Zoobiquity by Barbara Natterson-Horowitz

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Authors: Barbara Natterson-Horowitz
body. Barnacle penises, as long as they are, vary in their girth. Barnacles living in rougher waters sport thicker, stronger, and sturdier members. But those in calmer surroundings extend their longer filamentous penises in search of distant barnacle “vaginas.”
    Fleas and some worms also have hugely proportioned penises. And some animals have more than one.Several species of marine flatworms have dozens of penises.Some snakes and lizards are doubly endowed; switching between their two hemipenes during multiple copulations increases their sperm count by a factor of five.As for insects, so exuberantly inventive are their male genitalia that entomologists scrutinize them to classify entire species.
    If you haven’t thought much about the procreative thrustings of other animals, especially those you can’t see, you’re not alone. Many animals are nocturnal, extremely small, shy, or just very careful to mate where other animals (including curious biologists) can’t see them. Inaccessibility to these covert proceedings has been a barrier to the comparative study of sexuality. a But the challenges of achieving up-close analyses ofthese animals in flagrante delicto have meant gaps in knowledge and frank misinformation.
    The sexcapades of krill, for example, have been seriously underestimated. These tiny shrimplike creatures make up the bulk of the diets of important aquatic megafauna, including whales. It had long been assumed that krill reproduce by mixing their eggs and sperm near the surface of the water. In 2011, however, the journal
Plankton Research
reported the surprising discovery that Atlantic krill—all 500 million
of them—mate at depth. In these deep, dark, underwater orgies, krill use internal fertilization techniques that involve penetrative sex.
    Since arising more than 200 million years ago, all male mammals have had penises, each achieving erection in one of three ways.An actual penis bone, called a baculum, offers a stiffening assist to many male bats, rodents, carnivores, and most nonhuman primates.A rope of thick tissue running down the center of the shaft partially stiffens the fibro-elastic penises of pigs, cattle, and whales. (The popular chew toy sold in pet stores called a bully stick is made by drying out this bull penis structure.)
    But humans, along with armadillos and horses (not to mention several nonmammals like turtles, snakes, lizards, and some birds), have what’s called an inflatable penis. These organs thicken and harden using only hydraulics and internal compartments of spongy tissue that fill up with blood or other body fluids.
    From a biomechanical perspective, these inflatable kinds of penises are really quite extraordinary.As Diane A. Kelly, a biologist and penis expert at the University of Massachusetts, Amherst, explained to me, creating a structure adequately stiff for penetration that is also strong enough to withstand intravaginal thrusting is a tricky mechanical challenge. The steps that go into building a hard penis have an elegant flow that would please any professor of engineering.
    It starts with the deceptively inert, flaccid penis. A penis in repose,although it seems floppy and relaxed, is actually in a state of constant, moderate contraction. The tube of smooth muscle that runs down its center is mildly tensed. So are the linings of the thousands of tiny blood vessels that crisscross the organ. Further contraction of this muscle and the arteries is what accounts for shrinkage in cold weather or water. So although a penis in the process of erecting can seem like it’s springing into action, it’s actually submitting to a crucial, and opposite, process. First it must relax.
    The command to relax comes from the pudendal nerves. When the smooth muscle lets go, arteries deep in the penis dilate. The channels suddenly open up. Blood rushes in, straightening the vessels and filling millions of tiny pockets in the two tubes of spongy tissue (called corpus cavernosum) that

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