Size does matter — to female mosquito fish, anyway

Some would argue bigger is better, but in some cases, being too well-endowed can kill you.

Take the lake-dwelling creature known as the mosquito fish. His penis can extend to 70 percent of the length of his body, thereby interfering with his need to swim fast and escape deadly predators. How could such a thing evolve in a world ruled by survival of the fittest?

Such mysteries explain why there's a whole field of science devoted to study of the penis. "It's an important field," says Brian Langerhans, a biologist at Washington University in St. Louis who specializes in the male organ. "For some reason, male genitalia exhibit striking diversity in size and form."

Naturalists sometimes have to look at the penis to tell one species from another.

Scientists are just starting to figure out what evolutionary forces give rise to this assortment of penises, including the human one, which is apparently bigger than necessary. Form, in many cases, doesn't follow function.

In the case of the mosquito fish, the male's generous equipment makes him irresistible to females, says Langerhans. The downside is that the organ creates drag in the water, making the most well-endowed mosquito fish easy pickings for predatory fish.

Most fish don't have a penis — their idea of sex is to release sperm into the water. But a few fish do have intercourse and bear live young. The males, therefore, need an organ for "sperm transfer," says Langerhans. Technically, it's called the gonopodium, he says, and it evolved separately from the penises of mammals, but it's more or less the same thing.

Whatever you call it, there's no obvious reason it needs to be almost as long as the fish. A male can use only a small portion in mating. The situation would seem to contradict Darwin's survival of the fittest.

The solution comes back to Darwin, who proposed another evolutionary force called sexual selection, in which the sexiest, not necessarily the fittest, may pass on the bulk of the genes to future generations.

Sexual selection explains the beautiful tail of the peacock, which weighs down the males and can attract predators. As long as females keep choosing to mate only with the prettiest-tailed males, the tails can get longer and prettier than is practical. For peacocks and a number of other creatures, evolution is a trade-off between survival and looking good.

A similar trade-off seems to happen with mosquito fish, which only grow the most extreme male endowments in relatively safe waters. Where more predators lurked, Langerhans found the males had shorter organs and hence could swim faster.

Until now, biologists argued penis length was determined by its ability to implant sperm in the female. In species where females mate with multiple males, the guy with the longest organ might have a better chance of fertilizing an egg.

But in the extreme case of the mosquito fish, Langerhans suspects it has more to do with what the females prefer. To test his hypothesis, he put female fish in a tank and projected pictures of male mosquito fish on opposite sides. The pictures showed the same male, but in one Langerhans digitally extended the length of the penis by about 15 percent.

The females checked out both and then swam straight for the longer one every time, he says. He published his findings in the Proceedings of the National Academy of Sciences.

Could similar forces have shaped the male organs of other animals, such as human beings? According to physiology professor Jared Diamond in his book "Why Is Sex Fun?" the average penis for a human male is 5 inches, far more than a guy needs to procreate when our closest primate relatives get by with about 1 ½ inches.

"The 1 ½-inch penis of the male orangutan permits it to perform in a variety of positions that rival ours," Diamond writes, "and to outperform us by executing all those positions while hanging from a tree."

Faye Flam's Carnal Knowledge column appears Wednesdays in The Seattle Times.