Seize the Prey: Raptor Predation on Primates

Monica Stupaczuk

Writer’s Comment: Having the freedom to write on any raptor-related topic for AVS 115 (Raptor Biology), I had a hard time deciding on a topic with so many possibilities available. But given a world where humans seem to be increasingly separated from nature and the selective force of predation, I knew I wanted to explore the interactions between nature and culture. As a Wildlife, Fish, and Conservation Biology Major, I quite often hear about the effects that humans have on wildlife. I wanted to turn around this relationship by exploring the effects raptors may have had on early hominids via predation. I demonstrate the impacts this force likely had on the evolution of the human species and continues to have on other primates. It is fascinating to think that defining human characteristics and other traits we express today may be the retained products of predation events that occurred thousands to millions of years ago.

Instructor’s Comment: One of the hardest lenses for a scientific essay is that of the human animal.  It’s funny really.  Here is our most basic self–the furless, baby-headed primate.  We are intricately connected to the earth’s cycles and ecosystems.  We are as mysterious in our behaviors, our ecology, and in our evolution as any other animal species.  And yet we are still instructed from an early age to act like objective observers, to be non-participatory watchers in the magnificent community of species.  In “Seize the Prey,” Monica Stupaczuk runs right over this paradox and tells an entirely fresh story, not about man the objective, not about man the thinker, the watcher, nor even the conqueror.  Rather, Monica writes about man the edible.  No special pedestal here, merely a species as apt to provide daily nutrition for a bulky-winged, thick-taloned eagle as any squirrel, snake, or pigeon.  My thanks to Monica for shaking off her own cloak of objectivity and delivering a truly novel account of humans in the food chain.
—Allen Fish, Department of Animal Science

“During any given day, an animal may fail to obtain a meal and go hungry, or it may fail to obtain matings and thus realize no reproductive success, but in the long term, the day’s shortcomings may have minimal influence on lifetime fitness. Few failures, however, are as unforgiving as the failure to avoid a predator: being killed greatly decreases future fitness.”
(Lima and Dill 1990)

In considering the role of predation in human evolution, most people—including scientists—have tended to conceptualize humans as predators, not prey. But the emerging view is that primates, including early humans, evolved as prey of many predators, including wild cats and dogs, hyenas, bears, snakes, crocodilians, and even raptors (Hart and Sussman 2009). While it is not difficult to imagine hominids falling prey to large mammals and reptiles, as such attacks still occur around the world, the thought of hominids being subjected to regular predation from raptors is rather startling. For this reason, I assess the effects that raptors may have had on early hominids via predation. First, I examine the research on extant raptors to address general patterns and relationships of raptor predation on primates and whether raptor predation has any significant effects on primates. Then, I show how fossil evidence supports the possibility of raptor predation on early hominids, and I discuss the role this form of predation may have had on human evolution. 

Birds of Prey as Predators of Primates

There are 81 species of raptors known or alleged to prey on primates, if the raptor category is expanded from diurnal species of hawks, eagles, and falcons to owls and other miscellaneous carnivorous birds (crows, toucans, shrikes) (Hart and Sussman 2009). Some species, such as Crowned Hawk-Eagles (Stephanoaetus coronatus) and Harpy Eagles (Harpia harpyja), are primate specialists. Studies of the Crowned Hawk-Eagles in the Kibale forest of Uganda found that approximately 85% of their diet consisted of monkeys (Skorupa 1989; Struhsaker and Leakey 1990).

