Remember how James Carville, the Democratic strategist, kept the Clinton campaign focused before the elections of 1992 and 1996? "It's the economy, stupid!" was the mantra that spelled success. Well, apologizing for any unintended insult, we want to co-opt the James Carville tactic and reiterate: "It's the predators, stupid!"
It is a paradigm in the published research of animal behaviorists (called ethologists) that predator—prey relationships can best be studied from the perspective of the predator. They all know this—but primatolo-gists chose to reinvent the wheel where predation was concerned. "Hey, let's pretend that it doesn't count at all. We never see it!" (Small wonder since most predators choose not to attack their next meal in front of people.) "Every other branch of field research looks at the predators, but we'll only look at the prey. After all, we study primates." We don't claim to be tackling something in a brand new way when we state that the integration of research on predatory animals may help to answer some of the myriad questions regarding predation on primates. In fact, it is so elementary that you may wonder why we would make a point of it. So, we humbly mention that a look at primate predation from the viewpoint of current predator research could expand knowledge on this important aspect of primate life history. Many other primatologists have concurred that progress in understanding the importance of predation on primates will come only from this approach.28
Observation of only one group of one species (the typical parameters of primate research) provides limited data and often skews the perception of predation, whereas fieldwork on predatory species gives a broad view of several food chain levels. The home range of a solitary predator usually overlaps numerous prey groups and species; while the predator hunts on a daily basis, it may attack the primate group under study only occasionally.29 For example, the range of an African forest guenon, such as the blue monkey, is approximately 50 square yards; redfronted lemurs in Madagascar have a home range even smaller; while a chacma baboon troop may range just a few square miles.30
Compare those relatively minute areas with the predator's frame of reference. Home ranges for predatory species are large.31 Within the range of the typical leopard there may be not only many populations of one primate species, but many populations of many primate species, and the leopard may be preying on all of them. Leopard territories have been estimated to be as small as 4—8 square miles or as large as 250 square miles, and one male leopard in the Kalahari Desert had a home range of 500 square miles. Other carnivores hunt in equally vast areas that might contain scores of primate groups. Tigers defend a territory of 20 square miles, spotted hyena clans claim a territory of 6—25 square miles, jaguars hunt over an area of approximately 30 square miles, and a pack of African hunting dogs will range over 400 square miles. Birds of prey, of course, also have huge ranges in which they seek food. A harpy eagle pair may possess a 60-to-120-square-mile territory, Philippine eagle pairs have a home range of 15—30 square miles, and the total home range of a pair of crowned hawk-eagles is approximately 6—8 square miles.
Even a relatively small carnivore, such as a fossa (a relative of civets, genets, and mongooses), is estimated to hold a home range of nearly 3 square miles. Fossa fill the ecological niche of wild cats in Madagascar and were previously misclassified taxonomically as a cat species. By virtue of its cat-like claws, teeth, and jaws, the fossa looks much like a North American cougar and is equipped to kill mammals nearly its own size. Patricia Wright and her research team from Duke University tracked one individual fossa in the rain forest of southeastern Madagascar that was making regular attacks on four sifaka social groups, each separated by at least 1 kilometer. The sifaka is an acrobatic lemur that excels in vertical clinging and leaping. They jump in an upright position, sail loftily through the air, and land upright with all four limbs entwined around the intended trunk or branch. Pat Wright's research team concluded that this particular fossa's home range was large enough that it took 1 year for the fossa to cycle through the four primate territories.
Earlier in this chapter we mentioned the lions that move in and out of the Mahale Mountains National Park, Tanzania, and the chimpanzees that provide part of their diet. While researchers speculated that predation on chimpanzees could be heavy if the lions would permanently reside in the park, it may be that the Mahale Mountains are simply one portion of the lions' home range through which they periodically cycle.
