Monday, November 21, 2011

Sexual selection: Darwin musings

Being at a center named “Senda Darwin”, having just reread the Chiloe section of Voyage of the Beagle, and having tackled his biography – a 700 page tome – this summer, I have been thinking of the great man a bit. What most impressed me of his biography was not the theorizing he did; surely great, but also the part we learn so much about. What impressed me most was his experimentation. When he wanted to learn how plants settled distant islands, instead of immediately embracing completely ridiculous notions of land bridges which many contemporaries and later scientists did (remember that plate tectonics is very young, as theories go), he went about soaking seeds in saltwater for varying lengths of time to see if they would survive, which most did. Of course, as experimentation begets more questions, he found that even while perfectly viable, most of the seeds became waterlogged and sank – which makes drifting to islands pretty difficult.


But what he will be most remembered for, of course, is selection: both “natural” and “sexual”. After saying something along the lines of “the sight of a peacock’s tail makes me sick”, Darwin figured the puzzle out, which he put down as sexual selection in his book The Descent of Man, which I will not profess to have read the entirety of (unlike the Origin of Species and Voyage, Descent is not a short, easy read). Simply, he posited that the choosy sex (usually the female) chooses the opposite sex based on some character and thus that character can become quite ornate. Various hypotheses have been put forth to exactly why, one says that an ornate male – for instance, a peacock male that can survive with such a long tail – is a signal to the female that the male must have good genes.

Peacock, Santiago zoo
And when I was going back through some pictures, I realized I have quite a few pictures of sexual selection – or at least the products of it. 

Fork-tailed Flycatcher, Chascomus, Argentina

Male Fork-tailed Flycatchers display their absurdly long tails in this sort of acrobatic display (accompanied by noise - I'll not call it song) pretty much all day, or at least when any female comes by. 

Saffron Finch pair, Chascomus, Argentina
The Saffron Finches above show a fairly normal sexual dimorphism probably related to sexual selection that basically everyone is aware of: the male is the pretty yellow one on the right and the female is the (also pretty) brown one. Interestingly, in Peru, the same species is also common and both males and female show the bright yellow coloration. Explain that...

But since this is not just about birds, onto other taxa!

Cicada sp., Nantucket
Sexual selection acts not only on coloration and ornamentation, it also acts on behavior. Male cicadas, grasshoppers and katydids (among many, many other animals of many, many taxa) sing for mates, at an increased risk of predation because of this behavior. 

But onto my favorite example of sexual selection, bar none!

Strap-toothed beaked whale, male left, female right
This beast, Mesoplodon layardii, the strap-toothed beaked whale possesses what I believe to be the most ridiculous sexual ornamentation of any animal. Two teeth grow upwards from the lower jaw in males, eventually crossing the top and allowing almost no opening of the mouth, thus adult males limit the size of prey they can eat because of this ornamentation - which is thought to be important in fighting for mates. I talked to a grad student who had dissected at least one of these and she thought the musculature in the throat/lower jaw might be increased and thus allow it to suck food in with negative pressure. 

Let's not forget that we, too, are animals and are thus always trying to impress mates and will dress in ridiculous outfits and do ridiculous things to that end. But a tasteful portrait of costume and ritual:

Gaucho parade, Argentina, Nov. 2010

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