Mauka to Makai
A science blog for the massesArchive for September, 2009
Sperm Wars
It may be a dog-eat-dog world* out there, but nowhere is the competition fiercer than in the female reproductive tract.
Biologically speaking, the goal of every male is to produce as many offspring as possible. To do this, males need to have some kick-ass sperm, but according to a recent study, too much kick-ass sperm can cause problems.
Human males, thanks to thousands of years of evolution, now over-produce crazy-fast, majorly-aggressive sperm known as “super-sperm.” One super-sperm reaches the egg first. If another sperm binds to the egg after the winning sperm has lodged itself in the egg, the egg will die. Of course, there’s a defense system to prevent this from happening.
As soon as the winning sperm binds to the egg, a biochemical barrier begins to form around the egg. The barrier is complete in just a few minutes, but (BAM!) another super-sperm enters the egg before the barrier is sealed and the fertilized egg dies.
Over those thousands of years of evolution, women’s bodies have evolved as well. To prevent one overly aggressive second place super-sperm from ruining a perfectly good zygote, the female reproductive tract does everything possible to keep sperm out. To survive the sperm will have to become even super-er.
Human sperm are not the only sperm with a mission. Meadow voles, earthworms, damselflies, field crickets and red junglefowl depend on stellar sperm to win the sperm war. You see, in many species, a fertile female will mate with multiple males. All of the males are shooting for the ultimate prize (paternity), but only one—or in some cases, a few—will win.
Different species have different sperm competition strategies. Some, like chimpanzees and rhesus macaques, produce super-speedy sperm. Others vary the amount and quality of sperm they ejaculate based on the situation. If a meadow vole smells other males nearby, he will contribute more sperm than he would in a more private setting. Field crickets and earthworms give significantly more viable sperm to “experienced” partners than to virgins. And red junglefowl produce higher velocity sperm when they mate with attractive females than when they mate with unattractive females.
Some males—like male black-winged damselflies—play it safe no matter what the girl looks like. During mating, the male black-winged damselfly pumps his scrub brush-like penis up and down to remove 90 to 100 percent of the sperm from the female’s spermatheca (her sperm storage tank). Once he’s done cleaning, he deposits his own sperm.
Alas, males and their speedy my-sperm-is-better-than-your-sperm sperm don’t always get to control who scores paternity. A recent study found that female crickets control how much sperm they store from each of their mates. By storing more sperm from appealing males and less from unappealing (related) males, the female determines a male’s chances of fathering her offspring—no matter how “super” his sperm might be.
*Siblings kill each other, lovers eat their mates and parents eat their offspring, scoring a mate isn’t nearly as easy as it is for humans and making a baby is a downright dangerous proposition.
Hasson, O., & Stone, L. (2009). Male infertility, female fertility and extrapair copulations Biological Reviews, 84 (2), 225-244 DOI: 10.1111/j.1469-185X.2008.00068.x
BRETMAN, A., NEWCOMBE, D., & TREGENZA, T. (2009). Promiscuous females avoid inbreeding by controlling sperm storage Molecular Ecology, 18 (16), 3340-3345 DOI: 10.1111/j.1365-294X.2009.04301.x
Cool Critter: Sea Cucumber
The ocean is full of fabulously-named critters like slippery dicks and donkey dungs. Of course, fabulous names don’t always match up with fabulously cool critters. Slippery dicks are pretty boring. Donkey dungs are not.
The donkey dung is an aptly-named species of sea cucumber. Sea cucumbers aren’t vegetables.* They’re animals, echinoderms actually (like sea stars and sea urchins) and they breathe out of their butts.
A sea cucumber pulls water in through its anus, removes oxygen from the water using the respiratory trees that branch off its cloaca—which is a hole, kind of—and then expel the water. With all the water flowing in and out of a sea cucumber’s anus, the cloaca (here’s the Wikipedia page) becomes a nutrient-rich hideaway for critters looking for a little extra protection. Pearl fish, some crabs and a few polychaete worms hide in the sea cucumber’s butt until they are big enough to defend themselves in the big bad ocean.
Sea cucumbers don’t hide in anyone’s butt. They have much more creative mechanisms of defense…
All sea cucumbers can change form. A sea cucumber’s body wall is made of a special kind of collagen called “catch collagen” which allows them to stretch and shrink as much as they want to without damaging any tissue. When touched, a sea cucumber may turn itself into a flaccid gooey blob or a tight turd-like glob.
Some species startle their attackers by shooting some of their internal organs out through their anus. Presumably, the sea cucumber’s attacker is so shocked (or disgusted) by this behavior that they abandon any desire to eat such a wretched creature. The sea cucumber, of course, is fine. Its internal organs quickly regenerate and it goes back to doing the things that sea cucumbers do.
Other sea cucumber species attack their attackers using spaghetti-like strings called cuvierian tubules. One end of the tubules is attached to the sea cucumber’s respiratory tree and, when the sea cucumber is chillin’, the other end floats freely in the sea cuke’s body cavity. When the sea cucumber is threatened, the free-floating ends tear a hole through the cloaca and shoot out through the anus. The tubules become sticky when they come in contact with anything so, when they hit the predator, they stick to it, entangle it and eventually immobilize it. The sea cucumber drops the tubules, disconnecting them from its respiratory tree, and goes on its merry way. The tubules grow back in a few weeks. (Some species that discharge cuvierian tubules also discharge a soap-like chemical called holothurin that kills any animal nearby.)
Sea cukes live on—and eat—the ocean floor. Sure, a species that eats dirt and depends on its anus to breathe and protect itself may not sound like a big deal, but it is. Just as earthworms do on land, sea cucumbers** mix the sediment and recycle detritus by eating dead stuff and pooping out the waste, creating fodder for bacteria and enriching the substrate. In other words, these butt-breathers are crucial to the marine ecosystem.
*For the record, a regular cucumber is a fruit, but because of its flavor, it’s recognized as a vegetable in culinary circles.
**Also for the record, sea cucumbers do not eat with their butts. They use their tube feet to put particles in their mouths, which are at the opposite ends of their bodies from their anuses.
