Wild thoughts from wild.., p.25
Wild Thoughts from Wild Places, page 25
Let’s review a few facts and dates. After his five years of circling the world on the Beagle, during much of which he was seasick and homesick, Darwin set foot back in England in 1836. In 1837 he began his first notebook on the question of “transmutation” of species, and in 1838 he hit on his great idea: that natural selection was the mechanism by which it happened. Meanwhile he had taken up residence in London, which he hated as an “odious dirty smoky town” but which allowed him access to museums, libraries, scientific societies, and the dinner-table company of brilliant people. In 1839 he married Emma Wedgwood, his first cousin, who was not the dinner-table-brilliant type. Then, in 1842, he bought an old parsonage in the hamlet of Down, “a quiet most rustic spot,” two hours by carriage southeast of London. He and Emma moved there in September. For the next forty years of his life, which was all of it, Down House (as it came to be called) was the only home he knew or wanted.
He’d had a bellyful of travel. His health was bad, he was shy, he was busy, he was defensive and secretive about his most important ideas; for these reasons and who knows what others, Darwin became an obdurate homebody. On his lonesome he wrote books and on Emma he fathered a passel of children. He never again went abroad. He traveled around Britain occasionally, but not far, not often, and only for extraordinary personal or professional reasons, such as taking the water cure at a provincial spa. Mostly he stayed at Down, reading, writing, dissecting barnacles, running little experiments in his study, being a good husband and father. He dropped away from the scientific societies of London and made clear to his close colleagues that if they wanted to talk, they would have to come visit at Down. He did his more far-flung research through the mail, pestering scientific pen pals all over the world for bits of data. When his father died, in 1848, he missed the funeral. When his eldest and much-beloved daughter died, in 1851, he skipped the funeral. Both these funerals, you’ll notice, fell within the barnacle years. When the British Association held its famous meeting at Oxford—in 1860, a year after The Origin of Species had triggered an intellectual revolution—and T.H. Huxley debated Bishop Wilberforce about the new theory’s profound and scandalous implications, Darwin was conspicuously absent. Again he preferred to take an invalid’s excuse and let others report to him what had happened. No doubt he’d have skipped his own funeral, at Westminster Abbey in 1882, if allowed any choice in the matter.
Having traveled so many hard miles in his young manhood, Darwin had found his preferred spot—that old house in the hamlet of Down—and cemented himself to it. Virtually everything he wanted or needed (except quack doctors offering water cures) was there within reach. So why move? Small wonder, if you ask me, that he felt a special attachment to the Cirripedia.
• • •
AS THE Pacific bulge pushed itself back up into the Strait of Juan de Fuca and the rising tide nudged me toward high ground, the acorn barnacles of the uppermost littoral were the last creatures over which I could ruminate. I was intrigued by the way each one lives out the consequences of that single decision, made as a larva: where to attach. How do they choose? How do they know? How do they achieve such heroic resoluteness? Wet to the shins, I was still listening for the call of the barnacles.
But no, nothing. Only the susurrus of sea against land.
Maybe Darwin had heard it, I thought. Maybe he was patient and focused enough to apprehend their most private truths. I made a mental note to get hold of his big, boring, authoritative book, the one some people felt he shouldn’t have bothered to write. Then we hopped in the car and wandered on.
Voice Part for a Duet
Two intriguing statistics recently grabbed my attention. They concern that remarkable form of social behavior known as monogamy. The first is a testament to avian fidelity: Ninety-two percent of all bird species are monogamous, at least through a single breeding season. The second is another sort of testament: Among mammal species, in contrast, just three percent are monogamous. By my clumsy arithmetic, that makes monogamy about thirty times more prevalent in the avian world than down here on solid ground among the class of plodding beasts to which you and I and Elizabeth Taylor belong. This of course leads to a question about mammals versus birds: Who are we to call them flighty?
