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Sexual Selection

April 28, 2013



Sexual selection is far from being limited to human females posting personal ads looking for a mate who is tall, handsome and loaded with money; or to bar queens wearing spandex and smoking a cigarette and thrilling at the sight of two drunk guys pummeling each other for the chance to buy her ninth drink of the night; or to a rock and roll gal looking to score with the lead singer of a local band but having to settle for the guy wearing the colorful leather jacket who had gone to high school with the bartender; or to the more rhythmic females choosing to take home the guy who has demonstrated the best moves on the dance floor in the Saturday Night Fever stage of our evolutionary history. This sort of behavior is displayed throughout all parts of the organic realm that depend on sexual reproduction to further the species.

Sexual reproduction is essential if a species is to adapt to ever changing environments, whereas asexual reproduction (cloning) needs the same external conditions for long-term survival. Sameness leads to lameness. One of the byproducts of sexual reproduction is dimorphism – that is, two versions of the same species, with the female looking one way and the male looking another way, with the most prominent difference, aside from genitalia, being body size. There are examples in nature of the ladies as the larger sex, for instance spiders wherein the female will kill the male after he has performed his conjugal duty, but, in most species, the male is bigger, stronger and sometimes even prettier than the female. This dimorphism is the result of sexual selection, of females selecting male partners that stand out from the pack as a result of bigger, special, attractive qualities that are then passed down to the next generation of male offspring, a process that, when repeated through thousands of generations, create a phenotypic difference between male and female. (Ricklefs, 2007)

Charles Darwin meditated at length on sexual selection in his 1871 book, The Descent of Man. He used the term “secondary sexual characteristics” to describe the male qualities that take no part in actual sexual union, and in fact have no real utilitarian purpose other than to help individual males outshine each other for the chance to partake in coitus. He cites the male peacock’s tail as a shining – and gaudy – example of such superfluous display that helps the most ostentatious male get the girl (Gadagkar 2003). On the other hand, males are larger in size and sometimes equipped with weaponry, like the horns of antelopes, for the reasons of combat, not much different than our two drunk guys throwing each other against the jukebox for the chance to couple with the female barfly.

What follows is a review of various studies of sexual selection featuring males competing with one another for the approval of the discerning female and thereafter to a procreative event. Our examples will include how size matters in elephant seals and in tule elk; how eyes matter in stalk-eyed flies; how being the lead singer matters in field crickets; and how a colorful neck makes or breaks the male red-collared widowbirds. There will follow a conclusion that will debunk the human conceit that our romantic rituals are loftier than what goes on in the animal and insect kingdom.

Who gets the girls?


Two Swedes and a Scotsman, all biologists, set out to determine whether the huge size difference in male and female elephant seals have more to do with male-on-male competition than with the gals picking out the tallest guy.  Female pinnipeds tend to gather in groups, like an audience at an Oprah Show, except these ladies do not boo the males, but rather wait to see which male will beat back all the other males and thereafter convert the audience into his private harem – to the victor goes the polygynous prize.

What the scientists did was collect data from a host of literature and then run it through their own set of regression analysis. What they looked for was a direct correlation between male body size and harem size – did the biggest guy literally get the most girls and, moreover, keep them captive? The numbers backed up their prediction. They were also confident that it was male-on-male battle that selected for larger and larger males, which makes sense in that it is the heftiest guys who impregnate the harems, leaving the little guys to watch from the periphery. (Lindenfors, Tullberg, Biuw, 2001)

Tule Elk:

Four scientists spent time in the Owen Valley, California to study the “effects of antler breakage on mating behavior in the male tule elk.” What they wanted to know was whether a big rack meant improved fitness, and one way to determine that was to record antler size before the rut, during the rut and after the rut – i.e., the period when male deer get really horny and, as a result, engage in antler-clashing brouhahas over harem rights. They chose Owen Valley because its elk population experiences an abnormal amount of antler breakage. Here they would be able to tell if such Captain Ahabs, with their badge of de-manhood visible to the rest of the tule elk population, would fall as a consequence in the male hierarchy and lose their women.

