Nature's Nether Regions Read online

  Praise for Menno Schilthuizen’s Nature’s Nether Regions

  “A provocative voyage on the ‘vast ocean of sexual function beyond the quiet backwater that we humans find ourselves in.’ ”

  —Kirkus Reviews

  “A remarkable book that explores the evolutionary origins of sex organs succeeds in finding exactly the right tone. . . . Schilthuizen’s entertaining work reminds us not to take ‘the mechanics of sexual intercourse’ for granted.”

  —Publishers Weekly

  “Rather than furiously flipping through a stack of increasingly obscure science journals, those interested now have an easily digestible text to work with, the charmingly titled Nature’s Nether Regions . . . interspersed with a selection of the finest, and most scientifically accurate, sex jokes.”

  —VICE Motherboard

  “Evolutionary biology, it turns out, is a lot kinkier than you might have imagined when you first learned about Darwin’s theories. . . . Nature’s Nether Regions [takes] a closer look between the legs (or, in the case of the Australian velvet worm, on the head) to explore what the sex lives of various creatures can teach us about reproduction, diversity, and human sexuality.”

  —Salon

  “A sweeping look across the animal kingdom . . . [that] illustrate[s] an amazing diversity of genitalia that has been largely unappreciated, even among scientists.”

  —LiveScience

  “What distinguishes this parade of marvels from other ‘wow, is nature weird’ volumes is that Schilthuizen uses the creatures to offer an appealing introduction to the big themes in current research on how sexual parts, practices, and ornaments have become so elaborate and diverse across the domains of life.”

  —Science News

  “Schilthuizen whizzes between geographies and species, learning from apes, slugs, spiders. In the process he invites us on all kinds of interesting adventures, from finding ancient beetle genitals trapped in amber to examining barnacles depositing their sperm with eight-foot-long appendages. . . . This is a book about heterosexual sex between animals, but Schilthuizen hasn’t closed the case on other kinds of sex driving animal evolution, too. Ever excited, ever open-minded, he pushes towards new frontiers.”

  —Tess Taylor, Barnes & Noble Review

  ABOUT THE AUTHOR

  Menno Schilthuizen is a Dutch evolutionary biologist, ecologist, and permanent research scientist at Naturalis Biodiversity Center in Leiden and a professor of character evolution and biodiversity at Leiden University, in the Netherlands. From 2000 to 2006 he worked at the Institute for Tropical Biology and Conservation in Sabah, Malaysian-Borneo, where he studied land snail evolution in tropical forests, caves, and limestone habitats. He is the author of Frogs, Flies, and Dandelions: The Making of Species and The Loom of Life: Unraveling Ecosystems. When he is not extracting beetle genitals, he enjoys painting and hiking in the dunes near his hometown, Leiden, the Netherlands.

  ALSO BY MENNO SCHILTHUIZEN

  Frogs, Flies, and Dandelions: The Making of Species

  The Loom of Life: Unravelling Ecosystems

  PENGUIN BOOKS

  An imprint of Penguin Random House LLC

  375 Hudson Street

  New York, New York 10014

  penguin.com

  First published in the United States of America by Viking Penguin, a member of Penguin Group (USA) LLC, 2014

  Published in Penguin Books 2015

  Copyright © 2014 by Menno Schilthuizen Penguin supports copyright. Copyright fuels creativity, encourages diverse voices, promotes free speech, and creates a vibrant culture. Thank you for buying an authorized edition of this book and for complying with copyright laws by not reproducing, scanning, or distributing any part of it in any form without permission. You are supporting writers and allowing Penguin to continue to publish books for every reader.

  Illustrations by Jaap Vermeulen. Copyright © 2014 by Menno Schilthuizen.

  THE LIBRARY OF CONGRESS HAS CATALOGED THE HARDCOVER EDITION AS FOLLOWS:

  Schilthuizen, Menno.