Who's eating whom? Like building a pyramid, we needed to get at the foundation of predation on all primates before we could discuss predation on early hominids. We couldn't present a theory of Man the Hunted unless we could get the facts on other primates. If non-human primates are not prey, then that sets us and our relatives in a special category—so special that the laws of nature and predation do not affect us.
We set to work finding out all we could about predation on pri-mates.32 Until our research, no comprehensive attempt had been made to collect and summarize published and non-published empirical or anecdotal records of actual primate predation events in the wild. We examined the scientific and natural history literature from both the predator and prey perspectives. From our data we came to the conclusion that primates in general are vulnerable to predators. In fact, the chances may be just as likely that certain primate species will become the meal for a hungry predator as the chance that more "typical" prey— such as gazelles or antelopes—might be selected and eaten.
For all the debate about predation on primates (and by extension, predation on our hominid ancestors), a systematic count of observed predations on primate species had never been attempted. Based on anecdotal comments from colleagues and an initial cursory look at the writings of naturalists from the first half of the twentieth century, we felt there was reason to challenge the rather dubious axiom, "predation is rarely observed." Nevertheless, our own intuitive judgment that predation was an important feature of primate life was no more solid than the idea that predation is rarely observed. So we decided to do the sublimely obvious—simply count in a systematic fashion the number of times predation on primate species has been observed and described in the scientific and natural history literature. Published accounts from field researchers and naturalists who have actually witnessed predation on non-human primates were collected. These eyewitness accounts of predation events are dramatic and numerous, beginning in 1895 and continuing to the present. (Just as with other living primates, modern humans still fall prey to predators, and anecdotal descriptions also abound of man-eating lions, leopards, tigers, crocodiles, and pythons.
These unsettling instances accentuate the long history of humans as another species of primate prey.)
After compiling in excess of 550 eyewitness accounts of predation on non-human primates, we felt rather confident that primates occur in the diets of many predatory animals. Approaching the subject of predation on primates from this objective viewpoint, we found it evident that a divergence of views concerning the rarity of predation exists between primate researchers and their counterparts who study those species that eat primates. The evidence for primates in the diets of many carnivores, raptors, and reptiles is considered indisputable to researchers who study predatory animals.
The difference between data gathered from primate and from predator researchers can be dramatic. This was apparent from questionnaires sent out to the two different research communities. Less than 10% of the 227 primatologists who responded to questionnaires had knowledge of more than two predations on their study populations. Contrast this with the responses from predator researchers; known or observed kills by the predator they were studying averaged 20 primates, and one researcher had gathered information on 350 primate kills by leopards.33
Predation as a demographic parameter in primate populations has been one of the least studied areas because little empirical information on predation has been available. So, what was the exact empirical evidence gathered during our study of predation on primates? We found 176 species of confirmed or potential predators of primates in the four geographic regions inhabited by non-human primates (Africa, Madagascar, Asia, and the Neotropics).34 These predatory animals include diminutive birds all the way to huge mammals and reptiles. Primates are preyed upon by hawks, eagles, owls, and other predatory birds, and by wild cats, wild dogs, jackals, hyenas, and bears. They are preyed on by little-known species of small carnivores, such as civets, genets, mongooses, tropical weasels, raccoons, the marsupial opossum, large and small crocodiles, snakes, monitor lizards, tegus (Neotropical lizards), and even sharks! These predators range in size from 2.5 ounces (the hook-billed vanga of Madagascar) to in excess of 500 pounds (the Indian mugger crocodile, which measures over 9 feet in length).
We reviewed the scientific literature dealing with these predator species for quantitative and qualitative references to primates as prey.
Questionnaires we sent to field researchers were used to collect an additional set of quantitative and qualitative data. Both of these areas showed that primates may as likely be prey as most other groups of herbivorous or omnivorous mammals.
When describing the data we gathered, support was given to our argument that primates are, and have always been, prey animals. Much of the data collected for our study came from predator researchers who determined the frequency of primates in predator diets through analyses of feces, stomach contents, regurgitations, nest and den remains, skeletal components left at kill sites, and direct observation of kills—information regularly gathered in studies of predatory animals.