The tule elk practice the same polygynous mating system as the elephant seals. Thus the baddest elk in the valley spreads his seed far and wide throughout the genetic pool. The writers of the paper cite other writers as to how antler size does influence rank and harem size, but they qualify it by adding that big antlers come with big bodies, and that it was their hypothesis that big bodies in the end determine the alpha male.

What their study indicated was that the bigger elk enjoyed a further advantage with their telltale sprawling rack, as it signaled to other males that they should think twice before messing with a cervid who could support such a mighty weight. Still it was their mass and genuine ability to fight prior to and during the rut that settled the male hierarchy. Afterward it made no difference whether their antlers were broken, sometimes to the point of an Ahab-like stub. They had already made their combative mark on the other males, and those other males remembered the beating enough not to test the theory that maybe the elk had lost its ability to fight along with its rack. Such big-bodied elk, though now a little light on top, maintained their harem as would a rich man who still scores with supermodels even though he has lost his hair. The “point” was that, yes, antlers conveyed power and status, but it was the size of the body that wielded that power and enjoyed the company of the opposite sex. (Johnson, Bleich, Krausman, Koprowski, 2006)

The Eyes Have It:

In 1998, Gerald Wilkinson, along with research partners, Presgraves and Crymes, submitted one of the worst written papers in a field full of bad writers. But thanks to a thesaurus and the synthesizing talents of Robert Ricklefs, we were able to tease out the gist of their field study.

The male stalk-eyed fly is an interesting looking fellow in that he looks at the world from eyes that are as far apart from each other as his head is from his tail. Picture a six-foot-tall human man with a six-foot-long pole attached transverse-wise to his face, and at the ends of that pole are eyeballs. Now reconfigure his body into that of a skinny arthropod, and there you have the male stalk-eyed fly.

Wilkinson and friends traveled to Malaysia to test whether females preferred males with longer eye spans, and if it had anything to do with the species trying to correct its unbalanced ratio of there being only 35% males in the population. This is a promiscuous species, meaning there are no lasting bonds, just sex and the eventual brood.  As such, a girl mates with whoever catches her eye — in this case, the guy with the faraway eyes. In the experiment, this led to runaway sexual selection whereby the male eyes of each succeeding generation got wider and wider until they were so far apart as to appear like satellites. But it turned out that this ocular freak show had a correlation to viable sperm, and so, after 22 generations, there was a one-to-one ratio of male and female.  The ever abstruse Wilkinson called it “genetic quality by meiotic drive suppression.” A more apt term may be “prettier males lead to more males.” (Wilkinson, Presgraves, Crymes, 1998)

If you’re Sinatra, then you’re dead:

Imagine this choice for a guy: You can be in possession of the secret to get all the girls you want, but that same secret will result in an early, horrible death. This is the situation with field crickets, T. oceanicus, on the island of Kauai. A threesome of California biologists researched this development for fifteen years.

The mechanics are as follows: The male cricket rubs together its wings to create a song that attracts females, who, in response, come running toward the crooner like hysterical teenage girls in 1965 chasing after the Beatles. But here is the rub: the song also acts as a beacon to a parasitoid fly that ends up killing the songster. The scientists found that, in time, many of these normal wing males began losing their majority in the population to be replaced by a mutant flat-winged cricket that lacked the powers of the troubadour. The scientists collected 133 male crickets, and only 9% had normal wings, such was the kiss of death of romantic song.

That begged the question: How was the species to persist if female crickets only arrived for sex when the normal winged males put out the call, and those male flies were dying for their music? What happened was that there were still a few crooners in the population, and so all the flat-winged mutants would loiter within one meter of him during his melodious mating call to await the arrival of the hot-to-trot ladies. The crooner could not mate with all the groupies, so the girls paired off with the closest thing to a healthy male T. oceanicus, albeit one with flat, soundless wings – yes, just like the girl trying to score with the lead singer in a local band but having to settle for a plumber named Ed. The writers did suggest that female crickets would need to become less picky about mates if they were to realize their purpose of existence, reproduction. (Zuk, Rotenberry, Tinghitella, 2006)

Reds or Tails?

