Evolutionary biologist Bjørn Østman of Pleiotropy tackles one of the oldest articles of faith in sexual selection. (Emphasis his.)
Oh.. dear, I’m becoming one of those annoying obsessed types – with the peacock tail. For the I don’t know what time, someone is stating as fact that the peacock’s tail evolved via sexual selection. Here’s one Denis Dutton on TED:
The peacocks magnificent tail is the most famous example of this [sexual selection]. It did not evolve for natural survival. In fact, it goes against natural survival. No, the peacock’s tail results from the mating choices made by peahens.
I’ve said it somewhere before, but I’ll just go on and say it as many times as the editor [a.k.a. Østman] of this blog allows me: THE PEACOCK’S TAILSCARESAWAY PREDATORS! (I’m told all-caps gets the message across).
Now, I admit immediately that this is not a well tested hypothesis. Here are the two pieces of evidence that I have: I have myself seen a peacock raise its tail in San Diego Zoo when it was approached by… children. They were going too close, and as a result it raised its tail feathers (yeah, I know, correlation/causation – maybe the peacock was horny at the sight of humans its own size).
Source: Pleiotropy
He mentions that he’s seen peacocks behave the same way towards dogs and cats. He pulls videos from YouTube demonstrating the effect. One including an impossibly adorable (cinematically Disneyfied) video short of a pug vs. peacock encounter. If you don’t like cutesy animal videos that clip is still significant because even though the peacock is not in mating plumage it still exhibits the same tail-fanning behavior and it still scares the pug.
There’s also the business about the “eyes” in peacock plumes — many, many animals from fish to insects to birds have eye mimicry on their bodies, and based on effects on predators they appear to work at least on the margin.
Østman is very clear that peacock tails also serve to attract mates. I couldn’t agree more — as he says, “It is unlikely the the peacocks fan their tails at peahens in order to scare them away.” And no, really, truly, there’s good evidence that peahens prefer mates with big fans, and that those who get mates with bigger fans have more and healthier offspring
It’s also likely that “runaway” sexual selection is responsible for the elaborateness of current displays. After all, the video with the pug that the effect is still pretty, well, effective even when the birds are not in full plumage. That implies that while big fans might be more effective, small ones can be pretty effective too. (Against pugs anyway.)
But the (entirely testable) hypothesis that the display also seems to benefit survival against predators suggests it’s better to call it a metaphor for fitness “signaling” than the pure signaling-only play advanced by sociobiology and evolutionary psychology fetishists.
Which came first? I’m guessing plain old-fashioned evolutionary biologists and/or animal behaviorists can untangle that. Do related but largely plume-less species (e.g. guineafowl) have similar defensive displays of tail feathers? Do peahens flash their tail feathers at predators? Speaking of evolutionary psychology, do peacocks appear to exercise flash-or-flee discernment in the face of different kinds or sizes of predators? Are somewhat larger and stronger peacocks really at a flight disadvantage compared to peahens of similar health and age? The data to support both hypotheses is available on YouTube. If I was an enterprising young biologist, or even more likely an enterprising philosophy of science major, I’d try and get at least one paper out of the phenomenon.