Both raptors and non-human primates are widely distributed; however, the vast majority of non-human primates are contained between tropical latitudes of 25° N and 30° S (Napier and Napier 1967). This results in a fairly narrow region where primates co-exist with primate-eating raptors. Though raptors that prey on primates are found in the Neotropics (southern Mexico, Central and South America, and the West Indies), Madagascar, Africa, and Asia, a greater number of them (30 species) occur in the Neotropics than in the other regions. Raptors inhabiting the Neotropics also have the highest mean percentage of primates in their diets (36.65%) as compared to Madagascar, (17.2%), Africa (6.8%), and Asia (4.4%). Thus, in the Neotropics, the mean primate component in raptor diets is more than twice as high as the figure in Madagascar, more than five times that of Africa, and more than eight times that of Asia. Not only are there more primate-eating raptor species in the Neotropics, they also prey on more species of primates than such raptors inhabiting other regions. Madagascar though, while much smaller than the other regions, has a surprisingly high count of 17 raptor species that have been identified as primate predators. This region has the highest ratio of raptors to total predator numbers (58.6% of the total predator component) and the highest estimated predation rates due to birds of prey (Hart 2007).

The three massive raptors that have been historically claimed as the fiercest in the world are, in descending size, the Harpy Eagle of the Neotropics, the Philippine Eagle (Pithecophaga jefferyi) found only on the Philippine islands of Luzon, Samar, Leyte, and Mindano, and the Crowned Hawk-Eagle of Central, East, and Southern Africa (Hart and Sussman 2009). These three raptors are ecological equivalents; they are all large, rainforest-dwelling, primate predators with a well-deserved reputation for being fierce (Skorupa 1989). The Harpy Eagle is the largest eagle of the Americas (wingspan of up to 8 feet) and is considered to be the most powerful bird of prey in the world (Brown and Amadon 1968). It can attain speeds of 40-50 mph and can exert 13,500 foot-pounds upon impact with its prey, which is nearly 3 times the muzzle energy of a bullet from a heavy rifle. Raptors that prey on relatively large mammals have thick toes, well-curved talons, and large tarsi. Crowned Hawk-Eagles have exceptionally thick, powerful legs and short, thick toes that end in very strong, rigid talons. Their talons can wrap around a branch that is as thick as a man’s thigh and their hind claws are the diameter of a man’s little finger. The Crowned Hawk-Eagle is Africa’s second largest eagle, but it is the most powerful raptor on the continent and is able to kill the largest prey. It routinely kills antelope that are 40-44 pounds (nearly 5 times its own weight) (Hart and Sussman 2009).

If the physical capabilities of these primate-eating raptors do not intimidate you, then perhaps occasional reports of attack on humans may. Part of the skull of a young African human was once found in a Crowned Hawk-Eagle nest. In another carefully authenticated incident, a seven-year old school boy of about 44 pounds in Zambia was attacked by a subadult Crowned Hawk-Eagle (Steyn 1983). The boy was nowhere near a nest, so the attack was most likely an attempt at predation. 

Effects of Raptor Predation on Primates

Predation has long been thought to affect traits such as body size, group composition, ecological niche, vigilance, and vocal and reproductive behavior (Treves 1999). For primates, studies show that predation from raptors can affect foraging, behavior, and group composition.

Primate foraging strategy may be influenced, for example, by the “sit and wait” hunting strategy of some raptors. Traveling in a straight line—which might seem the most efficient strategy for retrieving food—is not an adaptive foraging strategy as it allows predators to predict group movement. A study on sooty mangabeys (Cercocebus atys) found that the monkeys often avoided a straight-line approach to trees carrying fruit they forage on (Zuberbühler 2007). 

Another effect of raptor predation is the development of anti-predator behaviors. After Harpy Eagles were reintroduced to an area of Panama where they had been extinct for 50-100 years, howler monkeys quickly acquired an adaptive anti-predator response to Harpy Eagle calls. The howler monkeys from the reintroduction area had increased rates of vigilance and scanned their surroundings at a significantly higher frequency than a group of howler monkeys that had also not been exposed to Harpy Eagles in the past 50-100 years. Less systematic observations also revealed that upon hearing the calls, adult females from the reintroduction area would pick up their infants and move towards more dense areas of the canopy, while adult males would move outwards to more open locations of the canopy and often give alarm calls. Such behavior was never observed in response to other avian vocalizations. Furthermore, the howler monkeys from the reintroduction area retained an anti-predator response to Harpy Eagle calls several months after the Harpy Eagles had been removed (Gil-da-Costa et al. 2003). This rapid acquisition and retention of an anti-predator response in primates to a newly introduced avian predator shows how strongly raptor predation can influence the behavior of primates.

