Darwin enjoyed the cooperation of administrators and keepers in conducting numerous experiments. He presented various yarns to weaver birds to test their color preferences for nest building and tried to make elephants weep. He provoked monkeys to test their reactions; he gave a doll to one individual to gauge its surprise and snuff to another to see if it would close its eyes when it sneezed (it did not). He presented a mirror to orangutan adolescents, watching them caper and pose before scampering away in alarm.
Darwin’s first encounter with an ape, in fact, had been in 1838 at the London Zoo. Jenny, an infant orangutan who wore a dress and lived in the heated giraffe house, impressed him with her human-like emotions, her understanding, and her ladylike deportment (she had been presented to the Duchess of Cambridge). Contrary to the descriptions of Cuvier and his predecessors, whose assessments of primate aggression were weighted with value judgments, Darwin noted in a letter to his sister Susan that when Jenny’s keeper teased her by showing her an apple and then taking it away,
she threw herself on her back, kicked & cried, precisely like a naughty child.—She then looked very sulky & after two or three fits of pashion, the keeper said, ‘Jenny if you will stop bawling & be a good girl, I will give you the apple.[’]—She certainly understood every word of his, &, though like a child, she had great work to stop whining, she at last succeeded, & then got the apple, with which she jumped into an arm chair & began eating it, with the most contented countenance imaginable. 13
Orangutans would come under Darwin’s scrutiny again almost twenty years later, when he read four articles on the apes by Wallace in the Annual Magazine of Natural History, published soon after Wallace’s return from the Dutch East Indies. Darwin’s younger colleague had observed adult orangutans in the wild, and once, after his companions shot a mother orangutan, he kept the baby in his camp. She eventually died from malnutrition, but not before impressing him with her intelligence and her emotional similarity to human children, including a tendency toward tantrums. (These accounts by Darwin and Wallace coincide almost exactly with Galdikas’s descriptions of infant orangutan tantrums, affection, dependency, and intelligence.)
In his later volumes, Darwin seized the opportunity to argue for sexual selection, which his colleagues had greeted with skepticism and he had thus minimized in The Origin of Species. In Darwin’s view, the evolution of any species, including humans, depends largely on sexual selection. Although his original genetic research had to be conducted with rapidly reproducing nonhuman species (especially mollusks and pigeons), in the Origin of Species, Darwin explained the mechanics of sexual selection with evidence spanning the whole animal kingdom, including crustaceans, insects, fish, birds, reptiles, and amphibians. The focus of The Descent of Man is the primate order: in arguing for human kinship with other animal species, Darwin relies heavily on fossil evidence of archaic humans and secondary data about other primate species. In the first chapter, by repeating Huxley’s arguments, adding a few details, and placing the whole discussion within the context of speciation, Darwin gives his stamp of approval to the theory that humans, like other species, are simply the most recent stage in an evolutionary chain that—granting a few missing links—goes back to an ape-like ancestor.
However, whereas Huxley’s morphological argument ends with an assertion of bodily likeness between humans and other apes, Darwin’s explanation begins with morphology and quickly moves on to arguing similarities in human and animal cognition, psychology, society, and culture. In fact, Darwin distrusted Linnaean classification, and in The Descent of Man, he notes especially the case of the New World capuchin monkeys, the various forms of which some naturalists rank as species and others as varieties: “If of a cautious disposition, [the classifier] will end by uniting all the forms which graduate into each other as a single species; for he will say to himself that he has no right to give names to objects which he cannot define.”14 For Darwin, the study of life should concentrate not on external form but on processes within the lives of individuals and over evolutionary time. Darwin’s arguments about animals, including humans, are informed by attention to behavior and even attempts to understand motivation. He reasoned that human emotions and human society provide clues for understanding the lives of other animals, especially nonhuman primates, who can in turn serve as a mirror for a deeper understanding of human behavior.
In Darwin’s view, then, it matters less how many millet seeds it takes to fill up a brain case than how the brain—or any other organ—works. In The Descent of Man, Darwin begins with taxonomy and morphology, but most of his additions to Huxley’s work on nonhuman primates are physiological rather than morphological details. In his discussion of skin and hair, for instance, Darwin suggests that the relative scarcity of hair on human bodies evolved partly as a result of sexual selection and perhaps also because relative hairlessness helps humans remain free of parasites (a function of social grooming among other primates).
One of Darwin’s sources of information for The Descent of Man was a study of New World monkeys made by a German physician and explorer of Paraguay, Johann Rudolph Rengger, whose interests, like Darwin’s, lay in function and behavior over form. Against the trend of the time, and in spite of obstacles, Rengger made keen observations about the behavior of capuchin monkeys and speculated about their physiological processes. Citing Rengger, Darwin argues that close ties between humans and other primates are strongly suggested by these monkeys’ susceptibility to human diseases—bad colds, tuberculosis, “apoplexy, inflammation of the bowels, and cataract in the eye. The younger ones when shedding their milk-teeth often died from fever.”15 More interesting than these maladies is the monkeys’
strong taste for tea, coffee, and spirituous liquors: they will also, as I have myself seen, smoke tobacco with pleasure. Brehm asserts that the natives of north-eastern Africa catch the wild baboons by exposing vessels with strong beer, by which they are made drunk. He has seen some of these animals, which he kept in confinement, in this state; and he gives a laughable account of their behaviour and strange grimaces. On the following morning they were very cross and dismal; they held their aching heads with both hands and wore a most pitiable expression: when beer or wine was offered them, they turned away with disgust, but relished the juice of lemons. An American monkey, an Ateles [spider monkey], after getting drunk on brandy, would never touch it again, and thus was wiser than many men. These trifling facts prove how similar the nerves of taste must be in monkeys and man, and how similarly their whole nervous system is affected. 16
Today, diseases transmitted from humans are one of the leading causes of captive primate death, and even a casual visitor to research and shelter facilities is sometimes required to provide documentation of a tuberculosis test. Darwin was clearly in-trigued by Rengger’s research—and both men would perhaps be puzzled by how often biomedical and behavioral researchers continue to test the simian nervous system by giving monkeys cigarettes, beer, and other addictive substances.
Other likenesses Darwin notes between the human sensorium and that of other primates include similarities in nose, eyes, ears, and vocal apparatus. Humans have more prominent noses than most other primates, but it is impossible to distinguish humans from all other primate species on this basis, since the Hoolock gibbon has an aquiline nose and some monkeys, such as the proboscis monkey, carry the nose “to a ridiculous extreme.”17 All primates have similar facial structures; Darwin does not mention binocular or trichromatic vision (his resources for investigating neurology were limited), but he does note that many nonhuman primates have human-like eyebrows. Like some humans, certain monkeys also have vestigial Mr. Spock points on their ears, and though a distant ancestor common to all primates probably had moveable ears, this trait has not been preserved in apes: “The ears of the chimpanzee and orang are curiously like those of man,” Darwin writes, “and I am assured by the keepers in the Zoological Gardens that these animals never move or erect them; so that they are in an equally rudimentary condition, as far as function is concerned, as in man.”18 (Some humans can wiggle their ears, however; it’s a good party trick.) Darwin goes on to speculate that the strength and arboreal habits
