The same argument might apply to kiwis. As a poultryman might choose small hens for minimal decrease in egg size with maximal decline in body weight, natural selection for smaller adults might markedly decrease the average body weight within a species with very little accompanying reduction in egg weight.
I believe that this general argument, applied to kiwis, may be defended on three strong grounds. First, as stated above, a general finding in allometric studies teaches us that within-species slopes for adults of one species are usually much lower than among-species slopes along mouse-to-elephant curves. Thus, any evolution of decreasing size along the within-species curve should produce a dwarfed descendant with more of the particular item being measured than an average nondwarfed species at the same body weight. Second, we have actual data, for domestic poultry at least, indicating that the within-species curve does have a substantially lower slope than the hummingbird-to-moa curve for our crucial measure of egg weight.
Proposed allometric explanation for the large egg of the kiwi. The kiwi probably evolved from a much larger bird by backing down the very shallow within-species slope (upper line). Most birds arrange themselves on the standard hummingbird-to-moa curve with its steeper slope (lower line). Therefore, a kiwi has a much heavier egg than predicted for a bird of its body size. BEN GAMIT. ADAPTED FROM JOE LEMONNIER. COURTESY OF NATURAL HISTORY.
Third, I have studied many cases of dwarfism, and I believe we can state as a general phenomenon—rooted in the first point above—that decline in body size often far outstrips decrease in many particular features. Dwarfs, in several respects, always seem to have much more of certain body parts than related nondwarfed species of the same body size. For example, I once studied tooth size in three species of dwarfed hippos (two fossil and the modern Liberian pygmy)—and found their molar teeth substantially larger, for each of three separate evolutionary events, than expected values for related hoofed mammals at their body size (American Zoologist, 1975).
In another example, the talapoin, a dwarfed relative of the rhesus monkey, has the largest relative brain weight among monkeys. Since within-species brain curves have substantially lower slopes than the two-thirds value for the marmoset-to-baboon curve, evolution to smaller size by backing down the within-species curve would yield a dwarf with a far larger brain than an ordinary monkey at the same body size.
Put all this together and a resolution fairly jumps at you for kiwis. Their enormous eggs require no special explanation if kiwis have evolved by marked decrease in size. Kiwi eggs exhibit the weight expected for backing down the within-species curve if natural selection operates only to decrease body size and no other factor intervenes to favor an active reduction in egg size—as we might anticipate in New Zealand, this easy land of no natural predators, where a female might waddle without fear as an enormous egg distends her abdomen during passage down the oviduct.
In this interpretation, if you ask me why kiwi eggs are so large, I reply, “Because kiwis are dwarfed descendants of larger birds, and just followed ordinary principles of scaling in their evolution.” This answer differs sharply from the conventional form of evolutionary explanation: “Because these big eggs are good for something now, and natural selection favored them.”
My answer will also strike many people as deeply unsatisfactory. It provides a reason rooted in history, pure and simple (with a bit of scaling theory thrown in)—kiwis are as they are because their ancestors were as they were. Don’t we want answers that invoke general laws of nature rather than particular contingencies of history?
I would reply that my resolution is quite satisfactory, that evolutionary arguments are often properly resolved by such historical statements, and that we would do well to understand this important and neglected principle of reasoning—for we might save ourselves many a stumble in trying to apply preferred, but inappropriate, styles of explanation to situations encountered again and again in our daily lives.