Another question carries more scientific import: Why is mammalian monogamy so rare? If there’s a telling pattern within the data set, it doesn’t show at first glance; the monogamous mammals seem crazily varied. The trumpet-eared bat is among them, and so is the beaver. Also the bowhead whale. A fair number of canid (dog family) species are monogamous—the wolf, the coyote, the golden jackal, the red fox, the arctic fox, the Cape hunting dog, and others—but no felid (cat family) species, no bears, no hyenas, and just one marsupial. The klipspringer, a dainty African antelope, is monogamous. So is the indri, the largest of the lemurs, an extraordinary animal whose ghostly yodeling haunts the rainforest of northeastern Madagascar. Most other mammals are not. Monogamy, then, is like flight: an exceptional phenomenon among mammals, a sort of defiance of gravity by alternate means. It occurs in certain species but not many, and the circumstances have got to be special.
What accounts for the low incidence of mammalian monogamy? Well, don’t jump to the wrong conclusion when I say that some biologists attribute it to the invention of the breast.
• • •
IN A PAPER published some years ago in the Journal of Theoretical Biology, Allen T. Rutberg wrote: “The possession of mammary glands by female mammals encourages heavy parental care by females and biases mammalian mating systems in favor of polygyny.” Rutberg’s polygyny is the precise biological term for one male’s matings with multiple females. Birds don’t experience the same sort of physiological encouragement, since a male bird can provide parental care—incubating an egg or dropping worms into the maw of a hatchling—in essentially the same ways as a female. The most vivid exemplar of that truth is the male of the emperor penguin, Aptenodytes forsteri. This steadfast papa spends two months in midwinter standing deserted, on the Antarctic ice, with his mate’s single egg balanced on the tops of his feet. He keeps the egg warm there, beneath his abdominal flap, while the female goes off on a feeding excursion to build her weight back up. When she returns, finally, he gets his own turn for an overdue meal. His reward for paternal reliability is higher reproductive success than he could expect if he mated with many females and abandoned them all. If he chose the latter strategy instead, he might fertilize six or eight eggs from six or eight different mothers, but probably not a single offspring would survive. Life on the ice is so hard that it takes two emperor penguins, cooperating from start to finish, pulling long shifts, to rear one chick. The male can do his full share because breasts don’t enter into it.
Among mammals, the canids aren’t unique in their inclination toward monogamy. The primates lean that way too—not overwhelmingly, but disproportionally relative to the overall ratio for mammals—with about fifteen percent of their species monogamous. That figure includes the indri but no other lemurs. It might or might not also include Homo sapiens, since humans can be seen as either monogamous or polygamous, depending on which human culture you look at. According to one ethnographic source, monogamy is the standard marital arrangement in just 137 out of 849 societies examined. So human monogamy, such as it is, stands as an odds-against exception (137/849 being the ratio of societies) within an odds-against exception (15/100 being the ratio of primate species) within an odds-against exception (3/100 being the ratio of mammal species) to prevailing patterns. Never mind the arithmetical bottom line. The point is that monogamy—among mammals in general and humans in particular—is an anomalous sort of behavior that can’t be taken for granted.
Why do the evolutionary odds seem to set themselves so strongly against monogamous behavior among mammals? Is there more to it than the matter of breasts? And why have some species, breasts and all, flouted those odds?
Unfashionable thoughts: Monogamy is daring and mysterious.
