Years ago, Danny Abrams heard about a strange phenomenon: Deer skeletons were being found beside trees in the forests of the Midwest. These male deer had apparently gotten their massive, unwieldy antlers caught in the branches, where they’d found themselves trapped. Unable to find food or flee predators, they quickly met their demise.
Abrams, a mathematics professor at Northwestern University’s McCormick School of Engineering, hadn’t thought much about evolutionary biology. But that tragic story got him thinking: What kind of model would predict an animal developing such unwieldy features?
“I was wondering: why do they bother with these things?” says Abrams. “I was just amazed that this is something that can happen.”
In fact, the mystery of how these detrimental traits evolve is something that baffled even evolutionary pioneer Charles Darwin. In his 1871 book The Descent of Man, and Selection in Relation to Sex, Darwin hypothesized vaguely that a female animal would seek out a mate that “pleases her most,” and thus “pomp” would be preferred in evolution to attract mates.
“It is not probable that she consciously deliberates; but she is most excited or attracted by the most beautiful, or melodious, or gallant males,” Darwin wrote. Unlike most of his other proposals, however, Darwin couldn’t find the evidence to back up this extraordinary theory.
Today, biologists have filled in some of the gaps. In 1975, biologist Amotz Zahavi theorized that animals continue to have these ornaments because individuals that can survive despite having to overcome physical handicaps (such as unnecessarily large antlers) must be very healthy and fit. Thus, these physical ornamentations are attractive to animals looking for the most ideal mate, and are passed on genetically when these “handicapped” males mate frequently.
“The idea is that advertising in the business realm is sort of similar to advertising in the biological realm,” says Sara Clifton, a graduate student at McCormick who studies mathematical models and works with Abrams. “You have to spend money to make money.”
“Only the fittest animals can afford to waste their resources,” Abrams adds.
Zahavi’s “handicap principle” is one of the most influential theories on animal communication today, but there is still little consensus about how mating preferences evolve, says Texas A&M University biologist Gil Rosenthal. Now, a mathematical model published by Clifton and Abrams in the journal Proceedings of the Royal Society B adds a new wrinkle to sexual selection theory, showing what happens to the animals left in the shadows of their more glamorous brethren.
After looking at 23 sets of data from 15 different species ranging from beetles to lions to crabs, Clifton and Abrams found that all of the animals tended to be “bimodally distributed” when it came to ornamentation. That is, they had either large, showy displays, or very minimal, subdued displays; there was no middle ground. It seemed that when it came to attracting mates, it paid to either go all out—or not try at all.
The distribution pattern they developed contrasts sharply with the familiar average bell curve that applies to many measurements in nature, Abrams said, from plant size to human intelligence. Yet if you go outside of biology, there’s an easy analogy to this kind of distribution: advertising.
“It’s good to be the name brand—the one that advertises [heavily],” Abrams said. “Otherwise it’s good to save your resources and be the generic.” So while some deer strive to be the Gucci of their species, others are perfectly fine with being Wal-Mart deer.
But don’t feel too sorry for the quiet ones in the corner just yet, says Abram. They aren’t completely out of luck: Though they may miss out on the best potential mates and have fewer offspring on average, these generics can still usually mate with females who aren’t able to mate with the ornamented males. Or, they can simply conserve resources and outlive their peers to become what Abrams calls “the only game in town.”
Abrams believes that his model could have real world applications for conservationists hoping to measure how healthy an animal population is, or for fish farmers hoping to encourage the best and largest fish to mate. “Understanding animal ornaments is one piece of the puzzle,” he says.
Yet Rosenthal, who was not involved in the study, points out that mating preferences are actually much more complicated than the study’s model portrays. These preferences can evolve for many reasons that have little or nothing to do with genetic benefits to a female animal’s offspring. “I think this is a really cool study,” Rosenthal added, “but they’re a bit mired in 1970s thinking about sexual selection.”
So don’t go growing your antlers out just yet.