To cite just one example where I learned, to my deep chagrin, that a peculiarity of history, rather than a harmonious generality, resolved an old personal puzzle: I had been troubled for a long time by something I didn’t understand in the inscription on the Liberty Bell—not losing any sleep to be sure, but troubled nonetheless, for little things count. This national symbol bears, like most bells, an appropriate quotation: “Proclaim liberty throughout all the land unto all the inhabitants thereof (Lev. 25:10). But the bell also says, “Pass and Stow.” I assumed that this line must also be a quotation, fit to the purpose of the bell (as selection fits the features of organisms to their needs)—part of the general harmony and chosen plan. I pondered these cryptic words quite a bit because I didn’t recognize the source. I consulted Bartlett’s and found nothing. I constructed various possibilities: This too will pass, as we stow courage for the coming conflict; oh ye who pass by, remember, they prosper that stow and do not waste; pass the grass and stow the dough. Finally I asked the attendant on duty in Philadelphia. Of course, I should have figured it out, but I was too busy trying to make intrinsic sense of the inscription. The bell was cast by Messrs. John Pass and John Stow. Pass and Stow is a statement about the particular history of the bell; nothing more.
My odd juxtapositions sometimes cause consternation; some readers might view this particular comparison as outright sacrilege. Some may claim that the only conceivable similarity between kiwi eggs and the Liberty Bell is that both are cracked, but I reply that they stand united in owing their peculiarity and meaning to pathways of history.
The Liberty Bell on display in Philadelphia, advertising its makers, Mr. Pass and Mr. Stow. THE BETTMANN ARCHIVE.
8 | Male Nipples and Clitoral Ripples*
THE MARQUIS DE CONDORCET, enthusiast of the French Revolution but not radical enough for the Jacobins—and therefore forced into hiding from a government that had decreed, and would eventually precipitate, his death—wrote in 1793 that “the perfectibility of man is really boundless…. It has no other limit than the duration of the globe where nature has set us.” As Dickens so aptly remarked, “It was the best of times, it was the worst of times.”
The very next year, as Condorcet lay dying in prison, a famous voice from across the channel published another paean to progress in a world that many judged on the brink of ruin. This treatise, called Zoonomia, or the Laws of Organic Life, was written by Erasmus Darwin, grandfather of Charles.
Zoonomia is primarily a dissertation on the mechanisms of human physiology. Yet, in the anachronistic tradition that judges biological works by their attitude to the great watershed of evolution, established by grandson Charles in 1859, Zoonomia owes its modern reputation to a few fleeting passages that look upon organic transmutation with favor.
The evolutionary passages of Zoonomia occur in Item 8, Part 4, of Section 39, entitled, “Of Generation,” Erasmus Darwin’s thoughts on reproduction and embryology. He viewed embryology as a tale of continuous progress to greater size and complexity. Since his evolutionary speculations are strictly analogous to his concept of embryology, organic transformation also follows a single pathway to more and better:
Would it be too bold to imagine that in the great length of time, since the earth began to exist…all warm-blooded animals have arisen from one living filament…possessing the faculty of continuing to improve by its own inherent activity, and of delivering down those improvements by generation to its posterity, world without end?
As the last sentence states, Erasmus Darwin’s proposed mechanism of evolution lay in the inheritance of useful characters acquired by organisms during their lifetimes. This false theory of heredity has passed through later history under the label of Lamarckism, but the citation by Erasmus (a contemporary of Lamarck) illustrates the extent of this misnomer. Inheritance of acquired characters was the standard folk wisdom of the time, used by Lamarck to be sure, but by no means original or distinctive with him. For Erasmus, this mechanism of evolution required a concept of pervasive utility. New structures arose only whe
n needed and by direct organic striving for an evident purpose. Erasmus discusses adaptations in three great categories: reproduction, protection and defense, and food. Of the last, he writes:
All…seem to have been gradually produced during many generations by the perpetual endeavor of the creatures to supply the want of food, and to have been delivered to their posterity with constant improvement of them for the purposes required.
In this long section, Erasmus considers only one potential exception to the principle of pervasive utility: “the breasts and teats of all male quadrupeds, to which no use can be now assigned.” He also suggests two exits from this potential dilemma: first, that male nipples are vestiges of a previous utility if, as Plato had suggested, “mankind with all other animals were originally hermaphrodites during the infancy of the world, and were in process of time separated into male and female” and second, that some males may lactate and therefore help to feed their babies (in the absence of any direct evidence, Erasmus cites the milky-colored feeding fluids, produced in the crops of both male and female pigeons, as a possible analogue).
