Attraction and Selection 2: Ornaments & Orientation


Welcome back to my illustrated reflective trifecta on sexual selection and self-perception!

Last time, I nattered on about about how attraction could be trait with evolutionary underpinnings, and used swallow tails and human faces as examples of fluctuating asymmetry’s role in mate selection. In both cases, it was male quality being assessed by females.

I would be remiss if I didn’t acknowledge that all three of these posts will be male-centric. I know, I know–classic paternalistic, chauvinist, phallus-gazing twaddle. Biology’s progress has long been hampered by men whose hypotheses [1], observations [2], conclusions [3], and laboratory culture [4] are warped by machismo. By only emphasizing studies investigating male aspects of selection studies, I contribute to prevailing perception that evolutionary biology is all about teh menz.

Actually, we sometimes ignore males all together in population ecology. This isn't a feminist conspiracy, it's just modeling practicality. Endosymbiosis, however, is a feminist conspiracy.

Actually, we sometimes ignore males all together in population ecology. This isn’t a feminist conspiracy, it’s just modeling practicality. Endosymbiosis, however, is a feminist conspiracy.

Moreover, culturally-reinforced body dysmorphia (my initial writing prompt) disproportionately affects women [5]. Anorexia and bulimia nervosa, while increasingly common in men [6], remain more prevalent among women [7], both domestically [8] and globally [9]. Male body insecurity may seem a small issue by comparison, like panicking about dolphins in tuna when we should be worried about tuna’s inevitable demise (PUT THE SANDWICH DOWN!). But all problems are relative, and body image issues are particularly salient for a subset of the male population: namely, the queer subset.

This isn’t news within the queer community, or among our straight allies. Or our antagonists, come to think of it. Gay body fascism has been part of the LGBTQ lexicon since at least the 1990s*, and has been written about critically by plenty of wise people. The most humorous and human writing regarding the subject can be found on Orlando Soria’s brilliant Homemaker blog.**

Our obsession with being either whip thin or hugely muscular (or both at the same time) is more than an irritating facet of gay culture. Higher body dissatisfaction in young gay men [10 & 11] leads to engagement in dangerous conditions and habits, including eating disorders [12] and anabolic steroid use [13]. Alarmingly, abuse of un-prescripted steroids*** is increasingly prevalent among gay adolescents [14].

Self-endangering practices and self-loathing may be partially attributable to our narcism. However, particularly in the case of teens, I am concerned that body fixation originates as overcompensation, a desire to “make up” for any lack of masculinity we imagine comes free-of-charge with our same-sex attraction.§

But what exactly constitutes masculinity? What is a masculine trait? Symmetry may be a trait by which mates assess males, but it is not a feature exclusive to males. To examine masculine traits, we first need to talk about sexual dimorphism, starting with the obvious.

Males and females of non-hermaphroditic (specifically, non-concurrently hermaphroditic) animals differ in their anatomy. Females must have the facility to produce ova, males the capacity for sperm production. The structures designed to produce, nurture, and deliver gametes are referred to as primary sexual characteristics, and present just a minimum of sexual dimorphism.

In addition to their reproductive organs, the sexes can differ in size, shape, life-span, coloration, vocalization, temperature preference–you name it. All these characteristics are called secondary sexual characteristics. The greater the additive dissimilarity of these secondary characteristics, the more sexually dimorphic we consider a species to be. Additionally, variation in the degree of physical dimorphism within a given clade can illuminate evolutionary trends in mating strategy.

A cursorial examination of how female/male body size dimorphism correlates to mating strategy in apes and pinnipeds. As size dimorphism increases, it appears that the number of females that share single male as sperm donor increases (polygyny increases as Mf/Mm decreases). I made this with an Encyclopedia of mammals as my guide, so it's really not something you can reference as real, but here are some papers about it.

A cursorial examination of how female/male body size dimorphism correlates to mating strategy in apes and pinnipeds. As size dimorphism increases, it appears that the number of females that share single male as sperm donor increases (i.e., polygyny increases as Mf/Mm decreases). I made this with an Encyclopedia of Mammals as my guide, so it’s really not something you can reference, but here are some papers about it.

Let’s consider secondary sex characteristics that contributes strongly to dimorphism as a standard for masculinity. The more a prominent a trait is in males relative to females, the more more masculine we’ll consider it.

