Sex, Love, and Banana Slugs

I love the second week of February. Darwin Day and Valentine’s Day? If you’re anything like me, you've had a serious case of sex on the brain. More specifically, the biological costs and benefits of sex. Sex seems like the most natural thing in the world; how else are we supposed to pass on our genes? But it turns out sex is only one reproductive option. Guys, count your lucky stars, because most species have very few males, and some have none at all.

Why are there so few males in, say, the insect world? Why are their roles minimalized in so many organisms? Well guys, I hate to say it, but you’re a cost of sex. The point of reproducing is to pass on genes. Daughters will not only have your genetic material, but will raise children with your genetic material. A son has your genes, but the only contribution he makes is to give sperm to a girl, and then he’s done. From an evolutionary standpoint, all that is needed from the male is testicles. He doesn’t need a body to create new babies, so why would he need one? Creation of and raising a male is a biological hardship. “But wait!” most of the people reading this post will cry, “We have benefits! We wouldn’t have evolved if we were just a cost!” Very true! Males add genetic recombination into the mix. If females just cloned themselves all the time, everyone would be exactly the same, barring random mutations. And if everyone is the same, we as a species would have a hard time adapting to change.

I matter, dammit! 

So everyone knows how humans reproduce (I hope), but how do other animals do it? Other mammals, birds, reptiles—we’re pretty familiar with those, and they’re all similar to us in that they all have sex with independent males and females. But when we go smaller, we find some seriously weird sex rituals.

Fully asexual species clone themselves. Some species lay eggs, some “bud,” which is when the offspring grows out of the side of the adult (Coral is a good example of this). Just take a minute to imagine how weird that would be if humans grew babies off of their shoulders. So anyway, asexual creatures don’t have to deal with producing males, and except for random genetic mutations, they don’t change.

The next step from fully asexual to fully sexual would be the hermaphrodites. A quick side note: humans cannot be true hermaphrodites because we can’t fertilize ourselves and have our own baby. True hermaphrodites can, but it is better for them to have sex (genetic recombination again!). We’ll use slugs as a good example of hermaphrodites. They seem to have it all in terms of benefits: they get to have genetic recombination, but can still carry offspring themselves. Why aren’t we all hermaphrodites? For one, we’d all have to maintain two reproductive systems. Second, accidental self-fertilization brings us back to the problems associated with cloning. Lastly, and most horrifying, hermaphrodites fight over who gets to wear the pants. Because being a male takes so much less energy, when slugs mate they try to impregnate the other slug without themselves becoming impregnated. In banana slugs, they will go so far as to try and chew off the other slug’s penis, to force them to become the female. Yes, you can find video links of this. No, I don’t want to have to look at one to post it.

Okay, fine. Here's a picture. You can use your imagination from here.

Many animals have females and males, but have basically reduced the males to a pair of testes (remember when I said all that was really needed from guys was the testicles? Well, these species have taken that to an extreme). Take the anglerfish. The male is much smaller than the female and is born with what in humans would be a very good sense of smell. He needs it, because if he doesn’t find a female almost immediately, he’ll die. If he does find a female, he latches on and dissolves the skin of his mouth and her side, fusing them together. Think of it as a less gross version of the “Human Centipede." Females can have several males attached this way. Other species have variations on this theme. Female Black Widow spiders eat their mates and male bees live for a week then die by forcible castration after mating with a queen. Finally, male mites only live long enough to impregnate their sisters inside their mother

So guys, as we come to Valentine’s Day, get her something nice and feel lucky that you’re not about to get eaten, attached to her by fusion, and that you even exist at all.