These data were drawn from the fieldwork of ornithologists, her-petologists, and mammalogists. We analyzed the data collected through questionnaires and literature searches and used the information on predators to provide a counterbalance to information provided by primate researchers. This, in essence, allowed us to cross-check between the two fields, a technique useful in establishing more accurate estimates of the level and impact of predation.
Known primate deaths, unsuccessful attacks, and suspected predations were tabulated. Data on almost 3,600 separate instances of predation were available from questionnaires and the scientific and natural history literature. Diurnal raptors, owls, and other birds were the top predators on primates (41% of the total), followed by wild cats (35%), wild dogs and hyenas (7%), reptiles (5%), small carnivores such as the fossa (33%). Unknown predators accounted for 9% of the total. Further fieldwork is critical to expand these findings since many of the 176 predator species have not been studied at all in their natural state.
No geographic region could be identified in which primates were free of predation. No variables of body size, night or day activity cycles, or strata (high forest canopy, low shrub, or ground level) could be identified that exempted primates from predation.
The limited data available from questionnaires indicate that adult primates are more often prey than other age groups and that males are more often prey than females. However, infant survival rates are identified by many researchers as being very low, and a good portion of this mortality may be due to predation.
Estimated predation rates were as high as 25%. (Predation rate is defined as the percentage of the population removed annually by predators.
A predation rate of 25% means that one in four individuals in a population is killed by a predator in a year.) An analysis of the estimated predation rates of primate species indicates that small, nocturnal, and arboreal primates may undergo higher rates of predation than larger, diurnal, and terrestrial species.
In studies we analyzed, the frequencies of occurrence of primates in predator diets ranged from miniscule to 90%; raptors and wild cats had the highest percentages of primates in their diets.
Eleven predators met our criteria of "primate specialists," defined by us as species that rely heavily on primates as food. These rapacious super-predators are leopards, harpy eagles of the Neotropics, African crowned hawk-eagles, the fossa of Madagascar, Philippine eagles, bateleur eagles of Africa, Henst's goshawks of Madagascar, Madagascar buzzards, Madagascar long-eared owls, African pythons, and reticulated pythons of Southeast Asia. Of these eleven species, four—the leopard, harpy eagle, crowned hawk-eagle, and fossa—were by far the most dedicated to hunting primates.35
While we narrowed the whole realm of predators down to these four dedicated primate specialists, none of the four specialists have what you would call a rigidly narrow food base. There are no mammalian predators, for example, comparable in their food selectivity to aardwolves that eat only termites or whales that must strain tiny crustaceans through a baleen filter in their mouths. Therefore, primates are what we have dubbed "generalist" prey. Despite the evidence found to identify certain predators as primate specialists, we take the position that primates are "generalist" prey in the sense that, as a group, they come in all sizes, from very small to very large; they inhabit geographic ranges throughout the tropics, subtropics, and a few temperate forests; they range from totally arboreal to totally terrestrial; and they include both nocturnal and diurnal species. Their successful radiation into many ecological zones carried with it the potential to interact with many predators that will opportunistically feed on a wide range of prey.
For a wider perspective, we also compared primates to other, so-called typical mammalian prey species, such as hoofed animals and large rodents, to ascertain if there were any overall similarities in rates of predation. Traditionally, primates have not been thought of as prey species in the same league as ungulates (deer, antelopes, and gazelles). However, when primates are compared to other species (particularly the grazing ungulates) inhabiting the same biomes and preyed on by the same predators, some similarities in rates of predation became apparent. Primates may be as much a food source for predators as other, more typical prey.
Predation is undoubtedly significant and may be the leading source of mortality in populations of primates.36 It was the main conclusion of our study that species of primates are influenced to varying degrees by predation and that major physical, ecological, and behavioral adaptations have evolved in response to predators. We hypothesize that some predatory species specialize on primates as a resource base and that many others kill primates opportunistically.
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