What about the size dimorphism in the illustration above? I was initially skeptical of this idea…mostly because I think short men are hot. However, a few conversations and a walk around the Internet provided disturbing evidence that many people do equate height with masculinity. Check out the Heightism Exposed Twitter feed some time. Exempli gratia:


Not everyone’s a hater, though. I’m not the only person out there who appreciates short men. Besides, smaller-framed men live longer, healthier lives than their larger counterparts [15], potentially allowing for more mating opportunities. [This is extravagant and erroneous supposition, but just run with it for now.]

Also, size dimorphism may be an example of males and females occupying separate ecological niches. This reduces resource competition between the sexes (intersexual competition) [16]. For example, male birds of prey are frequently smaller than the females [17], which reduces the overlap for preferred prey items and allows the larger female–nearly always the primary incubator–to be scary-as-hades. In a similar vein, the sexually dimorphic bills of recently extinct Huia (Heteralocha acutirostris) [18] and extant Green Woodhoopoe (Phoeniculus purpureus) [19] are hypothesized to allow for the males and females to forage for different invertebrates.

Note that the Green Hoopoe bill partitions based on size, while the Huia partitions/ed by size and shape.

Note that the Green Hoopoe bill partitions based on size, while the Huia partitions/ed by size and shape.

If we set aside niche-partioning traits (that is, any trait that could serve to reduce intersexual, intraspecific competition), what secondary sex characteristics does that leave us with as our benchmarks for masculinity? What if we only look at dimorphic traits that serve no other function than the attraction of and competition for mates?

Enter the the wondrous and incredible world of Male Adornment. Examples of sexual ornamentation abound in the animal kingdom. It would be impossible to cover all their variety here: wild colorationcumbersome plumage,  fleshy crests, seasonal antlers, aggressive eyestalks, and so on.

Sometimes, these ornaments look really gross.

The evolutionary path seems relatively straightforward for some of these traits. For males that physically compete for mates, structures that give an advantage in virile brawls will be selected for over time. The enlarged mandibles of stage beetles (Lucanidae), the more elaborate antlers in Cervidae (deer and their allies), and the massive horns of bighorn sheep (Ovis canadensis) all fall under this category.

The selective path for other ornaments is not so clear. Rather than being practical, sometimes sexual adornments seem damn inconvenient, or downright dangerous. For example, what’s the benefit of adding drag to your wings when you forage for flying insects?

Pennant-winged Nightjar (Caprimulgus vexillarius). Illustration originally published in Ibis 1863. © Wikimedia Commons

Pennant-winged Nightjar (Caprimulgus vexillarius). Illustration by Joseph Wolf originally published in Ibis 1863. © Wikimedia Commons

Why would the male of a tiny prey species adorn itself with bright colors that make them look tastier to predators [20]?

Female (left) and male (right) guppy (Poecilia reticulata). Doesn't that orange tail make him look scrumptious?  © Wikimedia Commons

Female (left) and male (right) guppy (Poecilia reticulata). Doesn’t that orange tail make him look scrumptious? © Wikimedia Commons

And seriously, what’s the point of the Narwhal’s tusk? Yeah, I know people claim they’re for sparring or breaking ice, but they are way too long for that bag. With only some inferential evidence [21], I’m going to categorize the tusk as another weird-ass, inconvenient, and possibly maladaptive secondary sexual characteristic.

Ronald A. Fischer, Public Domain image.

Ronald A. Fischer, Public Domain image.

We refer to these bizarre traits as Fisherian Runaways, and the process that created them as runaway sexual selection. They take their name from Sir Ronald Fisher, a prominent evolutionary biologist and statistician (and eugenicist†) in the early 1900s. Fisher was fascinated with these runaway traits–why didn’t natural selection eliminate ornaments that posed such hazards? Wouldn’t the expense of maintaining the trait select against it? The literature on this is enormous, but here are two of the most studied (& not mutually exclusive) hypotheses advanced to explain Fisherian Runaways.

