Wow! ... all those predators waiting to eat early hominids. Considering the fact that many of the smaller primates have more predators than do large monkeys or apes, it was fortunate that our ancestors were too large to be eaten by the mongooses, civets, genets, tayras, coatis, raccoons, opossums, and little birds of prey that eat our smaller cousins.
Was increased size a primate adaptation to minimize predation? It appears that the first primates were arboreal and small and that increased size was a later adaptation.6 "Large body size may be seen as an evolutionary response to greater predation risk that has resulted in lower predation rates," noted primate researcher Lynne Isbell of the University of California—Davis.7 And, indeed, the emergence of larger arboreal primates may have been, in part, an evolutionary response to predation in the trees. Observations presented in this chapter are consistent with the hypothesis that size increase was, at least in part, an evolutionary response to predation. However, it is unlikely that predation is the sole explanation for primate species' size increases because size increase only works up to a certain point to ward off predators.
Using body-weight data on primates and predators, we explored size relationships and size increase as an anti-predation strategy. Our data set consisted of almost 2,000 instances of recorded predation events (involving over 100 different species) gathered from questionnaires and the scientific literature where both the weight of the predator and the weight of the primate prey were available.8 Non-human primates range in size from 2 ounces to nearly 400 pounds; their predators range from 2.5 ounces to more than 500 pounds. In our research we asked the question: What correlations, if any, exist between the relative sizes of primates and the animals that prey on them? Our examination of the primate weights and predator weights suggested that most of the "gain" (escape from predators through growth) is in the 2-to-12-pound weight range of primates. Below this range, there must be advantages to small size that outweigh greater vulnerability to predation (perhaps better access to fruit and insects on tiny branches or small size simply makes it easier to hide?). Above this range, there is little gain in terms of safety from predators because predators are even larger, so it seems likely that size increase above 12 pounds was driven by different rewards.
But still, smaller primates are more susceptible to predation than larger ones. After all, the limits of many predators' abilities to kill and consume prey are based on size relationships. As a result, the number of predators on primate species should decrease as body weight increases.9
There are 81 species of raptors, owls, and other predatory birds known or suspected to prey on primates.10 Many of the smaller birds weighing under 2 pounds (the toucans, small owls, kites, cuckoos, vangas, crows, small hawks, and falcons) prey on only the very smallest primates.11 A clear consequence of most arboreal primate species' evolution of increased body size is that they are too large to be attractive prey to the very predators most active in their arboreal habitats—small predatory birds. Two dozen primate species weighing less than 5 pounds were preyed upon by 32 confirmed avian predators, while 2 dozen primate species weighing over 12 pounds had half that number of winged predators.
So, yes, size increase was an advantage initially in primate evolution. But, putting it bluntly, whether hominids weighed 35 pounds or 135 pounds, they were still prey to the large predators. Primates, including our ancestors, could outgrow most birds of prey (except the crowned hawk-eagle types who can kill prey many times their own weight), but that only eliminated a fraction of the potential attackers.
Unlike avian predators, most carnivores outweigh the majority of primate species, yet the weight of mammalian predators varies considerably by group. The weights of what we term "small carnivores"—mongooses, civets, genets, tayras, coatis—are lower than many primates. The single species in this category outweighing many primates is the fossa, the largest predator in Madagascar. Our research showed that many arboreal primates have outgrown the small, carnivorous mammals and are protected from them by increased size.
Few primate species are larger than wild cats, and even the largest primates—gorillas—are within the size range of a leopard attack. In fact we calculated that 94% of wild-cat species are larger than all but the top 10% of primates, plus cats are able to kill prey larger than themselves. Hominids have been in the past (and are right now) within a very suitable size range for a variety of cats, both extinct and living. The leopard is an illustration of this; a yummy 100-to-120-pound hominid is the perfect size for a leopard to attack, kill, and pull up onto a tree branch to eat.
Similarly, very few primates are protected from wild dog and hyena predation by size alone. The distribution of weights for dog species and hyenas is similar to cats: 85% of the dogs or hyenas are larger than all but the top 10% of primates, plus these fellows often hunt in packs.
