Chick (1998, 2001) proposed that playfulness, and play itself, signal non-aggressiveness by males toward others, especially females.
The contexts wherein play among adult males may serve as a signal of non-aggressiveness appear to be delimited as aggressiveness in males in prized in many other situations. Deaner and Smith (2012) provide evidence that men have been much more likely than women to engage in activities, such as warfare, large game hunting, and combative sports, that involve or require extreme physical aggressiveness both in our evolutionary and recent history. In a cross-cultural comparative study, Chick and Loy (2001) found that, in some societies, males are socialized for “hypermasculine” behavior characterized by high levels of physical aggression in addition to other markers that may include excessive drug or alcohol use, destructive behavior, low tolerance for delayed gratification, and generalized belligerence. Brown (1991) and Ghiglieri (1999) noted that, on average, men everywhere are more aggressive, more domineering, more competitive, less caring, and less sensitive to others than women.
Play and Playfulness in Females
Play is, in a majority of animals that engage in it, a characteristic of youthfulness. And youthfulness, in females, is a sign of fecundity (Buss, 1989). Thus, Chick (1998, 2001) proposed that play, as a manifestation of playfulness, is a signal used by adult women to indicate youth, health, and fecundity, qualities that are important to men.
Human females and males employ a variety of visual, olfactory, auditory, and behavioral signals that indicate their value as mates. Women in many societies attempt to enhance their secondary sexual characteristics, such as breasts and waist to hip ratio as well as their appearance of health and youthfulness, through figure enhancing garments and various forms of makeup (e.g., lipstick, nail polish, rouge, hair coloration). Men may exaggerate, or create, their V-shaped torso with shoulder padded garments or manipulate secondary sexual characteristics, such as the beard, in order to enhance appearance. As Puts (2010) noted, “Beauty, fashion, and physical fitness are so important in the United States that they have become multi-billion dollar industries” (p. 157).
Human cosmetic efforts may also be directed at enhancing male-female dimorphism. Compared to our closest primate relatives, such as orangutans, gorillas, and chimpanzees, humans display relatively little sexual dimorphism with adult (over the age of 20) U.S. males averaging about 16% heavier than females in 1999-2002 (Ogden et al, 2004). In contrast, adult Bornean orangutan (Pongo pygmaeus) males are about 98% heavier than females, male western gorillas (Gorilla gorilla) about 90% heavier than females and male chimpanzees (Pan troglodytes) about 35% heavier than females. However, male gibbons (Hylobates agilis) are only about 6% heavier than females (calculations based on data in Flinn  and Gaulin & Sailer ). Differences in sexual dimorphism are usually attributed to several factors. These include mating system (polygynous species tend to be more dimorphic than monogamous species), size (larger species tend to be more dimorphic than smaller species), habitat (terrestrial species tend to be more dimorphic than arboreal species, with the orangutan being a conspicuous exception), group composition (dimorphism is greater in social groups dominated by single males than in those containing multiple males), and, especially, sexual selection (Flinn, 2012; Gaulin & Sailer, 1984).
Finally, adult human females may signal youthfulness, and thus fecundity, through play but also must not appear too youthful, that is, failing to exhibit mature secondary sexual characteristics. These include developed breasts, a desirable hip to waist ratio, and altered distribution of weight and body fat (Pazhoohi & Liddle, 2012; Singh & Young, 2001).
Testing the Signal Theory of Adult Playfulness
While the content of the signals sent by males and females via play and playfulness, indicating non-aggressiveness by the former and youth and fecundity by the latter, differ, the ultimate message, that of being a desirable mate, is the same. Therefore, both men and women should prefer playful individuals as prospective long-term mates. In his masterful compendium on animal play, Burghardt (2005) supported the idea that adult play and playfulness depends, in part, on sexual selection. Similarly, in their recent review of biology and play, Graham and Burghardt (2010) acknowledged that sexual selection might influence sex differences in human play. However, empirical evidence must be the ultimate arbiter.