1: Sexy Sons

Female choice for a trait with selective benefits leads to its outsize exaggeration [22]. To use an example from last time, let’s say, many generations past, the outer retrices of the Barn Swallow (Hirundo rustica) were good predictors of flight performance. Slightly longer tail feathers made males more maneuverable, and females that preferred longer-tailed males had fitter offspring. Thus, female preference for this trait is selected for in subsequent generations. After a while, the preference for long outer retrices is ubiquitous and indiscriminate: females will always mate with the male with the longest tail, even when the feathers are so long they don’t confer any flight benefit, and are actually energetically expensive to maintain. Fisher hypothesized that–at some point–the trait runs up against its absolute maximum (when males just die because of it), and is maintained there, with females choosing the fathers that will give her the sexiest sonsmoreswallows

2: Honest Signal

Runaway traits are a honest signal of fitness BECAUSE they are expensive [23]. Sure, a male guppy’s bright colors require energy to maintain, and make it susceptible to predation. But if a male survives in spite of these costs, a female knows he’s fit enough to forage well and dodge predators. Thus, choosing the male with the most outsize ornamentation leads to fitter offspring over generations, and the ornament is maintained.

Even though fewer showy males survived predation events, the ones that do may have that "extra something" to contribute to offspring fitness.

Even though fewer showy males survived predation events, the ones that do may have that “extra something” to contribute to offspring fitness.

We’ll return to runaway traits again in Part 3, but for now let’s look to humans and whether or now we can be considered ornamented. Some of our dimorphism is reproductive practicality, like women’s broadened hips for child-bearing. Some of it may be due to physical intrasexual conflict–at least, this hypothesis has been advanced to explain men’s thickened face bones (I guess our grandfathers have been bashing each other in the faces for a long time [24]).

What ornaments do we have that serve no real purpose? One could argue that humans’ pendulous mammary glands (boobs) are strictly ornamental prior to lactation, but that’s complicated by our evolutionary history as extended breastfeeders [25]: pre-childbirth breasts could have originated as a signal (honest or otherwise) of the potential to feed infants.

To me, that leaves one glaringly obvious dimorphic trait; one that appears to serve no real purpose than to populate cafes and artisan doughnut shops in every hipster-mecca across the globe, that bizarre and variable aspect of the male pubescence: the beard.

This is the dominant phenotype for Richmond males aged 25-35. Photo by Juan Luis, shared under Creative Commons License.

This is the dominant phenotype for RVA males aged 25-35. Photo by Juan Luis, shared under Creative Commons License.

I’ve searched Web of Science and Google Scholar using the terms “beard”, “evolution”, “facial hair”, “Homo sapiens“, “male”, “mustache”, “selection”, and “female preference” in various combinations. To my surprise, there isn’t much consensus on when facial hair evolved in humans, or what conditions selected for it, perhaps because beards themselves are so variable.††

Most hypotheses regarding their function fall along these lines:

  1. Beards were face insulation during the last glacial maximum.
  2. Beards act as wicks and traps for pheromones.
  3. Beards are an intra-sexual (i.e., man-to-man) signal of age & dominance.
  4. Beards function as inter-sexual (man-to-woman) of mate quality (i.e., good genes/sperm).
  5. Beards are an inter-sexual signal of social quality (i.e, socially dominant, with resources to be a providing father), but not gene quality.
Evolution. Why beards? What beards do? Many hypotheses. Much conjecture.

Dear Evolution: Why beards? What beards do? Many hypotheses. Much conjecture. Love, Dan

Hypothesis 1 is dumb. Women were exposed to the same frigid conditions as men during the Pleistocene, and they didn’t evolve beards.

Hypothesis 2 may be less dumb, but beard hair, which is long and relatively straight, doesn’t share the physical characteristics with pheromone-wicking pubic hair, which is short and curly.

Hypothesis 3 may have something to it. I do feel an abnormal compunction to punch hipsters in the ironic beard when they bring up their NGO that teaches low-income urban youth to brew kombucha using mason jars and cultures from moldy single-speed bicycle seats.

But hypotheses 4 & 5 are where most behavioral research has focused. Findings have been pretty variable. In some cases, women identified beards as unattractive [26], or only signaling age and dominance [27]. In others, women did find stubble unusually attractive, particularly when most fertile [28]. One study even found weak evidence that preference for beards is genetic, with monozygotic (identical) twins’ sharing attraction to facial hair more often that dizygotic (fraternal) twins [29].

The current popular study attributes this variable beard attraction to negative frequency-dependent preferences, or NDFPs. If a trait is subject to negative frequency-dependent preference, its allure is inversely related to its prevalence. In other words, the more common a trait is, the less sexy it becomes.