  Nature’s nether regions : what the sex lives of bugs, birds, and beasts tell us about evolution, biodiversity, and ourselves / Menno Schilthuizen.

  pages cm

  eISBN 978-1-101-60810-4

  1. Generative organs—Evolution. 2. Sexual behavior in animals—Evolution. I. Title.

  QL876.S35 2014

  573.6—dc23

  2013047833

  Cover design: John-Patrick Thomas

  Cover lettering: Matt Vee and John-Patrick Thomas

  Version_2

  Contents

  Praise for Menno Schilthuizen’s Nature’s Nether Regions

  About the Author

  Also by Menno Schilthuizen

  Title Page

  Copyright

  Preliminaries

  Chapter 1 Define Your Terms!

  Chapter 2 Darwin’s Peep Show

  Chapter 3 An Internal Courtship Device

  Chapter 4 Fifty Ways to Peeve Your Lover

  Chapter 5 A Fickle Sculptor

  Chapter 6 Bateman Returns

  Chapter 7 Future Suitors

  Chapter 8 Sexual Ambivalence

  Afterplay

  Acknowledgments

  Notes

  Bibliography

  Index

  . . . yes and his heart was going like mad and yes I said yes I will Yes.

  —James Joyce, Ulysses

  Preliminaries

  Not too long ago, the Netherlands Natural History Museum was housed in a lofty, cavernous building in the historical center of Leiden. Generations of biology students took their zoology classes there, in its two-tier lecture theater over the monumental staircase. During the less captivating parts on crustacean leg structure or mollusk shell dentition, their gaze would have wandered off to the two features that made this lecture room unforgettable. First, its abundant display of antlers of deer, antelope, and other hoofed animals, hundreds of them, suspended from the walls. Second, the huge painting from 1606 of a beached sperm whale that hung over the lectern. On an otherwise nondescript Dutch beach lies the Leviathan, its beak agape, its limp tongue touching the sand. A smattering of well-dressed seventeenth-century Dutchmen stand around the beast. Prominently located, and closest to the dead whale, stand a gentleman and his lady. With a lewd smile, face turned toward his companion, the gentleman points at the two-meter-long penis of the whale that sticks out obscenely from the corpse. Centuries of smoke-tanned varnish cannot conceal the look of bewilderment in her eyes.

  These few square feet of canvas, strategically placed in the painting’s golden ratio, exemplify two things. First, the unassailable fact (supported by millennia of bathroom graffiti, centuries of suggestive postcards, and decades of Internet images) that humans find genitals endlessly fascinating. Their own, but by extension those of other creatures, too. The amazing diversity in shape, size, and function of the reproductive organs of animals has been an eternal source of wonder, making bestsellers of the 1953 book The Sex Life of Wild Animals, the 1980s classroom wall poster Penises of the Animal Kingdom (over twenty thousand copies sold), and the Sundance Channel series Green Porno—short films starring a sanguine Isabella Rossellini enacting the copulation of various animals.

  The second point that may be underscored with this seventeenth-century sperm whale penis is the curious observation that the public fascination with genitalia was, until very recently at least, not matched by equally intensive scientific inquiry. The lofty offices down the corridor from this le
cture theater housed scores of biologists quietly cataloging the world’s biodiversity. In good classificatory tradition, they would painstakingly draw, measure, photograph, and describe the minutiae of the genitals and distinguishing features of the reproductive organs of any new insect, spider, or millipede they would discover—and yet never stop to wonder how these private parts evolved.

  We really have Darwin to blame for this. In his next-greatest book, The Descent of Man, and Selection in Relation to Sex (1871), Darwin explains how secondary sexual characteristics—like colorful bird plumage, the prongs on beetles’ heads, and the antlers of deer—have been shaped not by natural selection (adaptation to the environment) but by sexual selection: adaptation to the preferences of the other sex. He denies the primary sexual characteristics entry to his theory by categorically stating that sexual selection is not concerned with the genitalia or primary sexual organs—which, after all, are merely functional, not fanciful. So the diversity of all those antlers on the walls of the museum lecture room had been a tradition of evolutionary biology since Darwin, but investigating the evolution of the business end of things—of which the centerpiece of that seventeenth-century painting is just one prominent example—hadn’t.