The distribution of reptile weights is also similar to wild cats and dogs in the sense that reptilian predators include extremely large animals. All snakes are possible tree climbers; however, only the smaller snakes (those under 6 feet in length) are truly arboreal. Furthermore, most snakes, like most raptors, tend to hunt prey significantly smaller than themselves. There is a very good reason for this size limit: snakes are inhibited in their intake of large prey both because they must swallow their prey whole and because "satiated immobilization" may occur after ingestion of animals above a certain ratio to the snake's own weight. The snake can literally be so stuffed it cannot move. Most primate species are too large to be attractive prey to the smaller snake species that hunt arbore-ally, but large terrestrial snakes—for example, the reticulated and African pythons—are major predators of terrestrial primates.12 Most primate species are also within the size range of monitor lizards, and all are within the size range for crocodiles.
Habitat is a critical consideration for any discussion concerning size of primates and their predators. There are differing theories concerning the effects and interplay of habitat and body size. One theory contends that terrestrial primates are more subject to predation than arboreal species because not only are terrestrial species at risk from raptors, snakes, arboreal carnivores, and large terrestrial carnivores in open territory, but they are also far from trees where they might run for safety.13
Arboreality, in and of itself, has been hypothesized to confer some protection from predation because all primates (with the exception of gorillas, the largest species, and modern humans) sleep in trees, cliffs, or off the ground in some manner.14
To summarize, examining the size relationships of primates and predators with arboreality in mind suggests that primates can avoid many arboreal predators—but few terrestrial predators—by being larger than they are. While the number of total predators will decrease, results of data analysis show that size increase may confer protection for primates only within some groups of predators. There is a clear tendency for primate species to be larger than most arboreal birds and small carnivores but smaller than most terrestrial predator species. The evolution of larger body size in arboreal primates may have conferred some protection from raptors and small carnivores, but seems to have had little effect on the levels of predation inflicted by cats, hyenas, dogs, or reptiles on primates living in the trees or on the ground.
Obviously, hominids did not outgrow any predators but raptors and small carnivores. Increased size does not seem to be the route we took to avoid predation. We got bigger over time—the famous australopithecine fossil, "Lucy," weighed only 60 pounds and may have stood only 3 and one-half feet tall, fully grown—but our subsequent size increase didn't do us much good predator-wise.
And before you can say, "Wait a minute! Lucy was a female!" we'll offer a little information about size differences between male and female apes and fossil hominids. Sexual dimorphism is a state of dissimilar size between the male and female of a species. Adult male animals are usually the larger, but there are many exceptions: hyenas, raptors, some of the civets and genets, and the large pythons are species in which females may be significantly larger. In primates, though, either the sexes are nearly the same size or the adult male is the larger. If the adult male primate (or hominid) is bigger than the female, does he play a special role as a defender? It's tough to give a definitive answer to that question. Alison Jolly, one of the most respected American primatologists, has proposed that there are two types of primates where defense is concerned: those in which the male seeks to lure the predator away while the females and young hide (an example is the patas monkey) and those in which the females and young draw in toward the males at the sign of danger (example, baboons).15 It probably doesn't much matter that the male patas is larger than the female—his job is ultimately to get eaten instead of her. In the case of baboons, there is much dispute over whether adult males position themselves to protect the troop or whether they run helter-skelter away from predators, leaving the females to protect themselves and their infants. Great deeds of individual chivalry have been noted in male primates, but the jury is still out on whether sexual dimorphism is an anti-predator device or just an attractive contrivance to seduce females.
From fossil finds, we know that Lucy's male counterparts could have been much larger than she. To further complicate the issue, Lucy, as an individual, is at the low end of the unearthed specimens of Australopithecus afarensis; those at the high end were individuals 5 feet tall who could have weighed much more than 100 pounds. There is considerable variation both between the sexes and between individuals of the same sex in fossils of Lucy's kind.16 Besides australopithecines, sexual dimorphism appears in gorillas, chimpanzees, and orangutans; it is not found, however, in bonobos, the pygmy chimps.
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