Observations of Sexual Dimorphism
Sexual dimorphism is the term used to describe when the two sexes of a species possess physical characteristics that differ from each other. These physical characteristics can include differences in height, weight, color, decoration, and pattern and can even include body parts unique to one sex or behaviors such as birdsong.
Evolutionary psychologists are interested in human sexual dimorphism (the observation that men and women tend to have different physical qualities), and whether it constitutes an evolved signal of some kind. There is a large body of evidence that this is the case for nonhuman animals, for example, the peacock’s tail which signals mate quality to the peahen; and the antlers of some deer species which signal to rivals the dominance of the deer. This chapter will report some of the evidence suggesting that similar signals occur in humans, as well as evidence that suggests they do not. Specifically, it will focus on two areas: male ornamentation, physical characteristics that males possess which seemingly do not help with survival chances but do help with reproduction chances, and costly signalling, the fact that signalling can use an animal’s nutrients from food to grow an ornament at the cost of maintaining the animal’s overall health or increase the risk of death by predation.
When observing nonhuman animals, sometimes the two sexes of a species will look noticeably different from each other. When each sex of a species has physical qualities that are unique to that sex, or that differ quantitatively between the sexes, this is considered sexual dimorphism (from the Greek dίmorphos meaning “having two shapes”). There are many types of sexual dimorphism in the animal kingdom. Sometimes, the two sexes are different sizes, such as with the blanket octopus, whose females can be up to 40,000 times heavier than the males, or the colors of the animal can be quite different between the sexes, such as the many species of birds known for the stark difference between their brightly colored males and milder-colored females.
Although it is not always clear why these differences exist, evolutionary theorists have gathered evidence of the process behind their evolution, namely sexual selection. Instead of characteristics being selected because they help the animal survive (natural selection), some characteristics are selected because they help the animal gain access to mates, and this is what is called sexual selection. Sexual selection can lead to characteristics that help an animal in the mating market by giving it the means of winning the competition with its same-sex competitors, or by making it more attractive to the opposite sex.
This assessment is reinforced by the observation that some of the differences between the sexes appear to be purely decorative. Which is to say that it seems the specific characteristics have no use for the animal’s survival, they just appear to be ornamentation. These ornaments appear far more often in the male of the species, rather than the female. This is driven by what is known as the males-compete/females-choose model of selection which most sexually reproducing animals follow. In such systems, males compete for access to females, and/or females choose which male to mate with. Under this model, ornaments do serve a function. They are communication devices or signals to other animals of the same species.
In order for a characteristic to be considered as an ornament, it needs to meet some criteria. First, it must be a secondary sex characteristic. Secondary sex characteristics are features that are used in the process of sexual selection but are not directly used for sexual reproduction. Second, it must correlate with some underlying quality of the animal. This could be an aspect of the animal’s health or an aspect of the animal’s behavioral temperament. And third, there needs to be evidence that the other members of the species actually use the signal as an indication of the underlying quality.
Ornaments can take on many forms. They can be qualities of the physical body, such as the bright colors of the mandrill’s face; elaborate patterns like those of the golden pheasant’s feathers; or strangely shaped body parts like the hooked jaw of spawning salmon and trout. They can also be behaviors, such as the birdsong of the Australian whipbird; or the elaborate dances of the peacock spider.
The peacock is the most commonly used example of male ornamentation. It has elongated tail feathers with some of those feathers looking like large eyes (ocelli). Research has shown that the length of the male’s tale feathers is positively correlated with overall body condition (Moller and Petrie 2002), so the peacocks with the longest tails are also generally the healthiest peacocks. Other research has discovered that during mating season, the peacocks with the most ocelli are the ones which mate with the most peahens (Petrie et al. 1991). With these two observations linked, it can be said that the peacocks with the longest tails are also more likely to have the most ocelli (more space on the larger tail to fit them in) and mate with the most peahens. So, we can see that the ornament is related to an underlying quality, and the opposite sex is using the ornament to make mating decisions, i.e., finding the healthiest peacocks in order to increase the chance that her chicks will be healthy too. This is an example of the ornament being used to communicate something to the opposite sex, but ornaments can be used to communicate to the same sex as well.
