Whos Eating Whom

One thing's for certain: there's nothing in a Stephen King novel that will beat the gruesomeness out there in the archives of the print media. It was quickly apparent as we gathered material for this book that entire volumes could be filled with gory exploits of man-eaters garnered from newspapers and magazines around the world. These bloody events are commonly given high profile in newspapers, and they often make the front page in a city that is thousands of miles from the predation itself. For example, "India Fighting Plague of Man-Eating Wolves," screamed a headline on the front page of The New York Times in 1996.1 Would this situation really be relevant to the average Times reader? Or, more precisely, would it be more relevant than an article about 300 million citizens in India who suffer from malnutrition (a tragedy which does not appear in headlines in New York or anywhere else in the United States)? Certainly, in today's world deaths from car accidents, heart disease, cancer, toxic chemicals in our environment, malnutrition or starvation, and warfare hugely outnumber deaths from predators. Even in areas of the world where predation on humans happens regularly, the paucity of predators compared to ever-expanding human numbers makes the impact of predators on overall human populations miniscule. When current-day predation is posed as a mortality factor to a global population in excess of 6 billion people, the percentage of humans killed by wild animals may not even be worth recording.

So, why the heightened media coverage? Might it be that we are fascinated by a deep recognition of what predation by other species has meant in our long evolutionary history? Hans Kruuk, a famous animal behav-iorist and authority on predators, feels that our revulsion, our curiosity, our fascination with gory stories of man-eaters is based on a hardwired instinct that these events are very scary to our whole species—scarier than many of the more obvious killers in our midst.2 After all, we evolved for millions of years being hunted and eaten by other animals, but we have only had to fear automobile accidents for 100 years (just a few generations). Tigers, bears, and wolves touch off much deeper neural pathways than Toyotas, Fords, or Volkswagens.

We readily admit that the relative fright levels of certain species of animals versus certain makes of cars might bring up accusations of purely speculative logic. The big question for us as anthropologists is: How do we get past speculation about the long-term evolutionary effects of predation on humans and into the domain of empirical data when the critical epochs in which we are interested were many million of years before historical records? As mentioned earlier, there are only two factual avenues that trace the path of hominid evolution—the fossil record and our closest relatives, the primates. The fossil evidence for predation on our species is pretty impressive, and we will go into detail about it. But first, we'll take a look at primates—our taxonomic cousins—to see if they can shed light on our big question.

A Tale of Two Families

Let's consider two tales of predation: The first appeared in a popular journal called Asiaweek in April of 1998.3 It concerned a mother's nightmare in the Auri district of Garwhal, a region in northern India. This mother was carrying an infant while her other offspring, an 8-year-old, trailed behind. They were making their way home when a leopard came out of the darkness and grabbed the eight-year-old by the leg. The big cat began to pull the child while the mother frantically tried to hold onto the baby and her older youngster's arm. The mother's grip loosened as the leopard pulled them all down an embankment in a macabre tug-of-war.

Shrieking, the mother lost her hold on the child and heard the youngster's cries as she was carried off into the night. The half-eaten body of the 8-year-old subsequently was found under a bush by villagers who tracked the dried bloodstains.

The second tale was told by Walter Baumgartel, a "white hunter" in colonial Africa (specifically in what was then the Belgian Congo).4 Baumgartel wrote a book containing another description of a family stalked by a leopard. The father was attacked unawares while asleep, but he had tried to fight off the leopard and in so doing had tumbled down a slope. His lifeless body was taken to Makerere Medical School for an autopsy. Two days later, the leopard was found feeding on another member of the family, a youngster. She had been killed in the same fashion as her father with a gash in the groin that severed an artery and, according to medical opinion, had resulted in instant death. Over a period of several months, more bodies from this same family were found, sometimes fresh, sometimes partly decomposed.

The first tale involved Homo sapiens. In the second tale, the family was a close relative—the mountain gorilla (Gorillagorilla beringei). These two accounts serve to depict how parallel are predation events of humans and other primates.