The tenacity of anomalies through centuries of changing beliefs can be truly astounding. As a consequence of writing these essays for so many years, I receive hundreds of letters from readers puzzled about one or another apparent oddity of nature. With so large a sample, I have obtained a pretty good feel for the issues and particulars of evolution that pose conundrums for well-informed nonscientific readers. I have been fascinated (and, I confess, surprised) over the years to discover that no single item has evoked more puzzlement than the very issue that Erasmus Darwin chose as a primary challenge to his concept of pervasive utility—male nipples. I have received more than a dozen requests to explain how evolution could possibly produce such a useless structure.
Consider my latest example from a troubled librarian. “I have a question that no one can answer for me, and I don’t know where or how to look up the answer. Why do men have nipples?…This question nags at me whenever I see a man’s bare chest!”
I was fascinated to note that her two suggestions paralleled exactly the explanations floated by Erasmus Darwin. First, she reports, she asked a doctor. “He told me that men in primitive societies used to nurse babies.” Finding this incredible, she tried Darwin’s first proposal for nipples as a vestige of previous utility: “Can you tell me—was there once only one sex?”
If you are committed—as Erasmus was, and as a distressingly common version of “pop,” or “cardboard,” Darwinism still is—to a principle of pervasive utility for all parts of all creatures, then male nipples do raise an insoluble dilemma, hence (I assume) my voluminous correspondence. But as with so many persistent puzzles, the resolution does not lie in more research within an established framework but rather in identifying the framework itself as a flawed view of life.
Suppose we begin from a different point of view, focusing on rules of growth and development. The external differences between male and female develop gradually from an early embryo so generalized that its sex cannot be easily determined. The clitoris and penis are one and the same organ, identical in early form, but later enlarged in male fetuses through the action of testosterone. Similarly, the labia majora of women and the scrotal sacs of men are the same structure, indistinguishable in young embryos, but later enlarged, folded over, and fused along the midline in male fetuses.
I do not doubt that the large size and sensitivity of the female breast should count as an adaptation in mammals, but the smaller male version needs no adaptive explanation at all. Males and females are not separate entities, shaped independently by natural selection. Both sexes are variants upon a single ground plan, elaborated in later embryology. Male mammals have nipples because females need them—and the embryonic pathway to their development builds precursors in all mammalian fetuses, enlarging the breasts later in females but leaving them small (and without evident function) in males.
In a similar case that illuminates the general principle, the panda develops a highly functional false “thumb” from the radial sesamoid bone of its wrist. Interestingly, the corresponding bone of the foot, the tibial sesamoid, is also enlarged in the same manner (but not nearly so much), although increase of the tibial sesamoid has no apparent function.
As D. Dwight Davis argued in his great monograph on the giant panda (1964), evolution works on growth fields. Radial and tibial sesamoids are homologous structures, probably affected in concert by the same genetic factors. If natural selection operates for an enlarged radial sesamoid, a bigger tibial sesamoid will probably “come along for the ride.” Davis drew a profound message from this case: Organisms are integral and constrained structures, “pushing back” against the force of selection to channel changes along permitted paths; complex animals are not a dissociable collection of independent, optimal parts. Davis wrote that “the effect seen in the sympathetic enlargement of the tibial sesamoid…strongly suggests that a very simple mechanism, perhaps involving a single factor, lies behind the hypertrophy of the radial sesamoid.”
In my view of life, akin to Davis’s concept of constraint and integration, male nipples are an expectation based on pathways of sexual differentiation in mammalian embryology.
At this point, readers might demur with the most crushing of all rejoinders: “Who cares?” Why worry about little items that ride piggyback on primary adaptations? Let’s concentrate on the important thing—the adaptive value of the female breast—and leave aside the insignificant male ornament that arises as its consequence. Adaptations are preeminent; their side effects are nooks and crannies of organic design, meaningless bits and pieces. This argument is, I think, the standard position of strict Darwinian adaptationists.
