Based on all that we have presented in this chapter, we came to the conclusion that very few raptor species were large or powerful enough to have influenced the evolution of hominids through predation. Until more paleontological discoveries are made, we have only the Taung child and current anecdotal observations to go on when we try to envision the effects of predation by a massive primate specialist—analogous to the crowned hawk-eagle—on early hominids. Fortunately, as with so many other aspects of human evolution, we can look at our relatives—the primates that endure intense predation from raptors—for clues. Any or all of their experiences may shed light on our past and what it meant to exist in the presence of raptor predation on our young. From our own research on non-human primates, we could see what the cumulative behaviors induced by raptors might be. For example, diving to the ground from a tree was a reaction directed only at raptors. Monkeys feeding in trees, immediately upon seeing an eagle, dropped out of the canopy and plunged earthwards to thwart predation. Additionally, about 75% of the time that primates fled through the canopy to a tree trunk, it was because raptors were present.
Intense predation has been speculated to be the cause of many behavior patterns observed in lion tamarins, a New World monkey weighing only 1 pound. Their constant state of alertness, readiness for swift flight, and descent to lower strata in the forest when sensing danger may be indicative of the large number of diurnal raptors that prey on tamarins. Early retirement to night shelters and the small diameter of shelter openings suggest that nocturnal raptors, such as the great horned owl, also put lion tamarins at risk.54
There is a proliferation of small hawks that live in Central and South American forests. Neotropical hawk species are twice as numerous as Old World species mainly because of the ubiquitous small-sized forest falcons of the genus Micrastur. The same pattern of long tail—short wing proportions found in crowned hawk-eagles and harpy eagles is repeated in smaller genera of forest—hunting New World raptors. These include, to name a few, bicolored hawks, grey hawks, roadside hawks, black hawk-eagles, ornate hawk-eagles, Isidor's hawk-eagles, barred forest falcons, slaty-backed forest falcons, and collared forest falcons. The hunting technique of small rain-forest hawks combines an interesting mix of active and inactive behaviors: they sit motionless and inconspicuous, but intersperse the inactivity with occasional swift and soundless flights from tree to tree. Some species pursue active hunting, such as the collared forest falcon, which actually runs along branches, through thickets, and even on the ground in pursuit of prey!55
Field researcher Sue Boinski saw this species make 29 predation attempts on newborn squirrel monkeys at Corcovado, Costa Rica. Squirrel monkeys live in large social groups averaging 20—75 individuals, although groups of more than 300 have been reported. Pregnant females give birth in synchrony, perhaps as an adaptation to the heavy predation by raptors. Predatory birds swoop through and around the groups of new mothers, trying to pluck newborn infants off the females' body. At the same time the collared forest falcons were preying on newborns, Boinski also saw grey hawks and roadside hawks make attempts. Additionally, chestnut-mandibled toucans joined in the melee and tried to scoop newborn squirrel monkeys up with their larger-than-life beaks. (Boinski reported that successful attacks on newborns were accomplished by the toucans and an ornate hawk-eagle, while a collared forest falcon killed one of the adult female monkeys.)56
This high level of predation pressure by raptors on small Neotropical primates is supported by other studies. John Terborgh of Duke University recorded one or more raptor attacks at Manu National Park in
Peru in almost all 3-week periods into which his study was divided. The attacks were more frequent against squirrel monkeys and tamarins than against larger primates, such as capuchin monkeys. In another study, also at Manu, tamarin groups underwent attacks by raptors about once every 1 or 2 weeks. In Brazilian Amazonia, raptors attacked a mixed group of two species of tamarins at the rate of about once every 9 days. Other researchers calculated that diurnal raptors attacked squirrel monkeys at Corcovado and at Manu at the rate of one attack every 6 or 7 days.57
Since ornithologists estimate that as a general rule birds of prey launch more successful than unsuccessful attacks58 (although, of course, researchers cannot always see the outcome of the event), based on the sightings of attacks listed above, it is our assumption that predation pressure on Neotropical primates from predatory birds may be even higher than field records indicate.