We contend that a true understanding of ourselves as prey is enhanced and expanded by how our closest relatives are affected by predation. Our research has established that quantifiable evidence for predation on primates exists, including ample evidence of predation on all of the great apes—gorillas, orangutans, and chimpanzees. Gorillas, a species much larger than modern humans and several times bigger than early hominids, are prey to leopards weighing half as much. Anecdotal data describing leopards preying on gorillas, as our tale above, are supported by current scientific research in Central Africa. Michael Fay, primatologist cum conservationist whose daring trek across unknown areas of tropical rain forest recently was featured in National Geographic, carried out his dissertation research on lowland gorillas in Nouabale—Ndoki National Park, Republic of Congo. Pygmies in this area held firmly to the fact that leopards preyed on gorillas (and chimpanzees and humans, also). Mike and his colleagues subsequently published evidence of predation on gorillas. The substantiating evidence they presented were intact gorilla toes found in leopard feces—the last remnants of a feline meal.5 The toes have since been sent

Leopard scat that contained two gorilla toes was found by researchers in the Central African Republic; one intact toe has been removed from the fecal material and is clearly visible on the right. (Used by permission of Michael Fay)

to the Field Museum of Chicago and were examined before our writing began. Eerie is the only word that aptly describes these artifacts—relics of predation that are startlingly human in appearance.

The famous zoologist George Schaller reported eyewitness narratives of predation on mountain gorillas in his landmark monograph, The Mountain Gorilla: Ecology and Behavior.6 Schaller's information came from, among others, the same colonial hunter we mentioned earlier, Walter Baumgartel. Baumgartel resided at the time in the famous Virunga Mountains, where Rwanda, Uganda, and Democratic Republic of Congo meet, the only home of the mountain gorilla. He later transcribed his adventures in the heart of Africa in a book entitled Up Among the Mountain Gorillas; in it he tells of the leopard, recounted above, who stalked and killed an entire family group of mountain gorillas.7 Baumgartel also wrote that the same or a similarly disposed leopard continued the killing spree until there was a single group of only two mountain gorillas left in the Congo portion of the Virunga Mountains.

The great red ape of Asia, the orangutan, is an elusive creature who, unlike the majority of primates, does not live in social groups. Authentication exists that this species is prey to both tigers and the reclusive Asian wild cats called clouded leopards.8 Once in the 1970s a clouded leopard happened on a conservation project in Sumatra set up to reintroduce seven juvenile orangutans back to the wild. The cat repeatedly raided the project's camp until all seven orangutans were killed. Tigers, being much larger and stronger than clouded leopards, can and do kill even brawny adult male orangutans.

Nevertheless, to reveal the ancient hominid state, we find the most compelling evidence in the substantial predation rates on chimpanzees. Chimpanzees share 98% of human DNA; they make tools, they learn and transmit unique behaviors (thus fulfilling for some the definition of culture), they—along with other great apes—learn American Sign Language and will use it to convey concepts.9 Significantly, over the course of past decades, chimpanzees were assumed to be far too much like humans to be preyed upon. But this has since been dramatically disproven.

Even in the relative safety of Gombe National Park in Tanzania, the site of Jane Goodall's famous chimp families, human researchers have frequently seen anti-predation behaviors directed against the occasional transient leopard.10 The reality that chimps in Gombe are almost always in the company of armed humans, along with the fact that visits by leopards are extremely rare, may make the park an artificially safe environment for chimpanzees. It was noted several decades ago that a "safe environment" is definitely not the case in the Parc National du Niokolo Koba, located near Mt. Assirik in Senegal, West Africa. French primatologist Caroline Tutin headed a team that found suspiciously cautious demeanor among the chimps of Mt. Assirik.11 First, Tutin's team ascertained that a full complement of predators were active both day and night—lion, leopard, spotted hyena, and wild dog all coexisted with the chimpanzees. These chimps had a healthy respect and fear for the carnivores they encountered, and their behavior reflected it. Compared to the relatively protected chimpanzees at Gombe, chimps at Mt. Assirik built nests higher in the trees, never engaged in hunting forays for monkeys or the small forest antelopes called duikers, and behaved nervously when traveling as a group on the ground.