• • •
BIOLOGISTS have always been interested in mating habits—and who can blame them, since the subject holds great prurient appeal as well as scientific significance. But for the past thirty-some years there has been increasing speculation about the origins of monogamy versus polygamy. Back in 1964, for instance, a researcher named Jared Verner published a paper titled “Evolution of Polygamy in the Long-Billed Marsh Wren,” which helped launch the discourse into its modern phase. The ornithologist David Lack continued it in his 1968 book, Ecological Adaptations for Breeding in Birds, contributing some semantic clarity by dividing the concept of polygamy into harem polygyny (one male monopolizing several mates for a period of time), successive polygyny (one male mating successively with several females), and polyandry (one female mating with several males). Lack also wrote of the pair-bond, an implicit behavioral contract that ramifies beyond the act of copulation itself and distinguishes polygyny and polyandry from sheer promiscuity. The pair-bond can be limited to a brief phase of consortship or to a single breeding season, though in some species (the Canada goose, the klipspringer, the indri) it lasts for a lifetime. About polygyny, polyandry, and promiscuity, Lack said something notable: “The ecological factors making these abnormal types of pairing more advantageous than monogamy are not clear.” The notable part is that he called them “abnormal.” Later biologists who focused on mammals more than on birds would reject Lack’s assumption that monogamy was normal.
Edward O. Wilson, having come to theoretical ecology and sociobiology by way of the study of ants, also rejected that assumption. In his compendious 1975 volume, Sociobiology, Wilson said that polygyny would be the natural arrangement for any species in which female sex cells (eggs) are larger and more expensive metabolically than male sex cells (sperm). Producing those big, nutrient-rich eggs requires the female animal to invest more bodily resources in each single act of mating than the male, who has little to lose by mating frequently. Wilson suggested that monogamy, on the other hand, might evolve secondarily in response to any of three ecological circumstances: (1) some sort of crucial habitat resource (a good nesting hole, say) is so scarce and localized that the cooperative effort of two adults is required to defend that resource against competition; (2) the physical environment is so difficult (as it is for the emperor penguin) that cooperative effort is required to cope with it; and (3) early breeding (at the first flush of spring, perhaps) is so advantageous that monogamy saves precious time that would otherwise be squandered on courtship. Wilson’s third condition is relevant to the evolution of long-term pair-bonding. Finding a new mate each year might be amusing but it’s also costly, and lifelong monogamy—at least for some species—is a more economical reproductive strategy.
The zoologist Devra G. Kleiman published a hefty review article, “Monogamy in Mammals,” in 1977. Kleiman’s piece is especially interesting for its attention to the social dimension, as well as the sexual dimension, of mating. She recognized that certain forms of gentle, sedate behavior—and not just the hot immediacy of courtship and copulation—play an important role among monogamous species. The simple act of resting together, for example, occurs conspicuously among beavers, among the antelopes known as dik-diks, and also among such monogamous primates as the marmosets, the tamarins, the siamang, and the gibbons. Another sort of amiable sociality is grooming—that is, picking carefully through another animal’s fur to remove parasites—which is common among primates generally but shows itself in a special way among monogamous primates: Male partners more often perform the grooming on females, a nice little token of the pair-bond. Kleiman also mentioned the monogamous titi monkeys of the genus Callicebus, of which mated pairs sometimes sit side by side with their tails twined together. Tail-twining doesn’t produce any new offspring or directly aid the survival of those already born, it doesn’t even get rid of parasites, but it may well serve some adaptive purpose in the long run. It’s another reaffirmation of the pair-bond.
Kleiman was intrepid enough to discuss the pair-bond between monogamous humans. She noted drily that, in Western society, “great emphasis is currently placed on maintaining high levels of sexual interactions in married couples” after they have started raising children. It’s the old how-many-times-a-week? question that young husbands and wives begin asking themselves, proudly or disconsolately, around their fifth anniversary. “Frequent sexual behavior is thought to contribute to the maintenance of a strong bond in humans,” Kleiman wrote, whereas “clearly this is not the case in many species of monogamous mammals where contact and affiliative behaviors, such as resting together and grooming, are more common than sexual behavior.” She and her titi monkeys were onto something.
In the two decades since Kleiman’s article, others have speculated further about the evolution of monogamy. Their ideas are multifarious. A pair of researchers named James F. Wittenberger and Ronald L. Tilson, in a 1980 paper, described three preconditions that must exist before monogamy can evolve and five hypotheses that might account for just why it does evolve when it does. They judged each hypothesis against empirical evidence from various kinds of animals—colonial birds, ducks, carnivorous mammals, antelopes, primates, reptiles, frogs and toads, wood roaches, dung beetles, horned beetles. Monogamy among the horned beetle Typhoeus typhoeus, you’ll be excited to learn, is attributable to hypothesis three, the one about individual males stashing individual females away so that no other males can get to them. Details and tape at 11:00.
