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If you’re a fan of Rudyard Kipling’s Just So Stories, you’ll recall “How The Elephant Got His Trunk,” “How The Leopard Got His Spots,” and the like. You probably also remember from basic biology the regnant evolutionary account of “How The Giraffe Got His Long Neck.” It lent itself to contrasting Lamarckian selection—in which early giraffes stretched their necks to reach higher leaves, thereby bequeathing their long neckedness to subsequent generations—with Darwinian natural selection, in which evolution favored those individual giraffes whose genetic background endowed them with longer necks, thus selecting for this trait. Although Lamarck remains in disfavor, research recently published in the journal Science strongly suggests that a different and far sexier variant of natural selection—appropriately termed sexual selection—has been operating.1
Time to revise the textbooks.
But not completely. It remains true that giraffes with longer necks get to munch on leaves that are beyond the reach of other, shorter-necked competitors. Also true is that the highest leaves tend to be more nutritious. Food matters: Without enough of it, living things—however well adapted in other ways—wouldn’t survive to project their genes into the future. Looking at giraffes (and who doesn’t like to do that?), it’s all too easy to see them as merely munching machines on stilts, although their extraordinary anatomy necessitates a number of evolutionarily expensive adjustments, such as providing sufficient blood flow to that skyscraper head, and preventing too much flow when they lower their heads to drink, leading to the question, if you’re a giraffe, do you really need to reach up quite so high, easily 6 feet above all other grazers? Are those long necks merely extended chopsticks?
Seemingly cuddly giraffes have an evolutionary history of being rather nasty to each other.
Some years ago, a rogue band of giraffologists stuck their own necks out and began proposing a kind of biological heresy: that giraffes got their long necks not merely to better fill their bellies, but all the better to bash each other with, my dear. Think of a medieval mace, with a hard, heavy object at the end of a chain or—in the case of giraffes—a long, flexible neck, made more impactful in proportion as that neck is elongated and capable of swinging the business end with great force. We think of our own necks, when we think of them at all, as something that connects head to shoulders and that enables us to turn and tilt that head. Necks are to be protected rather than wielded. But we’re not giraffes.
In Science, the researchers reported that an ancestral giraffe from the early Miocene (about 17 million years ago), recently discovered in northern China, had “a thick-boned cranium with a large disklike headgear, a series of cervical vertebrae with extremely thickened centra, and the most complicated head-neck joints in mammals known to date.” It turns out that those seemingly cuddly giraffes have an evolutionary history of being rather nasty to each other, especially the males. Nor are they likely to have been unique, at least in the deep past.
Sauropod dinosaurs such as Diplodocus and Brachiosaurus also had notoriously long necks, although their anatomy doesn’t suggest weaponization … with one notable exception. Apatosaurus had unusually robust neck architecture, with armored internal structure and trapezoidal cervical vertebrae that particularly lend themselves to vigorous downward chopping motions. Apatosaurologists debate whether their subjects’ heads were typically held high or mostly horizontal. However, the heads of Apatosaurs were, if anything, unusually small, although their necks were exceptionally thick. Could they have used their necks as weapons?
For a century after Darwin published his second best-known book, The Descent of Man, and Selection in Relation to Sex, the sexual selection part was largely overlooked in favor of its ecological counterpart (plain old natural selection), although both operate by the same mechanism: differential reproduction among genetic variants. The problem was that sexual selection is itself divisible into two components, female choice and male-male competition, and although both are now recognized as consequential, female choice of whom to mate with was considered unlikely, because it was widely misunderstood to require a conscious aesthetic sense on the part of those animals doing the choosing.
We currently understand that no artistic sensibility per se is required, only that females who make a more adaptive choice leave more reproducing offspring, regardless of what’s going on inside their heads. One consequence is that such female choice generates selection for males with characteristics that turn females on, which feeds back on female choice itself. And what turns them on? Research has shown females in most species preferentially mate with the dominant male, increasing the probability that their male offspring will have similar traits and accordingly, some likelihood of being successful both in male-male competition and (if the option exists) being chosen by the next generation of females.
There are situations in which females aren’t able to choose. Cow elk are not just defended by a dominant bull, but because they are zealously and jealously gathered together by the dominant bull, they are nearly always constrained to mate only with him. Similarly, Northern elephant seal cows are vigorously kept in land-based harems by much larger and highly aggressive bulls. Occasionally however, especially when the dominant male seal is busy defending his subjects against would-be encroachers, another male will sneak in and begin mating with a female, who responds unenthusiastically, using loud vocalizations (not otherwise employed) that attract the dominant male, who drives away the interloper.
Although male-male competition has long been far more obvious and less controversial than female choice, it may have been too much so, hiding its subtlety and diversity in plain sight, rather like Edgar Allen Poe’s Purloined Letter. Ever since Robert Trivers’ breakthrough 1972 book chapter on “Sexual Selection and Parental Investment,” evolutionary biologists have understood what should have been obvious all along: Males, as sperm-makers, necessarily invest less in any given reproductive endeavor than do females, who not only produce the much larger and more metabolically expensive eggs but, in mammals, invest yet more via pregnancy and lactation.
As a result, one male can inseminate many females, which in turn leads to considerable competition among males to be the ones who do the inseminating. Hence, male-male competition and a generally higher variance in male than female reproductive success. Herein lies a tale (and in giraffes, a neck).
There is little evolutionary payoff to being a Ferdinand among bulls.
The most obvious consequence of male-male competition in mammals is greater body size and muscle mass, even among those make-love-not-war bonobos, among which a single male will dominate a single female; this doesn’t happen often, however, because female bonobos cleverly team up.