Other, more focused studies have since taken place to measure the degree of natural predation by large cats on chimp populations. The results definitively clarify the relationship between chimps and predators. Two examples are pertinent: The chimpanzee population of the Mahale Mountains, Tanzania, has been the subject of long-term demographic research by Japanese primatologists. Predation as a mortality factor for chimps was assumed to be negligible. No researchers had guessed that any of the Mahale Mountain chimps fell prey to the king of beasts until the Japanese team noticed lion feces containing a lot of familiar black hairs. An analysis confirmed the presence of chimpanzee hair, bones, and teeth in four out of eleven samples of lion feces collected over widely spaced periods of time.12 As it turned out, lions annually killed an estimated 6% of the chimpanzee population studied by Takahiro Tsukahara in the Mahale Mountains during his research period.

In the Tai Forest, Côte d'Ivoire in West Africa, the level of predation on chimps by leopards is much the same. Swiss primatologist Christophe Boesch has tracked leopard predation over the years on the Tai chimpanzee population he studies. In 1985 he first noticed two or three sharply cut parallel wounds on an adult male chimp that could only have been from a leopard. He was soon aware that leopard predation was a constant factor for the chimps: "The tremendous power of the leopard's bite makes him a rapid killer and, if taken by surprise, even an adult individual seems unable to prevent it from the fatal biting. Thus, all age-sex classes may suffer from predation by leopards."13 Boesch found that the presence of humans was not much of a deterrent to the leopards; one chimpanzee was attacked only 30 yards from where the researchers were working. Boesch calculated that leopards kill more than one in twenty individuals (5.5%) in his study group of chimpanzees each year.

The Prey Flee, the Predator Pursues

It might be a good idea at this juncture to take a few moments for a discussion on what is known about predator—prey interactions. Stated succinctly, within the constraints of habitat and population density of prey animals, predatory behaviors are shaped by the forces of natural selection to maximize nutrient intake.14 In plain language—predators hunt when they are hungry, and they are unlikely to kill more than they will eat. There is no predation just for fun. The act of predation is costly in terms of time spent, calories expended, and possible injuries sustained. No smart predator is going to use up precious energy stalking and killing unless hunger is pushing her buttons to feed herself or her dependent young.

Conventional wisdom would picture predators formulaically thinning the size of their prey population—mountain lions eating just the right numbers of deer to keep the deer, in turn, from overpopulating. But that's just the short-term view, and natural processes don't operate over short time spans. Under natural conditions (that is, in environments devoid of human perturbations caused by agriculture or technology), it is the prey populations that delimit the ultimate size of predator populations.15

One of the most elegant studies of predator—prey relations was carried out by George Schaller, the zoologist. His study involved the lions of the Serengeti Plain and the animals (including baboons and vervet monkeys) they ate. He concluded that predation is not an important limiting factor on prey populations. It actually has little impact on the populations of prey because predators like lions most often take the surplus animals destined to die anyway from malnutrition or disease.16 Naturally, the chances are better at securing an easy meal than one that may outrun you, leaving you not only hungry but too worn out to pursue the next prey. (Or, to put it in suburban terms, when you need milk at 10:00 p.m. and your gas tank is nearly on empty, why drive to a faraway shop when you can guarantee your milk needs will be satisfied at the corner all-night mini-mart?)

Should we then worry that too many chimpanzees might be obliterated by their natural predators? Absolutely not. Any substantive and long-term drop in numbers of prey will arise from lack of resources. In other words, prey have a finite amount of food they can eat and when there are too many of them for each to get the proper amount of food, there will be deaths. The resource-induced decline in prey populations will eventually set off a decline in predator reproduction: fewer predators will seek mates, fewer matings will take place among those that do, and fewer young will be produced at longer intervals when the prey population drops. The prey population will rise when the food supply rejuvenates under the decreased pressure of fewer animals, and eventually the predator population will do likewise. The population level of the predators simply lags behind and follows the status of the prey species they eat, and the prey species they eat are regulated by the amount of food available to them.