  It took until 1979 for evolutionary biology to start paying attention to genitalia. In that year, Jonathan Waage, an entomologist from Brown University, published a short paper in Science on the damselfly penis. He demonstrated that this minuscule penis carried a miniature spoon that, during mating, cleaned out the female’s vagina, scooping out any remaining sperm from previous males. It was an eye-opener as well as a sperm-scooper. For the first time, here was proof that animal genitals are not just mundane sperm-depositing and sperm-receiving organs, but are sites where a sexual selection of sorts goes on. After all, during damselfly evolution, males with the best sperm-scoopers had left more descendants.

  The time was ripe for this paper. When I interviewed Waage about those early days, he recalled how, in the years leading up to his sperm-scooper discovery, he had been influenced by the quiet revolution that biology faculties worldwide were undergoing at the time—a sea change brought about by George C. Williams’s book Adaptation and Natural Selection and by Richard Dawkins’s popularization of it, The Selfish Gene. People began to do away with the false notion that evolution works “for the good of the species” (an outdated concept, echoes of which can be heard even today in nature documentaries). Instead, they began viewing evolution correctly, as the effect of a kind of reproductive selfishness, in which it is all about the success of an individual in carrying its genes into the next generation. Evolution does not “care” about the species. And if a sperm-scooper would scupper the chances of competing males, then that is what evolution would favor. Waage was one of the first scientists to start asking the questions that mattered for how evolution works. And since evolution is all about reproduction, no wonder Waage and other modern biologists would sooner or later find themselves closely inspecting genitals.

  In that same revolutionary era, other young biologists began asking similar questions. One of them was a certain undergraduate biology student who in the 1960s was earning some extra cash with menial tasks in the depot of Harvard University’s Museum of Comparative Zoology. His job was to top up the alcohol in jars with pickled animals and to organize unsorted spider specimens. Picking up spider identification guides, the student began wondering why spider species are so often distinguished by the way their genitals are formed. Asking around in the museum, he was told that that is just the way it is. The genitals of different species of animals, be they spiders, spittlebugs, or Spanish fly, are often widely different, even if the species are one another’s close relatives and look identical on the outside. Probably, his seniors told him, the genetic differences also accidentally affect the shape of the genitals. Very useful if you want to identify spiders, but probably quite meaningless biologically. The student, unconvinced but not in a position to argue, shelved the question in the back of his mind, graduated, and went on to become a productive and successful tropical biologist at the Smithsonian Institution’s Tropical Research Institute in Panama.

  That student’s name was Bill Eberhard. And when, many years later, the issue of Science with Waage’s damselfly penis article landed on his desk, that old conundrum from his undergraduate days let out a little muffled cry from beneath many layers of mental clutter. Perhaps genitalia, in spiders as well as other animals, differ so much because each is a different kind of sperm-scooper? As it happened, Eberhard was about to begin a six-month stint as a visiting scientist at the University of Michigan, which gave him the opportunity to spend some weeks in the library.

  There, he pulled off one of those rare feats of biological unification. It is often not realized that the basic source of inspiration in biology, namely the endless diversity of life, is also one of its greatest handicaps. Biologists, much more than, say, chemists or mathematicians, tend to be divided by invisible barriers. Those barriers are held in place by expertise with a particular kind of organism. More often than not, biologists identify themselves as entomologists if they work with insects, or as botanists if plants are their thing. Or even as copepodologists, coleopterologists, or cecidomyiidologists (if their creed be copepods, beetles, or cecidomyiid gnats, respectively). And each organism-based field has its own congresses, professional societies, and journals, further affirming separatism. Contrary to, for example, physicists, to whom a neutron is a neutron is a neutron, biologists are always unsure whether what applies to one kind of organism also applies to another—or, worse, they don’t care about broad applicability at all. As ecologist Stephen Hubbell lamented, if Galileo had been a biologist, he would have spent his whole life documenting the trajectories of different animals thrown off the Leaning Tower of Pisa without ever coming up with gravitational acceleration.