In a study of mate preferences among a sample of college undergraduates, Chick, Yarnal, and Purrington (2012) hypothesized that the characteristic of playfulness, along with presumably related concepts such as sense of humor and fun-loving, would be highly desirable in potential mates by both males and females. They also included secondary hypotheses that characteristics, including kind and understanding and easygoing, which may reflect non-aggressiveness, would be more preferred by females in males than by males in females while healthy, physically attractive, and good heredity, likely indicators of fecundity, would be more preferred by males in females than by females in males. Chick et al. (2012) added playful, fun-loving, and good sense of humor to a mate preferences survey developed by Buss and Barnes (1986) that included the following thirteen items: kind and understanding, exciting personality, intelligent, physically attractive, healthy, easygoing, creative, wants children, college graduate, good earning capacity, good heredity, good housekeeper, and religious. A sample of 254 students (164 males, 89 females) were asked to rate the 16 characteristics on a scale of 1 to 10 (where 1 =“not at all desirable,” 6 = “moderately desirable,” and 10 = “extremely desirable”) as well as respond to several demographic questions.
Playfulness ranked fifth overall but fourth as a trait desired in males by females. Having a good sense of humor ranked first overall, second as a trait desired in males by females and first as desired by males in females, and fun-loving ranked third overall and third by both males and females as desirable in a partner. These ratings support Chick’s (1998, 2001) general hypothesis regarding the importance of playfulness and related traits. With respect to their specific hypotheses, Chick et al. (2012) found that males rated physically attractive, healthy, and good heredity in females as more desirable than females rated them in males. Females rated kind and understanding as more desirable in males than males did in females, as expected. Although male and male preferences for easygoing did not differ significantly, the other results are consistent with predictions from the signal theory of adult playfulness.
There are caveats, however. First, the data collected by Chick et al. (2012) only support the prediction that the characteristics of playfulness, a good sense of humor, and being fun-loving are indicated as desirable in prospective long-term mates by a sample of college undergraduates. Results regarding the reasons why males and females prefer playful mates are consistent with Chick et al.’s (2012) secondary hypotheses but are not direct tests of them. Second, the authors assumed, but did not demonstrate, that having a good sense of humor and being fun-loving are associated with playfulness. There is a substantial literature on sense of humor and mate preference including Miller’s (2000a, 2000b) suggestion that humor evolved via intersexual selection. Miller suggested that variation in sense of humor provides information about individuals’ genetic quality and therefore a good sense of humor is important in mate choice (Bressler & Balshine, 2006; Miller, 2000a, 2000b, 2001). While men appear to use humor more often than women (McGhee, 1979), in mixed-sex groups (Robinson & Smith-Lovin, 2001), and in courting situations (Simpson et al. 1999), Feingold (1992) found no gender differences in a meta-analysis of 34 self-report studies of mate preferences that included good sense of humor as one of various traits. Chick et al.’s 2012) results are consistent with Feingold’s (1992). No other studies of mate preferences included either playfulness or fun loving among traits studied (but see Graham and Burghardt’s  review of behaviors that include smiling, laughing, and singing as they relate to play).
Chick et al.’s (2012) college student sample is open to criticism, as well (e.g., Henrich, Heine, and Norenzayan, 2010). While it seems very likely that most or nearly all of the informants were interested in sex, given their age range, many may not have been seeking long-term mates. Most important, however, mate preference does not necessarily dictate mate choice (e.g., Perussé, 1994; Pillsworth, 2008), that is, you don’t always get what you want.
Alternatives to the Signal Theory of Adult Playfulness
Sexual selection, as described by Darwin (1859, 1871), is asymmetric in that competition most often involves males and choice is usually the domain of females. This raises several questions. First, can sexual selection account for the evolution of ornamental traits and weaponry in females as well as for female-female competition when it does occur? As anyone who has ever attended high school knows, human females emphasize their secondary sexual characteristics, incorporate add-on ornaments (e.g., jewelry, flattering clothing, perfumes, hair and nail coloration), and often engage in vigorous competition over males. Human males also engage in ornamentation via jewelry, clothing, manipulation of secondary sexual characteristics such as facial hair, and compete through displays of resources, such as fancy cars. In other species, mutual ornamentation occurs in birds, such as the black swan (Cygnus atratus) (Kraaijeveld et al., 2004) and the rock sparrow (Petronia petronia) (Griggio et al., 2009), some insects (e.g., Prudic et al., 2011) and fish (e.g., Massironi, Rasotto, and Mazzoldi, 2005).
