Not surprisingly, when physicians study penises, we tend to focus on the human variety. But our world is abristle with phalluses and has been for at least half a billion years. Today and every day since at least the early Paleozoic era, in meadows, oceans, streams and the air, many trillions of erections preceded trillions of copulations, which preceded trillions of ejaculations. Some erections sprouted readily and penetrated easily. Others flickered to life and abruptly terminated. Some were measurable in yards. Others were microscopic. Some were stiffened by blood; others by a similar fluid called hemolymph; others by skeletal supports made of cartilage or bone. Some erections culminated in mere seconds; others lasted hours.

It wasn’t always this way. The earliest single-celled organisms on Earth simply cloned themselves. Some of their descendants still do. But as complex multicelled organisms evolved and eventually “discovered” the ability to mix their gametes, they gained a giant genetic advantage. Since these ancient creatures lived in the sea, the earliest sex was a straightforward process of spraying sperm and eggs into the water. The lucky few connected.

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In that massive free-for-all, the fittest sperm reached the eggs and were rewarded with the prize of bringing their DNA into natural selection’s next round. Sometimes the fittest sperm were the strongest swimmers. Sometimes they were the ones deposited nearest the eggs. Others developed ways to follow molecular scent pathways to find the eggs. Or they bundled together in teams to improve their timing and accuracy. As sperm perfected ingenious rudders, tails, chemical markers and swimming strategies, the genital hardware that ejected them was evolving, too.

One innovation was internal fertilization, which allowed males to place their sperm not only near females but right inside them, next to the eggs. This allowed both males and females a measure of control over their offspring’s DNA. Females could audition males before allowing them to mate. Sperm was less likely to be spilled on barren ground. And one invention that effectively accomplished this combination of choice and precision was the penis.

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The oldest penis on record goes back 425 million years. It belonged to a crustacean found preserved under ancient volcanic ash at the bottom of a sea that used to cover Herefordshire, in England. The paleontologists who found the shrimp-like creature named it Colymbosathon ecplecticos, from the Greek for “astounding swimmer with a large penis.” Before it was found, the oldest known phallus was 400 million years old. It belonged to a fossilized daddy longlegs from Scotland.

When dinosaurs roamed the ur-continent of Pangaea about 200 million years later, their penises roamed with them. Paleontologists have speculated about dinosaurs’ mating apparatuses and behaviors, using what they know of crocodilians and birds, today’s relatives of those prehistoric creatures. The erect penis of a male titanosaur, for example, may have been 12 feet in length. Experts speculate that the male sauropod, with a body the length of a school bus, approached the massive, receptive female from behind. Like his crocodilian and avian descendants, he likely inserted his penis from this dorsal position and, at climax, ejaculated sperm through a vessel running along the outside of his organ.

Nowadays, Earth’s penises exist in multivaried splendor. Spiny anteaters sport four-headed varieties that rotate between copulations. Although most birds don’t have penises, the phalluses of Argentine lake ducks are nearly eight inches long (almost as long as an ostrich’s), corkscrew-shaped, and festooned with dense, brushy spines that sharpen to hard spikes at the base. Despite a 33-inch member and a penis-to-body ratio of seven to one, Limax redii, a Swiss slug, doesn’t have the most impressive proportions in nature. That title goes to Balanus glandula, which wows the tide pool with its prodigious barnacle penis. Permanently cemented to a tidal rock, the barnacle sports a penis 40 times the size of its 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.”

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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. But the challenges of achieving up-close analyses of these animals in flagrante delicto have meant gaps in knowledge and frank misinformation.

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The sexcapades of krill, for example, have been seriously underestimated. These tiny shrimp-like 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 tons 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 fibroelastic 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.

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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 run the length of the penis shaft.

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Next comes a key chemical reaction. When arteries dilate anywhere in your body — whether in your cheeks when you blush, your gut when you eat, or your genitals when aroused — they release nitric oxide. In the penis, this very special molecule (not to be confused with nitrous oxide, your dentist’s laughing gas, or nitrogen dioxide, the air pollutant) signals the smooth muscles to relax even further. More blood rushes in. By this point, the penis is crowded with liquid, and the increased volume compresses nearby veins, blocking their blood from flowing back out again. The chamber becomes tenser and tenser with trapped liquid, assisted by other structures that tighten and constrict. Pressure soars inside the fleshy tube. Most erections reach an internal pressure of one hundred millimeters of mercury — comparable to that which a boa constrictor might use to suffocate its prey.