Nonetheless, predators are responsible for many aspects of the morphology (form, shape, and size) and behavior of the animals on which they prey. There is basic asymmetry in the evolutionary rate of prey defenses and the predatory mechanisms challenging them. Behavioral ecologist John Endler named this imbalance the "life versus dinner" principle: A missed predation attempt saves the life of the prey, but only loses a meal for the predator, so defense mechanisms of prey species are more strongly selected for than are predator counterdefenses.17

Coevolution between predator and prey is a case of "the deer flees, the wolf pursues." In other words, if prey evolve a new way to elude predators, predators evolve in the direction of overcoming that new strategy. Any major destabilization in the balance of predators and prey comes about because the prey have evolved some new way to elude predation; the predator then has to counteradapt or give up eating the newly elusive prey.18 We contend that these same natural principles were in place as our human ancestors emerged. Hominids evolved new ways to elude predators and those adaptations include many of the most basic human behavioral traits. Predation is an important source of evolutionary change.

We find the coevolution that occurred between predators and their non-human primate prey is visible in the behavioral and anatomical adaptations of some species of monkeys. Adaptations are even traceable to specific predators. For example, aggregations of two or more different species who feed, travel, or rest together occurs with many New World monkeys and African forest monkeys. Nonetheless, these "polyspecific associations" only exist in geographic regions inhabited by monkey-eating raptors—harpy eagles of South America and crowned hawk-eagles of Central and West Africa. The birds of prey have provided a strong evolutionary incentive for monkeys to cluster into the largest groups possible.19 Harpy eagles have exerted such strong selective pressure on many Neotropical (Central and South American) primates that the effects are manifested in large body size in some species and in behaviors such as group living and cryptic concealment (freezing to avoid detection) in others.20

To reiterate, if anti-predator defenses have evolved in prey species in response to predators, populations of prey species will not be unduly affected by predation under normal environmental conditions. If, however, the slow process of coevolution has not occurred between predators and prey (specifically in the case of exotic or foreign species introduced into an ecosystem by humans), prey species may lack proper defenses and could suffer high mortality to the point of extinction.21 There are, unfortunately, hundreds of examples of these human-arranged debacles. The current endangered status of all Hawaiian ground-dwelling birds is a result of the importation by European settlers of pigs, cats, and mongooses to the islands where no terrestrial carnivores had ever lived. A lesser-known situation is the high mortality inflicted by domestic dogs (and feral cats to some degree) on primates living in many parts of Asia and South America.

Who Are These Primate Prey?

Inhabiting the world currently are more than 250 other species of primates besides humans. Non-human primates are a diverse-looking bunch; their physical differences are staggering. Primates range in size from a couple of ounces to nearly 400 pounds. (The gorilla, at 390 pounds, appears to have no outward resemblance at all to the tiny, aptly named mouse lemur, weighing 2 ounces.) Some primates actually resemble cats; in fact, during any given time period watching primates in a zoo setting, you will hear visitors mistaking ringtailed lemurs for cats. Some primates resemble rodents; the mouse lemur at first glance looks very much like a wide-eyed gerbil. Some primates resemble no other living creatures—the tiny 4-ounce tarsier has a combination of enormous eyes (each eye weighs as much as its brain) and exceedingly long, powerful hind limbs. The tarsier also moves its head 180 degrees in each direction instead of moving its eyes, just like an owl. Perhaps the most bizarre of the primates is the aye-aye of Madagascar, which has evolved colossal rodent-like gnawing incisors with a consequent dwindling and disappearance of other teeth; in addition the species has one long crooked digit on each front limb that is used to dig out insects from rotted wood, much like a woodpecker's lifestyle. The great eighteenth-century classifier of plants and animals, Linnaeus, included bats and colugos (commonly

A selection of representative prosimians: (a) fat-tailed galago; (b) slow loris; (c) sifaka; (d) ringtailed lemur; (e) mouse lemur; (f) ruffed lemur; (g) aye-aye (not to scale). (C. Rudloff, redrawn from Napier and Napier 1985)

called flying lemurs) in the primate order, and until quite recently tree shrews had been mislabeled as primates also.

Indeed, it was not until the mid-twentieth century that the British anatomist W. E. Le Gros Clark provided a modern definition of the primate order.22 Le Gros Clark characterized primates by the retention of generalized limbs tipped with five grasping digits; the replacement of some claws by nails; expansion and elaboration of the brain; emphasis on vision as the primary sense with subsequent de-emphasis on smell; loss of some teeth from the ancestral mammalian state; delayed maturation; and reduction of litter size to a single offspring that receives a high degree of socialization from adults. As primates, humans fit this description just as well as the other 250 or so species.