NDFP has been used to explain how the variety of colorful guppy tails is maintained in wild populations [30, 31, 32]. If we expect preference for only one tail type, the others would disappear from the population in a few generations. However, if females preferentially mate with the least common tail type–or males with tails different from previous mates–the proportion of tail types will fluctuate over time, maintaining the population’s polymorphism. [Predation has a hand in this, too]

Negative frequency-dependent preference in action, in guppies, in love.

Negative frequency-dependent preference in action, in guppies, in love.

Janif et al. (2014) demonstrated that attraction to male beards my operate on a similar principle [33]. To do this, they showed women (and men) photographs of men’s faces with a range of facial hair, and had the participants rate their attraction to those faces. Additionally, researchers manipulated the proportion of beardy faces that participants viewed. Some participants saw lots of stubble, others saw lots of clean-shaven faces, while others primarily saw of bearded countenances.

Participants ended up rating the phenotype they saw less frequently as more attractive. If they were exposed to many clean-shaven faces, they rated the bearded faces as more attractive. If there were plenty of beards, they perceived the hairless chins as sexy.

Janif et al. (2014) study, recreated with unauthorized sketches of my boyfriend. He normally sports designer stubble.

Janif et al. (2014) study, recreated with unauthorized sketches of my boyfriend. He normally sports designer stubble.

So good news for men everywhere! To figure out what facial hair is currently sexiest, you just need to quantify polymorphisms around you. Go with the least abundant variety, and you’ll be the belle of the ball. Or the beaux of the ball.

That is, unless you’re like me. While my father and younger brother both sport healthy beards, I think my Y-chromosome was baked at the wrong temperature. I can go about 5-6 days without shaving, with my face staying relatively smooth. Sometime around day 7, hideous tufts of red-brown wire erupt in uneven patches from my jaw and upper lip. To add insult to injury, it itches something fierce.


Alas, another masculine trait not available to me. But never, fear–phenotype is so much more than anatomical structures. A great wealth of phenotype can be found in behavior. More on that in Part 3.


*The earliest use of the term “body fascism” in reference to gay workout and physical norms I can find is a Guardian article by Richard Smith on 1993.10.23 titled THINGS THAT GO PUMP IN THE NIGHT: More gay man hours are spent in the gym than the nightclub. Why? Because muscle has pulling power. (1) Gosh, I love rambling headlines. (2) Gosh, I love bad puns & Aerosmith references. (3) Gosh, I would love to link to this article as well as the author’s website, but I found it through Lexis Nexus, and several men with the name of Richard Smith write for the Guardian, and I can’t be sure which one penned this paper. If you know who/where he can be found, let me know and I’ll hyperlink to him.

**Mr. Soria sarcastically concludes saying that body obsession will lead gay men to inherit the earth. They will, but not because of any mental or physical superiority. The Queer Illuminati have powers beyond heterosexuals’ wildest imaginings.

***Anabolic steroids have legitimate medical uses, treating hormone deficiencies, weight loss from cancer, and wasting associated with AIDS.Turns out queer guys are much more likely §Not saying feminine or non-masculine guys ain’t great. We are. The issue is that we think we ain’t.

† Yes, once upon a time, many brilliant people were ardent eugenicists. Yes, there was a journal called Eugenics Review, and it occasionally published papers with reputable science. I’m working on a post about the frightening history of eugenics & evolutionary biologists, but for now, I’ll leave it at this: eugenics is a monstrous and evil philosophy. It mixed scientific theory with myopic, classist, and racist perceptions of society to create reprehensible policy.

††Facial hair is a variable trait between populations: Mediterranean populations are notoriously hirsute, while facial hair is considerably dense and abundant in some East Asian lineages. We can’t be certain of the evolutionary history for each of these populations, and thus our inferential power is limited.


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28. BJ Dixson, JC Tam, M Awasthy. 2012. Do women’s preferences for men’s facial hair change with reproductive status? Behavioral Ecology ?

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32. AL Eakley and AE Houde. 2004. Possible role of female discrimination against ‘redundant’ males in the evolution of color pattern polymorphism in guppies. ProcB 282(1801): S299-S301.

33. ZJ Janif, RC Brooks, and BJ Dixson. 2015. Negative frequency-dependent preferences and variation in male facial hair. Biology Letters 11(1): 1-4.


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