Raptors may even elicit anti-predator behavior in primates without regularly subjecting them to predation. Strong stereotypic anti-predator behaviors have been exhibited by diurnal lemur species on Madagascar toward birds of prey, despite the absence of regular predation by modern raptors in the area. Although 17 species of primate-eating raptors currently inhabit Madagascar, most predate upon small (weighing less than 2 kg) primates that are arboreal and nocturnal (Hart 2007). Raptors, such as Black Kites (Milvus migrans), Madagascar Harrier-Hawks (Polyboroides radiatus), and Madagascar Buzzards (Buteo brachypterus) are relatively small and would have extreme difficulty in subduing and killing an adult lemur. So why do raptors evoke strong anti-predator behavior in the lemurs? The answer to this question may lie with bird bones discovered in southwestern Madagascar and belonging to a large raptor of the genus Aquila, which is thought to have gone extinct relatively recently (only about a few thousand years ago) and that would have been capable of hunting adult lemurs. The lemurs’ anti-predator behavior toward raptors may be a retained response to predation from the extinct eagle that is reinforced by rare acts of predation from modern raptors. (Goodman 1994). This research indicates that raptor predation is a strong selective force that retains anti-predator behaviors and conserves them to a high degree.

Group composition can be altered by raptor predation as well. Coevolution between raptors and their primate prey is evident in primate polyspecific associations, which are groups of two or more species that feed, travel, or rest together (Hart and Sussman 2009). The geographical occurrence of primate polyspecific associations corresponds to regions that contain large, monkey-eating raptors. Mixed groups are regularly observed in the forests of South America, occasionally in Central America, and rarely in Madagascar and southeast Asia. In areas like Madagascar and southeast Asia, where large raptors capable of taking adult diurnal primates appear to be absent, polyspecific associations are infrequent and of short duration (Terborgh 1990). The threat of raptor predation may be the major factor responsible for driving the evolution of polyspecific life. What anti-predation benefits do these multi-species aggregations provide? First, individuals living in large groups run a smaller risk of being singled out by a predator than individuals from small groups. Second, the chance of detecting a predator likely increases as a function of group size and may result in increased individual foraging time due to a decreased need for vigilance per individual. Finally, since different species are represented, multiple males are present, which may deter some predators. Why do these primate species not simply increase their own group sizes? Competition for food is probably higher in monospecific than in polyspecific groups since mixed species associations are more likely to be composed of species that occupy separate foraging niches (Zuberbühler 2007).

Though raptors can affect primates in various ways through predation, it is important to consider that most organisms have several different predators and that they may each put different, even opposing selection pressures on a particular trait. For instance, small body size might be advantageous for escaping pursuit hunters like chimpanzees (Pan troglodytes) but disadvantageous for encounters with raptors (Treves 1999). 

Evidence for Predation on Early Hominids

The possibility of raptor predation on human ancestors is evident in one of the most studied hominid skulls ever, known as the Taung child (Australopithecus africanus). This three-and-a-half-year-old ape-like child was discovered in South Africa in 1924 and was presumed to have been killed by a predatory cat. The cause of its death remained unquestioned until 1995, when Ron Clarke and Lee Berger published an article hypothesizing that a large bird of prey was responsible for killing and collecting not only most of the fauna found at the Taung site, but the Taung child as well (Berger and Clarke 1995). This research led to much debate, including a correspondence in Nature about whether a predatory bird the size of a Crowned Hawk-Eagle (weighing on average 4.3 kg) would have the strength to lift and carry the young hominid (Hedenstrom 1995; Berger and Clarke 1996). Such a feat initially seems doubtful, as the child was estimated to weigh more than double the bird (10-12 kg). Biomechanical information about bird load-lifting capacity indicates that a raptor the size of a Crowned Hawk-Eagle would be capable of carrying prey of only up to 6.1 kg for a short anaerobic sprint or up to 1.7 kg for a sustained flapping flight (Hedenstrom 1995). Observations in the field, however, have recorded Crowned Hawk-Eagles preying successfully on large primates such as adolescent mandrills (Mandrillus sphinx) (adults reach 60 pounds) and young bonobos (Pan paniscus) (adults reach nearly 100 pounds) (Hart and Sussman 2009). Since eagles typically dismember their prey quickly and take pieces back to the nest, the ancient raptor’s load-lifting capacity might not be an issue (Science Daily 2006).

