Predation Risk versus Predation Rate

We like a pithy statement from tropical ecologist John Terborgh who summed up predation rather concisely: "Successful predation is a rare event—at most it can occur only once in the lifetime of a prey."37 Whatever the overall impact of successful predation may be to the species or population, it does not modify the behavioral strategies of an individual primate living at constant risk from successful predation.38 Behavior is predicated on predation risk not predation rate because animals react to unsuccessful attacks, but they die from successful ones.

Animals strive to reduce their predation risk because it represents the sum total of all past unsuccessful encounters with predators plus their perception of the likelihood of future attack.39 Predation risk involves the entire spectrum of compensations primates must make to offset predation. Direct observation by researchers substantiates that predation risk is constant in the daily lives of primates.40, 41

Predation rate, on the other hand, involves the annual mortality of a certain percentage of a primate population due to predators. Predation rates on primates are variable, but as Connie M. Anderson, a physical anthropologist at Hartwick College, noted, "events have a powerful selective effect if the event completely eliminates an individual's genetic contribution to the next generation. ... [F]or a dead individual, any [predation] rate above zero is highly significant."42

Primate species have slow reproduction potential due to long gestation periods, birth of a single infant rather than a litter, and long

The tiny mouse lemur is the smallest primate. It can withstand a high predation rate from owls because of its unusually elevated reproductive capacity. (J. Buettner-Janusch)

intervals between births. John Terborgh postulated that predation was, of necessity, a rare event at the level of a whole primate group. He speculated, for example, that a South American capuchin troop could not sustain more than one loss to predation per year since the replacement level (the birthrate) is only one to two infants per year.43

How is it possible then that one-quarter of the little mouse lemurs in Madagascar are eaten each year by various species of owls? This 25% annual predation rate for mouse lemurs44 seems to belie Terborgh's cautionary comments. Higher predation rates are acceptable, however, if—and only if—the species' potential reproductive rate is high enough to compensate for losses to predation.45 Unlike the other prosimians, monkeys, or apes, the mouse lemur is able to sustain a predation rate of 25% because the species is rare among primates in having a high reproductive capacity to offset predation. The genus Microcebus ("tiny monkey" in Latin) has two "litters" of two infants per rainy season, resulting in an average annual production of four offspring per breeding female. Even with a 50% infant-survival rate, an average of two out of the original four babies per year survive, and this overachievement in offspring provides the mouse lemur with the ability to rapidly replace its population.46

But, you may legitimately ask, how does a discussion of mouse lemurs relate to human evolution? We find these predation rates relevant to our central Man the Hunted not Man the Hunter theme because of one highly publicized situation that is used as evidence to support conjectures that man (and it is limited to the male of our species) is a natural-born killer. The particular situation is the systematic killing sprees by male chimpanzees in Gombe National Park, Tanzania. The chimps' quarry is mainly another primate species, red colobus monkeys; the calculated pressure is a predation rate of 17—33% per year inflicted on the colobus by their chimp predators.47 The reproductive capability of Microcebus makes life with a 25% per year loss to predation possible. Red colobus monkeys do not have this high reproduction rate.

Irwin Bernstein, a highly respected primatologist, has challenged the idea that chimpanzees naturally hunt with such gusto.

The murderous Gombe chimpanzee hunters are said to have killed 30% of the red colobus monkeys each year for twenty years. Even if this was the only cause of death for red colobus, if half the animals were female and 60% were reproductive, then each female would have to have one live birth every year just to satisfy the chimpanzees.48

Without a uniquely high (and quite unprimate-like) reproductive capacity, how can such high rates of chimpanzee predation on red colobus at Gombe (up to a whopping one-third of the population killed yearly) be anything other than an aberrant situation? It is, therefore, obvious that this chimpanzee predation on monkeys is a recent and unnatural phenomenon. And, if it is aberrant and unnatural, then much of the punch is taken out of the "killer ape/man the hunter" equation.

Predation risk, not predation rate, nonetheless, is what drives an animal's anti-predation behavior. Low and high predation levels, of course, are relative terms. Nonetheless, scientists have assigned qualitative definitions to low, medium, and high predation risk: Low risk entails the presence of predators but no actual or attempted predation has been observed or suspected; medium predation risk is associated with occasional predation attempts but infrequent predation observed or suspected; high predation risk involves frequent or regular, actual or attempted, predation observed and suspected.49

Unsuccessful predation attempts—that is, the failure of predators to catch prey—underlie selection for defense behaviors. Guy Cowlishaw of University College London is one of a handful of researchers who are attempting to measure predation risk in a field setting. He constructed a field experiment to measure perceived versus actual risk in chacma baboons. He found that females in small groups are expected to be at the greatest risk from predators, but males had equal or higher mortality rates. These results suggest that females practice more anti-predator behavior, such as vigilance, and this may compensate for their susceptibility to predators due to their smaller size or pregnancy or infant-carrying. These kinds of insights into the individual and evolutionary impacts on primates from predation will come only from increasingly sophisticated field studies that emphasize the interrelationship between primate prey and their predators.50

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