In red-collared widowbirds, dimorphism occurs in two ways: the males sport longer tails and larger flaming red patches on their necks than the females, while the ladies assess those two qualities in prospective mates. That is one theory. Three biologists from Sweden conducted a study to determine if the size of the red patch had less to do with attracting females than in advertising male-on-male aggression and hence a sizeable territory, which, of course, still resulted in the same thing – pregnant widowbirds.

It was an elaborate experiment that entailed painting bird models three different colors, yellow, orange and red, with each color coming in three different patch sizes. There was further variation in that some were perched while others were “flown” on a wire, and of these airborne models, some had their tails folded and others keeled. The aim was to gauge the response of real male widowbirds when the fake birds sat near, or were flown over, their territory during pre-nesting, nesting and post-nesting time.

The results of the experiment pointed to these facts: Males were more aggressive during the nesting period. The perched models elicited little aviary hostility. The keeled-tail models were attacked more than the folded-tail ones – the conclusion being that the open tail operated as the proverbial middle finger to the feudal king bird. At that point, though, a fight would be avoided or escalated depending on the hue of the collar, with the yellow-necked models getting a beat-down and the bright red-necked models usually passing without contact. On the other hand, the real widowbirds with the largest red patches were more aggressive than those of smaller, paler areas of color, and also tended to reign over the largest territories.

The writers concluded that females are attracted to long tails; that the final choosing of a mate may come down to a big, flashy rear protrusion, but that for the male to even be considered a candidate for sexual union he must first battle it out with the competition and thereby rule a fiefdom. The question remains: What has the color red to do with ultimate fighting? The answer is simple: The color is derived from carotenoid pigments that can only be synthesized through diet, meaning that the male must be able to eat better than his fellow males through territorial dominance. A large red patch is visible evidence – to other males and females – that this man is strong and healthy enough to have secured a monopoly on the resources necessary to survive in this Darwinian realm, much like a human male driving a red Ferrari demonstrates to society that he has the wherewithal to afford such an expense. (Pryke, Lawes and Andersson, 2001)


There was a recent story on entitled “Why Beautiful Women Marry Less Attractive Men.” It quoted an MIT professor of behavior economics as saying that men are “sensitive to women’s attractiveness. Women seem to be sensitive to men’s height and salary.”

What, then, could be better proof of dimorphism through female sexual selection than how ladies usually want a man taller than themselves and taller than other men? It is no coincidence that the standard longtime phrase “tall, dark and handsome” begins with height. It is no coincidence that up until the last Presidential election the taller candidate always ended up in the Oval Office. The BBC ran a story two years ago that related a man’s height to his salary. Height in humans is no different than male size in elephant seals and tule elk, with the larger male gaining access to the prettiest girls. Height indicates health, dominance, of standing head and shoulder above the male competition and thereby gaining the attention of the discerning female.

Human ladies will also go for men who stand out in other ways, for example a guy with a strong piercing gaze – such was Rudolph Valentino’s power over women — not unlike the wide-eyed fly. Then there are the male singers, the equivalent of the song-making crickets, whether a one-man acoustic act who gets the prettiest girl with literary pretentions in the coffee shop, or a full-fledged rock star like Mick Jagger who once answered the question Why do you date only supermodels? by saying, “Because I can.” The ultimate effect of sexual selection is to push men to greater and greater efforts to stand out among the pack, to do ridiculous things like drive a bright red Hummer (the equivalent of the red patch on the red-collared widowbird), or jet past a group of bikini-clad girls on the beach in their long-keeled speed boat (similar to the keel of the same male bird).

But in the end it comes down to salary, which makes affordable the Hummer and the speed boat in the first place. A man’s salary is just another word for territorial dominance in the animal kingdom, of controlling large resources and forever defending their corporate place in their chosen industry against other males. The lady pinniped and tule elk opts to share the alpha male because he will insure the best resources for her to raise her brood to healthy adult status. The same kind of choice, albeit in monogamous terms, devolves to the human female when starting a family of her own. It requires money to bring up a kid, what with food, clothing and the cost of living in the “right” neighborhood (territory), to say nothing of college. These resources will come with the right man, the dominant man, the tall guy who towers over the rest to gain power and the right to call the shots within a given population, even if it means looking the other way from time to time when he invades a weaker man’s nest to enjoy the pleasures of another woman, thereby getting in touch with his polygamous evolutionary roots.

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