To protect the organ from rupturing under this intense force, a complex net of collagen fibers surrounds the outside of the penis, under the skin. As Kelly describes, the collagen strands are arranged in deeply folded, alternating perpendicular layers along the length of the penis. This allows them to pleat open efficiently when the erection is under way. Not only does this collagen “skeleton” strengthen the erection, it gives the structure a resistance to bending that engineers call “flexural stiffness.” (Kelly says it’s a trick shared by pufferfish, whose expandable skin also contains highly crimped, alternating strands of collagen.) When the penis is not being used for copulating or mating displays, the erective construction has the added benefit of folding away for neat storage. Being able to stow your penis provides more than simple convenience. A study on certain fish that cannot retract their reproductive organs — because they’re modified, permanently stiffened anal fins — showed that males with longer ones suffered higher rates of predation than those with organs that were less obviously on display.

When the erection is complete, and stimulation has reached what doctors poetically if vaguely describe as the “point of no return,” a spinal cord reflex causes a sudden burst of muscle contractions throughout the genital area, starting with the neck of the bladder. In rippling chains of contractions fueled by massive outflow from the sympathetic nervous system, the muscles around the testes and scrotum tense, followed by those of the epididymis, vas deferens, seminal vesicles, prostate gland, urethra, penis and anal sphincter. The rapid clamping and unclamping of these muscles, at intervals less than a second apart, spurts semen out of the urethra. A few slower spasms may follow that initial explosion of muscle activity. This sequence has been preserved across a wide spectrum of mammalian species.

The comparative study of ejaculation has focused mostly on primates and rodents. But all male mammals descend from shared ancestral ejaculators. The penises of mammals from narwhals to marmosets to kangaroos propel semen in nearly identical ways. And the ejaculation of a male human today even shares basic physiology with reptiles, amphibians, and sharks and rays. Ejaculation isn’t new. In fact, the human seminal propulsion system has ancient origins. This makes it not only intriguing but plausible that the human male’s experience of ejaculation may be shared by other animals. With the mechanics being so similar, the question is, do other animals experience the intense pleasure that drives so many men to such good and bad behavior?

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The experience of orgasm is not only legendary but also measurable. Electroencephalograms show brain-wave shifts, including an increase in slow-frequency theta waves, which are associated with deep relaxation. Many men describe a feeling of euphoria intriguingly similar to what heroin users describe experiencing when they plunge a needle into a blood vessel and discharge the drug into their system. The brains of ejaculating male rats are known to release powerful chemicals, including heroin-related opioids, oxytocin and vasopressin. Taken together, the muscle contractions, brain changes, chemical rewards and relaxed feelings add up to create the male orgasm.

After ejaculation and orgasm, a process called detumescence, or deflating, begins. Neurohormonally, this sequence is essentially simply the reverse of erection. The smooth muscle of the penis shaft contracts. So do the penile arteries. Blood flow to the penis decreases. With less pressure pushing them shut, the veins open up and normal drainage resumes. Chemicals associated with the sympathetic nervous system begin to take over. And before you know it, the penis is back in its resting state of slight contraction.

Clearly, a lot has to happen for this amazing structure to build on cue. But with that many dependent steps, a lot can go wrong. To complicate matters, human erections can be achieved in essentially two ways: Through thought or through touch.

As most men can attest, the penis is perfectly capable of achieving an erection purely from direct stimulation. It’s called a reflexogenic erection and is regulated by nerves in the lower spine. Reflexogenic erections are well known to prepubescent boys, men in deep states of REM sleep, and men with spinal cord injuries (in whom the nerves connecting brain to penis have been severed). Reflexogenic erections are as unconsciously controlled as digesting and breathing; they can spring up when a man least suspects or wants them.

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Reflexogenic, early-model proto-erections, in species such as barnacles and mollusks, evolved long before reptilian or mammalian penile stiffening. While effective at penetration and sperm delivery, these erection 1.0s lacked what more evolved erections offer: opportunistic engorgement and strategic deflation.

An important advancement in the evolution of erection was the addition of input from the brain. This allowed the brain to send signals to the penis through the spinal cord. From an evolutionary perspective, these psychogenic or “cerebrally elicited” erections are a savvy improvement on the reflexogenic type. Involving the brain in a process as intricate and crucial as an erection expands the animal’s reproductive opportunities and physical safety. It allows him to judge and respond to his environment before firing up or shutting down an erection. It enables sensory inputs like seeing, smelling, touching, or even thinking (fantasizing) about someone or something sexy to trigger the erection cascade. And it facilitates nearly instantaneous shutdown when a predator — or, more likely, a competitor — reveals himself.

And this is true whether the male is a moose, a mole or a man.

Excerpted from "ZOOBIQUITY: What Animals Can Teach Us About Health and the Science of Healing" by Barbara Natterson-Horowitz M.D., and Kathryn Bowers." Published this week by Alfred A. Knopf. Excerpted by permission of Knopf, a division of Random House, Inc. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.