While the debate about the lifting capabilities of raptors continued, a number of scientists began studying the collecting habits of raptors around the world. Most notably, William Scott McGraw and fellow researchers from Ohio State University conducted a study examining prey remains of the Crowned Hawk-Eagle. Over 1,200 animal bones were collected from underneath 16 Crowned Hawk-Eagle nests in the Ivory Coast’s Tai rainforest. Their study revealed punctures and scratches found on many of the monkey skulls that suggest raptors might have preyed upon our human ancestors. According to McGraw, Crowned Hawk-Eagles leave very distinctive beak and talon punctures around the face and eye sockets. Gouge marks in the orbits were identified as being critical to identifying large bird of prey feeding behavior and the skull of the Taung child has these same kinds of marks (Science Daily 2006, McGraw et al. 2006). 

While the skulls alone provide evidence for raptor predation on early hominids, an examination of McGraw’s full collection of bones offers further support. Approximately half of the bones gathered from underneath Crowned-Hawk Eagle nests came from primates. While most of the bones belonged to smaller monkey species, weighing from 2.5 to 11.5 pounds as adults, a third of the bones belonged to larger species that would weigh from 13.5 to 24 pounds. Most of the larger bones were from mangabeys, which are not only the largest species in the Tai forest but which live primarily on the ground like our human ancestors. Mangabeys occur at lower densities than other monkey species in the Tai rainforest and are presumably harder to find and attack by a raptor than arboreal species. Despite this, their bones occur in nests more often than by chance alone, suggesting that Crowned Hawk-Eagles are specifically targeting these larger primates (Science Daily 2006; McGraw et al. 2006).

Raptors are not only thought to have preyed upon our primate ancestors, but to have preyed upon them often (Viegas 2010). Fossil remains of primates on Rusinga Island, Kenya from the Early Miocene (about 16 to 23 million years ago) indicate predation from both mammals and raptors. While the presence of gnawing on the ends of fossil long bones as well as multiple tooth pits along the shafts is consistent with damage produced by modern mammalian carnivores, irregular puncture marks found on fossils resemble bone damage caused by modern raptors when de-fleshing carcasses (Jenkins 2010). Considering that our human ancestors were frequently exposed to predation, we may have to alter the long-running perception that humans evolved as hunters. Robert Sussman, professor of physical anthropology at Washington University, argues that the idea of humans as hunters is a false one that arose from a Judeo-Christian ideology of humans being inherently evil and aggressive. Humans may have actually evolved as prey animals rather than as predators. Predation can affect the behavior, group structure, and body size of a species, so if humans evolved as prey rather than predator, it is possible that humans today have retained certain behaviors associated with prey status. Sussman states that features such as intelligence and cooperation present in modern humans developed from attempts of ancestors to outwit predators (Viegas 2010).

Studies show that present day raptors do affect behavior and group structure in primates and that ancient raptors likely preyed, at least occasionally, upon early hominids. While such research suggests that raptors may have been an important selective force in primate evolution, early hominids were likely exposed to various other predators, each one possibly exerting unique selection pressures. Therefore, we can view the limited fossil evidence for raptor predation, such as the Taung child, as either a sample from a series of isolated predation incidents or as a representative sample of widespread predation by raptors. Even if raptors did not prey upon early hominids often, research shows that modern primates retain strong anti-predator responses to raptors even in the absence of regular predation. Thus the threat of predation may sometimes be as powerful in modifying behavior as the act of predation itself.


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