  Biology really moves ahead when somebody dares to cut across all those different subfields and look for general patterns. And that is precisely what Eberhard did when he cloistered himself in the University of Michigan library and began pulling books off the shelves on the genitalia of mice and moles, snails and snakes, weevils and whales. Four years later, in 1985, what had started as a little hobby project had turned into the 256-page Harvard University Press classic Sexual Selection and Animal Genitalia. In it, besides dazzling his reader with a sheer endless parade of wondrously shaped animal willies, Eberhard made two points. First, that genitals are bafflingly complex systems, far too complicated for the relatively simple task of depositing and receiving a droplet of sex cells. The male chicken flea, for example, has a “penis” that is actually a profusion of plates, combs, springs, and levers and looks more like an exploded grandfather clock than a syringe—whereas the latter should suffice if the organ’s role were just to squirt sperm into the female. And the second point he made was that no body part in the animal kingdom evolves as fast as genitalia.

  In his book, Eberhard argued that the reproductive organs of animals are under constant, intensive, and multitarget sexual selection—including the kind revealed by Waage, but certainly not limited to that. This is why they are so complex. This is also why they differ so much from species to species—a phenomenon that taxonomists (that special breed of biologist whose task it is to circumscribe, describe, name, and classify biodiversity) had been happily using throughout the twentieth century as an easy way to differentiate species. Animals’ nether regions are the stages where an evolutionary play is performed that would have made even Darwin blush. An evolutionary play that had been totally ignored by generations of biologists—even though genitalia are probably the best body parts to illustrate the power of evolution.

  And yet the evidence had been, quite literally, staring us in the face. Humans and our fellow primates do not shy from Eberhard’s accelerated genital evolution. Forget forebrains, canine teeth, and opposable big toes: the largest anatomical differences between us and our closest relative, the chimpanzee, are
found in our nethers. The human vagina is flanked by two pairs of skin folds, the labia minora and the labia majora. The clitoris is a two-winged structure lying along the walls of the vagina, and only the relatively small glans is visible externally, covered by the clitoral foreskin and lying at the point where the labia minora join. The chimpanzee vagina, on the other hand, lacks labia minora, has a larger and downward-pointing glans of the clitoris, and contains specialized tissue that makes the labia and the foreskin of the clitoris swell dramatically during the fertile phase of the menstrual cycle, causing the vagina to bulge out and increasing its operating depth by 50 percent. And on the other side of the sexual divide the differences between these two sister species are no less striking. The human penis is thick and blunt ended, boneless, has a ridge around the smooth glans, and has a foreskin. It has two corpora cavernosa, the sponge-like tissue that swells during erection. The chimp penis, by contrast, is thin and sharply pointed, carries a penis bone (baculum) inside, has no glans, no foreskin, and only one corpus cavernosum. Oh—and it carries lots of tiny tough spines along the sides.

  In other words, the exaggerated diversity—biodiversity—in genital shapes that Eberhard had highlighted carries right up to our own species. The evidence for this pattern throughout the animal world is available in large quantities of respectable nineteenth- and twentieth-century tomes on comparative anatomy and systematic zoology, and yet before Eberhard nobody had bothered to explain it.

  But this is not a book about Bill Eberhard. Rather, it is about the band of disciples that followed in his footsteps. Hundreds of scientists all over the world, myself included, have been inspired by Eberhard’s book. Together, with our lab experiments, fieldwork, and computer simulations on a wide variety of organisms from primates to pack rats and from sea slugs to sexton beetles, we have nursed to life a brand-new discipline of evolutionary biology: a science of the genitals, if you will. And, as disciples and disciplines are wont, we have increasingly come to dispute the exact workings of genital evolution. Are penises internal courtship devices, as Eberhard would have it? Or are they used to combat rival males on the female’s turf, as Waage showed? Or are male and female genitalia perhaps at loggerheads over who is in charge of fertilization, as people like the English zoologist Tracey Chapman think?