Ornaments can also communicate something about the animal’s temperament or expected behavioral patterns. The fallow deer is a species found in many places around the world, due to its introduction from European origins. The mature bucks grow a new set of antlers every year before their mating season. The antlers are shovel-shaped, with some spike-like growths around the edges. Research has shown that the total number of these spikes and the dominant behavior of the bucks are positively correlated (Pélabon and Joly 2000). This finding, combined with the fact that, although the bucks do use the antlers for fighting, they are the main source of determining status in the male hierarchy without fighting (Lincoln 1972). Due to the likelihood of injury during a battle, it can be more effective to use the antlers as a signal of which bucks to avoid. There has also been some conflicting evidence regarding the symmetry of the antlers, with Malyon and Healy (1994) finding that fluctuating asymmetry correlated with dominance, but Pélabon and Joly (2000) finding that it did not. It is also important to note at this point that the antlers are not strictly ornamental, as they do serve a function in fighting; therefore, the argument could be made that they are technically armaments.
Humans are dimorphic as well. There are differences in the average weight, height, bone density, muscle and fat distribution, and facial structure between the sexes (Burke and Sulikowski 2010; Gangestad and Scheyd 2005). Just as biologists and ethologists use theories of evolution to explain the function of dimorphism in nonhuman animals, evolutionary psychologists can use the same theories to explain the function of dimorphisms in humans. However, data to date suggest that humans are more complex than other animals in this regard, and it is not always clear what constitutes an ornament in humans. This may be because although they still exist, human ornamentations are less extreme than those seen in some other animals. While there are strong body shape dimorphisms in humans, which reflect greater muscle mass and strength in males, and differential fat distribution and pelvic structure in females, many of these differences might plausibly be related to being better able to perform sex-typical roles in traditional societies, and so, even though there is evidence to suggest that these differences may operate as signals/ornaments (e.g., hand-grip strength, Gallup and Fink 2018; waist-to-hip ratio, Singh 1993), the emphasis here will be on facial dimorphisms, since they are more likely to be pure ornamentation.
Facial masculinity is one possible ornament in human males. When the overall composition of the face, including the bones, fatty tissue, skin, and facial hair characteristics, aligns with those which are more likely to be found in men’s faces, the face is considered to be masculine. Generally speaking, men will have wider jaws, larger chins, larger width-to-height ratio of the maxilla (the top section of the jaw), and a larger brow ridge relative to a woman of the same body size. Compared to women, less fat is deposited in the cheek area, skin tone is generally darker, as are eyebrows, and of course, men are much more likely to be able to grow beards (Gangestad and Scheyd 2005).
Researchers have found that masculine facial characteristics are an indicator of underlying sex hormone ratios. Testosterone (found in greater quantities in men) is involved in the building of bone structures, while estrogens (found in greater quantities in women) suppress bone growth (Kung 2003; Neave et al. 2003). Additionally, masculine facial characteristics develop fully in structure during puberty, when there is an increase in sex hormone levels. This is another indication that facial masculinity is an ornament, as ornaments tend to develop in other animals as they reach reproductive maturity.
Based on these findings, it can be said that the level of facial masculinity seen in a man’s face may be a signal of the measure of testosterone circulating in his system. This is important to prospective mothers and future children because testosterone promotes both morphological and behavioral differences.
A greater level of circulating testosterone has been found to lead to more aggressive behaviors and a reduction in pro-social behaviors. It has been shown that as circulating testosterone levels increase, so do levels of physical aggression, verbal aggression, anger, and hostility. Additionally, as levels of circulating testosterone increase, levels of altruistic and empathic behaviors decrease (Harris et al. 1996). As it would be in a prospective mother’s best interest to provide a safe and nurturing environment for future offspring, it would be reasonable to assume that she would avoid those males signalling greater levels of aggression. However, in our ancestral past, a certain level of aggression would have been adaptive. An aggressive male would have stood a better chance of climbing the social hierarchy, thus making him more attractive; but he also would have stood a better chance protecting his family from other men wishing to take his mate and/or resources.
More evidence supporting this hypothesis is the fact that both women and men will judge images of highly masculine male faces as belonging to a more dominant person, while less masculine faces are judged as less dominant (Windhager et al. 2011). However, although the relationships between testosterone and both aggression and physical dimorphism are well established; perceived dominance shows a relationship with testosterone which is not entirely clear. Neave et al. (2003) studied a group of males by taking measurements of both prenatal levels (2D:4D ratio) and current circulating levels of testosterone (salivary measure). They found that prenatal levels of testosterone are related to perceived levels of dominance, but the current circulating levels of testosterone are not. This indicates that greater levels of prenatal testosterone do indeed organize the bone structure of the face to make it appear more masculine and dominant. But, as the levels of prenatal testosterone and current circulating levels were also not related, this calls into question the relationship between facial masculinity and actual behavioral outcomes.