Besides the crowned hawk-eagle and the harpy eagle, which are able to navigate so elegantly through thick forest, there are many large eagles that hunt primates by scanning for prey on the ground from a perch or while soaring. In South Africa black eagles are known to prey on baboons and vervets, but it was unclear whether the capture of an animal as large as a baboon was just an anomaly. That question was answered when documentation was published of this same huge eagle attacking hamadryas baboons in the Eritrean highlands four times in 4 days.59
Madagascar harrier hawks have special morphological adaptations of the legs that enhance their ability to hunt primates. The bones in their legs are articulated in a fashion that allows them to bend and probe into narrow tree cavities and behind tree bark to find resting mouse lemurs during the day.60
Barn owls, along with the Madagascar red owl and the Madagascar long-eared owl, are also predators of small, nocturnal prosimians, especially the mouse lemur. Until research carried out by Steve Goodman, at Chicago's Field Museum of Natural History, no one guessed that the geographically widespread barn owl was such a major predator on primates. Goodman's breakthrough field study has been especially instrumental in defining the scope of owl predation on lemurs. The barn owl is the most common and widely distributed nocturnal raptor in Madagascar. It has unparalleled acoustic ability and is able to detect, locate, and catch prey in utter darkness. Mouse lemurs are small, solitary, nocturnal foragers that are particularly vulnerable to barn owl predation.
How do you tell exactly what an owl has eaten? Well, they provide a very reliable tool to ascertain the composition of their diet. At regular periods (for example, every 2 or 3 days) owls regurgitate a pellet that is a compacted accumulation of undigested bones, hair, or fur. Because they swallow their prey whole (rather than tearing it apart as do the diurnal raptors), the pellets of owls are particularly handy for the identification of prey. At one nest of Madagascar long-eared owls, 50% of the prey identified from pellets were sportive lemurs. Mouse lemurs constituted a range of 17—44% of the prey killed by pairs of Madagascar long-eared owls when four other nests were evaluated. Goodman's conservative calculation, when the combined predation pressure on mouse lemurs at Beza Mahafaly Reserve by barn owls and long-eared owls was considered, estimated a removal rate of one-quarter of the mouse lemur population each year.61
What about large-owl species that might be able to prey on small or young hominids? Verreaux's eagle owl, a huge nocturnal African bird, has two hunting strategies that depend on the age of the individual owl. Adults sit singly on an elevated perch with a wide field of vision over open ground. Subadults, however, roam more than adults, actively seeking out prey by gliding from perch to perch. Although it is the largest of the African owls, the Verreaux's eagle owl is not as powerful as the large African eagles. Nevertheless, they can catch diurnal prey up to the size of vervet monkeys, though nocturnal primates, such as bushbabies, are probably more common in their diet.62 Owls come in large sizes, but they do not have the strength of eagles. Notwithstanding their great size, their almost-artistic predatory mechanisms, and their proclivity for primates, we calculate that the influence of owls on hominid evolution was slight if any.
That is not the case with eagles in our opinion. The crowned hawk-eagle is capable of killing 66-pound prey—this is as large as Lucy, an adult female Australopithecus afarensis.
If we were to make an educated guess based on the fossil and living primate evidence, we would unhesitatingly state that eagles with the power of the present-day crowned hawk-eagle were significant predators on young hominids and may even have played a role in evolutionary adaptations that served to protect infants. On this topic we'll defer to the expert and let Leslie Brown speak in his own words:
I would now even reserve judgment on such thoroughly unlikely tales as that eagles take babies—not that it happens nowadays, but it may have happened once. I would not put it past an eagle to take and kill an infant left out in the open, perhaps uncovered and wriggling. ... A baby is not as big as a bushbuck calf or a duiker, both of which can be killed by large eagles. And though I will not nowadays believe that a swaddled baby in a pram in Switzerland will be snatched up into the air by a non-existent Lammergeier [the magnificent bearded vulture], I will accept that at some dim and distant time in human memory some such thing may have happened as a factual basis for fable.63
The list of predators on living primates numbers over a hundred species right now and probably replicates the predation pressure on early hominids—running primates down over land, ambushing them, exploding out of the water while they drink, dive-bombing them from the air. But so far we have only told the predator's story. Our hominid ancestors survived quite well as a species because all predation attempts are not successful. Our ancestors weren't passive creatures who were just waiting to be eaten—they had a few tricks of their own to evade being consumed.
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