Human sperm-making machinery is a bit lazy. How else to explain the fact that 90 percent of the sperm in a man's ejaculate are deformed? Two heads, two tails, huge heads, pinheads, coiled tails - the list of common deformities is a long one.
It's the price of monogamy, Dr. Niederberger says. "For those species where more than one male's sperm can find itself in a female at the same time, the sperm are much more uniform in appearance," he says. "In humans, Joe and Sam's sperm don't usually find themselves in Betty at the same time."