I could defend the importance of structural nonadaptation with a long and abstruse general argument (I have done so in several technical papers). Let me proceed instead by the most compelling route I know by presenting a second example based on human sexuality, a case entirely comparable in concept with the origin of male nipples but differing in importance for human culture—a case, moreover, where the bias of utility has brought needless pain and anxiety into the lives of millions (where, indeed, one might argue that Freudian traditions have provided a manifestly false but potent weapon, however unintentional, for the subjugation of women). Consider the anatomical site of orgasm in human females.
As women have known since the dawn of our time, the primary site for stimulation to orgasm centers upon the clitoris. The revolution unleashed by the Kinsey report of 1953 has, by now, made this information available to men who, for whatever reason, had not figured it out for themselves by the more obvious routes of experience and sensitivity.
The data are unambiguous. Consider only the three most widely read of extensive surveys—the Kinsey report of 1953, Masters and Johnson’s book of 1966, and The Hite Report of 1976. In his study of genital anatomy, Kinsey reports that the female clitoris is as richly supplied with sensory nerves as the male penis—and therefore as capable of excitation. The walls of the vagina, on the other hand, “are devoid of end organs of touch and are quite insensitive when they are gently stroked or lightly pressed. For most individuals the insensitivity extends to every part of the vagina.”
The data on masturbation are particularly convincing. Kinsey reports from his sample of 8,000 women that 84 percent of individuals who have ever masturbated depend “primarily on labial and/or clitoral techniques.” The Hite Report on 3,000 individuals found that 79 percent of women who masturbate do so by directly stimulating the clitoris and surrounding vulva, while only 1.5 percent use vaginal entry.
The data on intercourse affirm this pattern. Shere Hite reports a frequency of orgasm with intercourse at 30 percent and often attained only with simultaneous stimulation of the clitoris by hand. She concludes: “not to have orgasm from intercourse is the experience of the majority of women.” Masters and Johnson only included women who experienced orgasm with intercourse in their study. But they concluded th
at all orgasms are identical in physiology and clitoral in origin. These findings led Hite to comment that human copulation “sounds more like a Rube Goldberg scheme than a reliable way to orgasm…. Intercourse was never meant to stimulate women to orgasm.” As Kinsey had said earlier with his characteristic economy and candor: “The techniques of masturbation and of petting are more specifically calculated to effect orgasm than the techniques of coitus itself.”
This conclusion should be utterly unsurprising—once we grasp the proper role and limitation of adaptationist argument in evolutionary biology. I don’t believe in the mystery style of writing essays: build up suspense but save the resolution until the end—for then readers miss the significance of details along the way for want of proper context. The reason for a clitoral site of orgasm is simple—and exactly comparable with the nonpuzzle of male nipples. The clitoris is the homologue of the penis—it is the same organ, endowed with the same anatomical organization and capacity of response.
Anatomy, physiology, and observed responses all agree. Why then do we identify an issue at all? Why, in particular, does the existence of clitoral orgasm seem so problematic? Why, for example, did Freud label clitoral orgasm as infantile and define feminine maturity as the shifting to an unattainable vaginal site?
Part of the reason, of course, must reside in simple male vanity. We (and I mean those of my sex, not the vague editorial pronoun) simply cannot abide the idea—though it flows from obvious biology—that a woman’s sexual pleasure might not arise most reliably as a direct result of our own coital efforts. But the issue extends further. Clitoral orgasm is a paradox not only for the traditions of Darwinian biology but also for the bias of utility that underlies all functionally based theories of evolution (including Lamarck’s and Darwin’s) and, in addition, the much older tradition of natural theology that saw God’s handiwork in the exquisite fit of organic form to function.
Bully for Brontosaurus Page 12
