stop it, your bib is turning me on

Male house sparrow (gardening-for-wildlife.com)

We all have those certain “turn-ons” that get us going. For me, a solid set of shoulders and manly forearms catch my eye. And a little stubble never hurt anyone. And dark hair. Oh, and strong hands.  And a defined jaw. But I digress. Unlike human males, male house sparrows have black patches on their chin and upper chest, known as badges. It looks like they’re wearing little bibs. Some males have large ones, some have smaller ones (he he). Now, when one sex has a characteristic that the other doesn’t, one must always question if it plays a role in sexual attraction.

My taste for square shoulders and strong forearms is clearly linked to fitness. The latent cavewoman in me sees a potential mate who can build shelter and fight off threatening people or animals. Therefore, it is in my and my future offsprings’ best interest for me to be attracted to a male who is physically strong. But how is a black bib linked to fitness? Does it function in camouflage? Make the bird healthier in some way? What’s the deal?

A study by behavioral zoologist Anders Moller took a closer look at house sparrow badges. Come to find out, males with larger badges occupied prime real estate with more nesting sites. Territory defended by males with big badges had safer nests and fewer hatchling fatalities. So this showed that the bigger the badge, the more successful the offspring. But on top of that, Moller found that the big-badged males were… well, pimps. He pumped some females full of estradiol to get their lebidos up, and they were all over those big-badged males like a cheap suit. Poor little small-badged males were just sitting there, dejected and alone. Like me at my eighth grade dance.

So, back to the question. How does a badge function in fitness? Moller’s study reveals that the badge is a signal of fitness, but not a direct determinant. Which is really neat, I think. These types of characteristics- ones that signal but don’t directly function in fitness- show the nuances of evolution that go deeper than “survival of the fittest.” That black bib didn’t make ancient sparrows more likely to survive. Somewhere along the line, by chance, ancestral sparrow males with bigger badges happened to have more fit offspring frequently enough to make it significant. Then genetic drift began to waltz with selective pressure and chance, and before you know it, having a big black throat patch makes you an irresistible house sparrow.

Human sexual attraction seems to be so much more complicated than house sparrows. Do we have fitness signals? If so, what might they be? How do they compare to badges of the house sparrow?

Here's the Moller paper. Ya' know, for fun.

Badge size in the house sparrow Passer domesticus

BEHAVIORAL ECOLOGY AND SOCIOBIOLOGY

Volume 22, Number 5 (1988), 373-378