I decided to do some posts about parental investment when my pathological curiosity motivated me to investigate the reproductive lives of sticklebacks.
I’m start with peacocks because they’re the classic example. They troubled Darwin – what possible survival function could be served by that MASSIVE train? – and his solution, sexual selection, was never as widely regarded as natural selection. But oh, it should be. If natural selection is “survival of the fittest,” then sexual selection is “reproduction of the fittest.” And reproduction is more important than survival; reproduction is survival into next generation, which is what really matters, evolutionarily.
In Darwin’s terms, sexual selection consisted of male intrasexual competition and female intersexual choice: males beating or showing each other up, and female picking the ones they’re willing to mate with, based on that display.
Research now shows that in peacocks male competition and female choice results in more and fitter offspring, because males have evolved trains that accurately advertise their fitness, and females have evolved mechanisms to detect and prefer particularly fit trains.
Males compete with each other to win access to the females; and the females pick the males they want.
Let’s just take a minute here to appreciate Darwin. The VISION required to generate the idea of sexual selection… I mean, for a Victorian gentleman to suggest female choice of sex partner as a crucial element of reproduction… to SEE the dynamics underlying the behavior that, prima fascia, seems otherwise senseless and easily (trivially) explained by the theories like “God made birds and flowers beautiful for humans to enjoy”… I mean, holy jesus god. If it doesn’t make you tremble to realize of the POWER of his INSIGHT, well then… then you’re not me. It’s on par with Newton inventing calculus.
But this dynamic – male competition and female choice – is not how it works for many species; indeed, it’s not how it works for humans.
Parental investment is a big piece of the answer. The social functions of sex are another, but we’ll leave that for later.
Parental investment was first theorized by another holy-shit genius, Robert L. Trivers. Its first publication, “Parental investment and sexual selection” in Sexual selection and the descent of man, post-dated Darwin’s (The Descent of Man and Selection in Relation to Sex) by fully 100 years.
(I’m skipping a whole lot of history of science, and simplifying the science itself. See the remarkably readable The Ant and the Peacock for more.)
Parental investment, very briefly: time and energy and food and everything else a parent gives an offspring, they’re all limited resources; your goal as an organism is to maximize your resource-to-offspring ratio. You want to invest as little as possible while still successfully producing offspring that survive to reproductive age. Your ultimate goal is to be a grandparent or die trying. Literally.
In peafowl, sperm is the only investment the peacock makes in reproduction. The peahen gestates, lays, and incubates the egg, then raises the peachicks. So the female invests a great deal, you see, in parenting. Peacocks, in contrast, invest almost nothing – biological spare change, really. Parental investment. Dig? (I looked and looked for any suggestion that peacocks invest in parenting and found none; can anyone find any?)
What a peacock put his energy into instead is growing this massive metabolic hog of a train, which puts him at risk for predation and also reduces his immune functioning. It’s a GIANT cost. But he can’t afford not to invest in it because successfully persuading a female to allow him to fertilize her eggs is his one and only gateway into the next generation.
So. The more different the parental investment between the sexes, the more different the mating strategies between the sexes. Peacocks represent quite an thorough example of dimorphism in the service of parental investment and sexual selection.
The intrasexual competition/intersexual choice dichotomy is where we’re we’re starting in this occasional series of posts about parental investment and how it shapes sexual systems.
Wait til we get to the part where I say that sex is a parenting behavior in humans. People sometimes feel like that’s creepy but there is nothing more interesting to me!