Allen T. Rutberg, whose observation about mammary glands I quoted earlier, divided the question of monogamy into two processes of choice—female choice about how to distribute themselves on the landscape, male choice about how to react to the females’ distribution—and then analyzed the factors that can influence each choice. C.P. Van Schaik and R.I.M. Dunbar listed four hypotheses, three of which were reshaped versions of what Wittenberger and Tilson had proposed, one of which was more fresh, and from each hypothesis generated a handful of testable predictions. Van Schaik and Dunbar’s paper was titled “The Evolution of Monogamy in Large Primates: A New Hypothesis and Some Crucial Tests.” The two authors matched their predictions against known facts about the behavior of gibbons, indri, baboons, humans, and other primates.
What does all this theoretical work tell us? It tells us that monogamy isn’t one behavioral pattern but many, and that it might or might not arise for different reasons in different situations. The scientific literature on the evolution of monogamy is intricate, ingenious stuff. You could read your way through a fair portion of it, as I’ve lately done, and be almost assured of replacing your ignorance with confusion.
• • •
BUT MAYBE I can minimize your confusion, and relieve my own, by lumping all the hypotheses and preconditions and predictions together and rendering them down to just a few simplified points. First, monogamy is more likely if females of the species spread themselves thinly across the landscape than if they gather together in big sisterly aggregations. Why? Because if the females are far enough apart, a male will run himself ragged trying to maintain a harem. Second, monogamy is more likely if males of the species have the physical or behavioral capability of making some crucial contribution toward the gestation and the rearing of offspring. That contribution might be direct, as in male birds or canids who carry food back to their young, or indirect, as in the male klipspringer, who stands lookout for predators while the female eats. It might involve the male of a given species defending a territory of rich habitat, in which his mate enjoys the right to feed without competition from other hungry females or interruption by randy males. It might even take the form of the male protecting his offspring from infanticide by rival males, murderous interlopers who covet his mate, his territory, or both. If the circumstances are right, the trend toward male contribution will be promoted by natural selection, as males who do contribute leave more offspring than males who don’t. And the logical extreme of the trend is an exclusive helpmate relationship with one female.
Here’s a third point: Monogamy is disproportionally common among primates because male parental investment is both more possible and more crucial than it is among other mammals. Remember, primates have dexterous hands and big brains. The big brains grow slowly and entail longer periods of juvenile dependency during which noninstinctive behaviors must be learned — and longer dependency requires more parental investment. The dexterous hands allow males to contribute in a variety of ways, despite their pathetic, embarrassing lack of breasts. A male zebra just isn’t capable of making himself as useful, paternally, as a male marmoset who carries his young through the treetops. So the zebra has no better option than to mate with multiple females and hope that some of his progeny will survive. The marmoset plays a different gamble.
Marmosets are small-bodied animals, and so are most of the other primates among which males carry the young or bring them food. Large-bodied monogamous primates, like the nine species of gibbon or the indri, generally don’t provide that sort of direct paternal contribution. But they do form durable pair-bonds, which are manifested in various ways: traveling with the mate, sleeping beside her, grooming her, performing soulful singing duets with her. According to a tabulation by Van Schaik and Dunbar, seven of the nine gibbon species, as well as the indri and the Mentawei leaf monkey, engage in duetting. The duetting seems to be a means of reaffirming to each other, and announcing to the wider world of predators and competitors, an irrefrangible mutual commitment. What’s the ultimate impetus for that commitment? Van Schaik and Dunbar offered a single answer, the “new hypothesis” of their paper’s title, persuasively supported with logic and evidence: Among larger primates, pair-bonding prevents infanticide.