Male-male competition is revealed in many ways, including a strong cross-species correlation between sexual dimorphism (male-female difference) with regard to body size and the degree of polygyny—the average number of females who are sexually monopolized by one male. The poster child at one extreme is the southern elephant seal, in which males outweigh females by a factor of 8 to 10; males have been recorded to reach 3,700 kilograms compared to females, who are typically between 400 and 800 kgs. Not surprisingly, this is a species in which males fight each other for mastery of a harem that can be upward of 30 females; hence, selection for male bulk.
By contrast, among the mostly monogamous monk seals, males and females are comparable in size. A similar pattern obtains in the deer family. Elk are highly dimorphic and harems are large, compared to monogamous roe deer which are essentially monomorphic as well. This pattern is found in essentially all mammals, including nonhuman primates: Male gorillas, for example, maintain harems and silverback males are considerably larger than females, whereas the mostly monogamous gibbons are mostly the same size.
For bull elephant seals or elk, the situation is close to winner-take-all. A successful male will breed with all the females in his harem, consigning the large number of unsuccessful males to resentful and nonreproductive bachelorhood. For the females, by contrast, everyone is to some degree a winner—although no one wins big time. An incidental result is that since they are largely freed from the tribulations of same-sex competition, females often get a curious benefit: They are more likely than males to be at the ecological optimum when it comes to body size. Males, on the other hand, since they are constrained by the rigors of male-male competition, are more likely to be too big for their own good, except for the payoff they receive via male-male competition.
As anthropologist/primatologist Sarah Hrdy points out:
The virtues of large size are not limitless. Even though most bulls don’t live in china shops, there are, nevertheless, costs attached to that much bulk. Limitations to male size include availability of food and the restrictions of gravity. Orangutans are among the most arboreal of apes; yet, a fully grown male (weighing up to 165 pounds) may become so large that the forest canopy no longer supports his weight and he is forced to travel long distances by walking along the tangled, leech-infested floor. … Slowly, persistently, endlessly, the shaggy, phlegmatic red titan consumes vast quantities of unripe fruits and mature leaves, the junk foods left by more discriminating females. The female orang, because she is smaller, can afford to be a picky eater, selecting the nutritious shoots of new leaves and the ripest fruits.
Why, then, are males pushed into being supersized? Simply put, because the competitive victors are nearly always those that are larger and stronger, which in turn selects for larger size in the more competitive sex, which is nearly always males. Polyandry—females keeping harems of males—is so rare that when zoologists find such a case, it makes scientific headlines.
In addition to greater size, in many cases males are outfitted with intimidating anatomy that contributes to their potential success: horns, antlers, large canines, sharp claws—and evidently, in the case of those giraffes with which this discussion began, long weaponized necks.
None of these accouterments make sense, of course, unless their possessors are inclined to employ them. It wouldn’t do for a bull elk or seal, no matter how large and imposing, to refrain from using his bulk or his weapons when it comes to competing with other bulls. There is little evolutionary payoff to being a Ferdinand among bulls. Accordingly, just as evolution often generates sexual dimorphism in physical size, it works similarly with regard to behavior, and for the same basic reason. As with the male-female difference in physical size, male-female differences in aggressive behavior vary with the degree of polygyny and of male-male competition.
There’s more. In species evincing substantial male-male competition, females become sexually and socially mature at a younger age than do males. This is superficially counter-intuitive, because when it comes to reproducing, females are the ones who bear the greater physiological and anatomical burden, whereas the biologically mandated demands upon males are comparatively trivial: Just a squirt of semen.
We might expect that if anything, males should breed when younger whereas females should delay sexual maturation until they are proportionally larger and stronger. But not only are female mammals typically smaller than males, they become sexually mature earlier rather than later because of yet another consequence of the power of male-competition. It is highly disadvantageous for males to enter the competitive fray when they are too young, too small, and too inexperienced. A male who seeks to reproduce prematurely is liable to be thrashed by his older, larger, and more savvy competitors, whereas early-breeding females—who don’t have to deal with the same kind of socio-sexual free-for-all—don’t suffer a comparable penalty.
The result is “sexual bimaturism,” whereby female mammals in which male-male competition is pronounced breed when younger than males. Sure enough, sexual maturation occurs at roughly the same age for males and females in primate species that are monogamous, such as Latin American marmosets and owl monkeys. Among polygynous species such as rhesus macaques, squirrel monkeys, gorillas, and, indeed, nearly all primates, including humans, males reach social and sexual maturity later than do females. The sexual bimaturism so familiar to observers of Western teenagers (and to those teenagers themselves) is a cross-cultural universal.
Although we don’t do a lot of head slamming, a la giraffes, there is no doubt that Homo sapiens is no stranger to precopulatory male-male competition via their bodies. This is strongly suggested by our sexual bimaturism and men being on average 20 percent larger and heavier than women. Discounting fat reserves (more abundant in women because of the advantage it confers during pregnancy and lactation), men are roughly 40 percent heavier and carry 60 percent more muscle mass, with a whopping 80 percent difference in arm musculature.
Like the males in many animal species, the men in ours compete to win the love of a mate, and victory is often sealed with a nuptial gift. Scorpion flies offer the protein-rich bodies of other flies, men offer a ring. Male birds engage in territorial singing contests, and men woo with songs too. Then there’s the conspicuous mano-a-mano competitions commonly found among excitable animals—regardless of the length of their necks. 
David P. Barash is an evolutionary biologist who has specialized in animal behavior and is now emeritus professor of psychology at the University of Washington. His most recent book is Threats: Intimidation and its Discontents.
Lead image: Henk Bogaard / Shutterstock
References
1. Wang, S-Q., et al. Sexual selection promotes giraffoid head-neck evolution and ecological adaptation. Science 376, 6597 (2022).
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