Female ornamentation has generally been regarded as the result of genetic correlation with sexually selected male ornaments because female ornaments are often smaller or less conspicuous than their male counterparts (Lande, 1980). More recently, however, researchers have proposed that female ornamentation may be due to female-female competition or male preference (Amundsen, 2000). While there is ample evidence that females choose mates based on signals, including ornaments, that indicate their genetic quality (Andersson, 1994; Andersson & Simmons, 2006), male choice may be driven by signals of variation in female fecundity which may also include ornaments (Amundsen and Forsgren, 2001; Griggio et al., 2005; Massironi et al., 2005).
Additionally, females of some species, such as the spotted hyena, have evolved weaponry as well as aggressive behavior. Tobias, Montgomerie, and Lyon (2012) summarize recent research and theory on female ornaments and weaponry and point to growing evidence that female traits are more important in mediating competition for resources rather than for acquiring mates. In order to accommodate female ornamentation, weaponry, and aggressive behavior, Tobias et al. (2012) provide two possible solutions: “One is to expand the concept of sexual selection to include all mechanisms related to fecundity; another is to adopt an alternative conceptual framework—the theory of social selection—in which sexual selection is one component of a more general form of selection resulting from all social interactions” (p. 2274).
West-Eberhard (1983) described social selection as “differential reproductive success (ultimately, differential gene replication) due to differential success in social competition, whatever the resource at stake” (p. 158). Sexual selection is therefore a “subset of social competition in which the resource at stake is mates” (West-Eberhard, 1983, p. 158). According to West-Eberhard (1983), social selection can involve ornamentation, weaponry, aggressive behavior, and conspicuous displays as in sexual selection and convey similar information about the quality and condition of individuals. Social selection involves all aspects of social interaction, including play and playfulness, while removing the emphasis on male-male competition. Problems in dealing with female-female competition as well as female ornamentation, weaponry, and displays are thereby reduced or eliminated.
If adult playfulness can be regarded, at least in part, as an ornament or an analog to an ornament designed to provide information about mate quality, it must be considered to be sexually monomorphic since, while the signal is similar for both males and females of playful species, the message may differ by sex. Since social selection theory was designed to address monomorphic ornamentation and to include sexual selection, it may, therefore, provide an appropriate basis for the examination of play and playfulness.
Even if play is a sexual signal among adults, we know little about how often, honestly, and reliably it indicates mate quality in terms of non-aggressiveness in males and fecundity in females, the degree to which it correlates with responsiveness on the part of receivers, and how costly both the signal and the response may be. Additionally, we know nothing about the degree to which response to the signal is mediated by individual (such as age, health, or social status) and cultural characteristics of the sender or the receiver.
In summary, accepting play and playfulness as sexual signals requires them to be forms of mutual ornamentation or display but with somewhat different functions for males and females. Men may utilize play as a means of acquiring mating opportunities by indicating their non-aggressiveness to females. Women may use play to attract men, perhaps those with resources or the ability to acquire them, by demonstrating their youth, health, and fecundity, a view that requires an expansion of sexual selection theory to include female-female competition and ornamentation or recourse to social selection theory. Both theory development and gathering a great deal more empirical evidence are necessary before stronger claims can be made about the degree to which playfulness as an individual trait and play as its behavioral manifestation signal others about fitness as a mate. Adult play is a legacy of human evolutionary history but is not well explained by theories applied more successfully to play among juveniles. Therefore, its examination in light of sexual and social selection theory appears to have merit.
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