The males-compete/females-choose model has been cited as the reason behind ornamentation of males, but what if the roles were reversed? As predicted by Trivers (1972), when the parental care roles are the opposite of the norm, males can be choosy and females can evolve ornaments. For example, some female pipefish, seahorses, and seadragons are more ornate than the males. It is the males that carry, give “birth” to, and care for the young, and therefore, it is the females that work for the attention of the males.
A second model of sexual selection can also result in female ornamentation. The mutual mate choice model predicts that when both sexes compete intrasexually, and both sexes are choosy about mate selection, then both sexes could use ornamentation. This is the case in humans. Just as facial masculinity is considered an ornament in men, facial femininity is considered an ornament in women. As mentioned previously, testosterone is related to facial masculinity; but for facial femininity, there is a similar relationship with estrogen and progesterone (Law Smith et al. 2006). In women, these two sets of hormones are found in greater quantities, and they are responsible for maintaining and controlling the menstrual cycle. In other words, they are related to a woman’s fertility, and so the femininity of a woman’s face is a potential signal of her fertility.
As has been explained, ornaments are a type of signal. In evolutionary theory, signal has a specific meaning. It is a characteristic of the animal that is used to communicate a specific message. It is done “intentionally” by the signaler (but rarely consciously), and its message is intended to be clear to the receiver, resulting in a behavioral change that is beneficial to both individuals. Cues, on the other hand, are unintentional, may be detrimental to the animal sending the cue, and may not always be clear in their message. However, if the cue turns out to be a useful communication device, there is the possibility that it evolves into a signal, which is known as the “signaler precursor route” (Laidre and Johnstone 2013). Take, for example, a wolf baring its teeth. This is a definite signal that the wolf is aggressive and about to attack. It is quite possible that this began as a cue, simply because a wolf needs to draw back its lips so it can bite. Those wolves that saw this cue and avoided conflict with the aggressive wolf would have lived longer, and thus reproduced more pups that tended to avoid other wolves when they saw teeth being bared. Additionally, those wolves that bared their teeth before attacking would have also avoided conflict, lived longer, and reproduced more pups that tended to bare their teeth when becoming aggressive. This is a simple example of a mutually beneficial signal, but signaler/receiver psychology can become more complex when the “cost” of the signal becomes high.
In line with the previous example, in order signal to evolve, the benefits must outweigh the costs. One “cost” of the signal is the amount of physical energy or nutrients/resources required to generate the signal, with ornament growth shown to decrease when food sources are low (Zeh and Zeh 1988). Costs are also associated with the increased risk of predation or being attacked by same-sex rivals due to the signal increasing the visibility of the animal. Another potential cost is the opportunity cost of missing mating opportunities because of the signal. The “benefit” is defined as the increase in reproductive fitness – an increase in the number of offspring the animal produces. In short, if the development of any signal leads to a handicap for the animal’s survival, then there must be a greater advantage to the animal’s reproductive chances to warrant the cost; otherwise, the genes which guide development of the signal will never make it into the next generation (Zahavi 1975).
Returning to the wolf example, it is reasonable to believe that the cost of baring the teeth would be quite low. The energy cost would be minute, and the likelihood of attack from competitors would be low as well because the signal is expected to actually reduce the likelihood of violent conflict. However, consider the signal this sends to prospective mates. If the wolf were to bare its teeth excessively, then it may be seen as an overly aggressive individual, which puts the safety of future pups at risk. Therefore, the interpretation of the signal must be consistently reliable and accurate – an “honest” signal. That is, a signal must truly be signalling what it is “intended” to signal in order for the communication system to work; otherwise, the system would collapse. So, it can be seen that the cost of a signal needs to be finely tuned in accordance with the benefit to reproductive fitness. For female mammals especially, the cost of favoring a “dishonest” signal can be high from a mating perspective, as mating with the wrong individual can lead to the extremely high cost of gestating, giving birth to, and feeding young which have a lowered chance of going on to reproduce in the future.