Sixty-five million years ago, the first almost-primates were probably small, arboreal, nocturnal insect- and fruit-eaters, resembling present-day tree shrews.23 As primates evolved, some became terrestrial, some diurnal, some became fruit-eaters, some specialized their diets even more and became leaf-eaters. Throughout millions of evolutionary years, the solitary early primates adopted complex social structures. Last but not least, body size increased along with these other adaptations. Modern prosimians—the earliest primates to evolve (the lemurs, galagos, and lorises)—are thought to be more like the first primates 65 million years ago than are monkeys and apes. Many of the prosimians are found only on the island of Madagascar, although the galagos and a few other prosimians inhabit Africa and lorises live in tropical Asia. Prosimians depend more on a sense of smell than the rest of the primates. (In fact, their wet nose, or rhinarium, is a trait shared with dogs and cats.) Many prosimians are nocturnal and arboreal, and their social groups generally are not as large or complex as the monkeys or apes.

New World monkeys include squirrel monkeys and capuchins (think organ-grinder monkeys), the tiny tamarins and marmosets that can easily fit in your hand, and the large spider and howler monkeys. All New World monkeys, except the owl monkey, are active only during the day, and many have a prehensile tail that acts like a fifth limb during treetop travel and feeding.

Old World monkeys range throughout tropical Africa and Asia. Baboons and macaques are the most numerous of these primates. Macaques also have successfully colonized temperate areas in Japan and the mountains of Morocco. The frequently filmed Japanese macaque (or snow monkey) lives at a higher latitude than any other non-human

Swinging Tamarind Monkey

Representative monkeys and apes: (a) siamang; (b) colobus monkey; (c) tamarin; (d) drill; (e) saki monkey; (f) gorilla; (g) spider monkey (not to scale). (C. Rudloff, redrawn from Napier and Napier 1985)

primate species and has evolved many behaviors such as bathing in hot springs to help it counter the cold climate. Another group of Old World monkeys, the leaf-eating colobines, has adopted the primate equivalent of bovine behavior. Sacculated stomachs (much like the four stomachs of a cow) help digest the roughage in their diets; this makes for a more lethargic lifestyle in these primates than the fruit- and seed-eating baboons and macaques.

Besides the great apes—gorillas, orangutans, and chimpanzees—there are the lesser apes, the gibbons and siamangs. Found only in Southeast Asia, gibbons and siamangs are rather unique. One, they are frequently monogamous, contrary to the generic promiscuous primate state. Two, they are true brachiators—traveling fast and furiously through the rain-forest canopy by swinging hand-over-hand under the branches.

The Dichotomy about Death

Pick a side! Regarding the subject of predation on primates, the prima-tology community holds entrenched—and divergent—views. Opinions range from a belief that the role of predation has been minimal to theories that predation has been a powerful force in shaping social patterns.24 Predation as a demographic parameter often is discounted by primate researchers under the assumption that few instances have been observed or recorded.25 We noticed that variations on the statement "Predation is rarely observed ..." are frequently employed by scientists as a caveat when the topic of primates as prey is under discussion. It is not an exaggeration to say that the statement has come to be accepted as an axiom within the pri-matology community and is sometimes used to convey in shorthand that the evolutionary consequences of predation on primates are as incalculable as the unknown magnitude of predation. Furthermore, the significance of apparent anti-predator patterns (physical, social, and behavioral) is disputed because most of these adaptations can also be explained plausibly as responses to sexual selection (the competition between males for available breeding females) or feeding competition.26 Because observations of predation in the scientific literature have often been anecdotal rather than quantitative, there has been a tendency to minimize both the possible frequency of predation on primates as well as the pervasive influence that predation has had on the behavior and ecology of primates.

However, the pendulum may be slowly swinging. Significant changes in field research methodology have been called for, such as night observations and the study of primate predators along with primates themselves, to accomplish a more accurate appraisal of predation.27

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