Nobody Puts Birdie In A Corner

Study Shows Birds Sensitive to Testosterone Can Do More Complex Mating Dances
May 14, 2015 

The natural world has more in common with bad cinema than many of us would like to admit. The scores of films featuring mindless shuffling zombies resemble the parasitic cordiceps fungus, which takes over the brains of ants and bees, forcing them to spread the parasite.

Top-predator movies such as Jaws, Jurassic Park and Alien sequels — I can’t in good conscience call the originals “bad cinema” — and anything involving alligators in sewers are all retellings of the invasive species archetype.

Zebra FinchNow, thanks to a new study from Wake Forest University, we have an analog for teen dance films from the 70s and 80s in the mating displays of tropical birds. Much like the hormone-fueled leading men of Dirty Dancing, Grease, and Footloose, the birds that are more sensitive to testosterone are better dancers.

Mating displays are common in the animal world. A male will perform some feat in front of a female in order to get the female to choose him as a mate. The feat can take many forms. Peacocks fan out their massive tail feathers, male lions fight each other, frogs try to croak louder than other frogs and some species of birds even build their prospective mates a house.

No matter what the male is doing, the goal is the same — to communicate to the female that he is a worthy manly mate that will make strong children. Species actually evolve this way, as females select mates with specific traits. Instead of natural selection (survival of the fittest), scientists call this sexual selection (survival of the sexiest). And in the world of many birds, the best dancers are the sexiest.

Biologist Alex Fuxjager watched the mating dances of six species of tropical birds, searching for indications of fine motor control and coordination, such as cartwheels and somersaults. Some species performed markedly more complex routines than others.

Previous studies have shown that androgens — testosterone and other “male” hormones — control the amount and types of muscles in the body through androgen receptors (ARs). These ARs can be found on many types of muscle cells, and when they bind to androgens they stimulate muscle growth and a specific ratio of fast twitch to slow twitch muscles.

Fuxjager hypothesized that species with more complex mating dances had more ARs in their muscles than species with simpler routines. The idea is that when mating season comes, the androgens start flying around the boy birds’ bloodstreams while they sing a jaunty chorus of “Summer Nights” and mambo on a fallen log.

ManakinThe ARs in the boy birds’ muscles pick up on that surge and remake the wing muscles to put on the best show they can at the final dance of the season. Then when they see Birdie in a corner, they yell “LET’S DANCE!” and start their mating display. The birds with more ARs should have been able to more finely tune their muscles for dancing and finally do that lift they had been practicing all summer.

After testing muscle tissue from the birds’ wings, Fuxjager determined that indeed, the better dancers had more ARs and thus were more sensitive to testosterone. Further, since within species the females choose the males who dance best, sexual selection favors the birds that are more sensitive to testosterone, which may help to explain why “The Brain” is the only guy without a girlfriend at the end of The Breakfast Club, but that might be a movie best left for another natural phenomenon.

This study reveals a concrete physiological trait tied to the sexual selection of mating dances. Fuxjager says he will expand this work by investigating how testosterone sensitivity plays into the sexual selection of other animals that choose mates based on different behaviors, such as fighting or singing. With any luck we’ll then be able to place Dirty Dancing, Amadeus and all the Twilight movies into one biological category.

The paper describing Fuxjager’s research was published in the journal Functional Ecology. 

— Daniel Lane

Daniel Lane covers science, engineering, medicine and the environment in North Carolina.


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