For the facial masculinity of human males, one of the greatest costs comes from the negative effects of greater levels of testosterone. Exposure to high amounts of testosterone has been shown to impair the immune system and make it more difficult for the human body to fight off pathogens (Foo et al. 2017). According to the “good genes” theory of mate choice, the display of high levels of testosterone is seen to signal genetic quality (Penton-Voak et al. 2004). The theory predicts that for a male to be signalling his compromised immune system, then his genes must be of a high quality in order to compensate for this disadvantage; and it is these genes that the female values to maintain health of future offspring. This theory was put to the test by Zaidi et al. (2018) who used a measurement of a genetic marker of immune system competence known as the major histocompatibility complex, and a measurement of facial masculinity in close to 2,000 participants. They found no relationship between the two measures. This means that the level of a man’s facial masculinity may have nothing to do with his intrinsic levels of health. It is also worth noting that this finding and failures at explaining other mating behaviors (e.g., Jones et al. 2017) have led to the founders of the good gene theory to retract their support for the concept (Buss and Schmitt 2019).
There is another set of ornaments that is, as far as we know, unique to humans. Miller (2000) makes the argument that displays of intelligence such as those made through music, art, and humor are ornaments. He states that they have no other evolutionary function outside of mate attraction. In terms of costs involved in producing these signals, there is a massive resource cost in growing and maintaining the human brain, which is responsible for these displays. But there is also a social cost when these displays are delivered poorly. Take self-deprecating humor for example. This style of humor is received and interpreted through the context of the status of the person delivering the joke. When a higher status person makes a self-deprecating joke, it is seen as a mismatch with reality and the signaler’s status is unaffected because it is believed they can “afford” the cost. However, if a lower status person uses self-deprecating humor, the results are different because they appear as self-defeatist or depressed (Greengross and Miller 2008).
Many sexually reproducing species exhibit sexual dimorphisms, including humans. Some of these dimorphisms are considered “ornaments” because they appear to serve no survival purpose; they are purely for increasing access to mates. These ornaments are typically displayed by the male of the species, but there are exceptions such as humans. Ornaments can be body parts, or behaviors, and are in essence signals that advertise the quality of the individual who possesses them. Each signal presents a cost to the animal’s ability to get its genes into the next generation, which must be weighed against the benefit to the animal’s overall reproductive fitness.
- Buss, D. M., & Schmitt, D. P. (2019). Mate preferences and their behavioral manifestations. Annual Review of Psychology, 70(23), 1–34. https://doi.org/10.1146/annurev-psych-010418-103408.CrossRefGoogle Scholar
- Gangestad, S. W., & Scheyd, G. J. (2005). The evolution of human physical attractiveness. Annual Review of Anthropology, 34(1), 523–548. https://doi.org/10.1146/annurev.anthro.33.070203.143733.CrossRefGoogle Scholar
- Greengross, G., & Miller, G. F. (2008). Dissing oneself versus dissing rivals: Effects of status, personality, and sex on the short-term and long-term attractiveness of self-deprecating and other-deprecating humor. Evolutionary Psychology, 6(3), 393–408. https://doi.org/10.1177/147470490800600303.CrossRefGoogle Scholar
- Harris, J. A., Rushton, J. P., Hampson, E., & Jackson, D. N. (1996). Salivary testosterone and self report aggressive and pro social personality characteristics in men and women. Aggressive Behavior, 22, 321–331. https://doi.org/10.1002/(SICI)1098-2337(1996)22:5<321::AID-AB1>3.0.CO;2-M.CrossRefGoogle Scholar
- Jones, B., Hahn, A., Fisher, C., Wang, H., Kandrik, M., Han, C., … DeBruine, L. (2017). Women’s preferences for facial masculinity are not related to their hormonal status. BioRxiv Preprint, 1–20. https://doi.org/10.1101/136549.
- Penton-Voak, I. S., Jacobson, A., & Trivers, R. (2004). Populational differences in attractiveness judgements of male and female faces: Comparing British and Jamaican samples. Evolution and Human Behavior, 25(6), 355–370. https://doi.org/10.1016/j.evolhumbehav.2004.06.002.CrossRefGoogle Scholar
- Trivers, R. (1972). Paternal investment and sexual selection. In B. Campbell (Ed.), Sexual selection and the descent of man. Chicago: Aldine-Atherton.Google Scholar
- Zaidi, A. A., White, J. D., Mattern, B. C., Liebowitz, C. R., Puts, D. A., Claes, P., & Shriver, M. D. (2018). Facial masculinity does not appear to be a condition-dependent male ornament in humans and does not reflect MHC heterozygosity. BioRxiv, 322255. https://doi.org/10.1101/322255.