Relationships between primates and snakes are of widespread interest from anthropological, psychological, and evolutionary perspectives, but surprisingly, little. What is the effect of chimpanzee predation on the populations of their prey animals, These parties vary in size, in relation to the abundance and distribution of the food At Gombe, nearly 40 % of the kills of colobus monkeys occur in the dry. Vertebrate predation by primates: a review of hunting patterns and prey Carpenter C.R.A field study in Siam of the behavior and social relations of the gibbon.
In the early 's, when Dr. Jane Goodall began her now famous study of the chimpanzees of Gombe National Park, Tanzania, it was thought that chimpanzees were strictly vegetarian. In fact, when Goodall first reported this behavior, many people were skeptical and claimed that meat was not a natural part of the chimpanzee diet.
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Today, hunting by chimpanzees at Gombe has been well documented Teleki ; Goodalland hunting has also been observed at most other sites in Africa where chimpanzees have been studied, including Mahale Mountains National Park Uehara et al.
At Gombe, we now know that chimpanzees may kill and eat more than small and medium sized animals such as monkeys, wild pigs and small antelopes each year. Chimpanzee society is called fission-fusion, to indicate that there is little cohesive group structure apart from mothers and their infants; instead, temporary subgroupings called parties come together and separate throughout the day.
These parties vary in size, in relation to the abundance and distribution of the food supply Wrangham and the presence of estrous females who serve as a magnet for males, Goodallso the size and membership of hunting parties vary greatly, from a single chimpanzee to as many as The hunting abilities of the party members as well as the number of hunters present can thus influence when a party hunts as well as whether it will succeed in catching a colobus.
Chimpanzee Predatory Behavior After three decades of research on the hunting behavior of chimpanzees at Gombe, we already know a great deal about their predatory patterns. We know that although chimpanzees have been recorded to eat more than 35 types of vertebrate animals Ueharathe most important vertebrate prey species in their diet is the red colobus monkey.
But Gombe chimpanzees do not select the colobus they will kill randomly; infant and juvenile colobus are caught in greater proportion than their availability Stanford et al.
Females also hunt, though more often they receive a share of meat from the male who either captured the meat or stole it from the captor.
Although lone chimpanzees, both male and female, sometimes hunt by themselves, most hunts are social. In other species of hunting animals, cooperation among hunters may lead to greater success rates, thus promoting the evolution of cooperative behavior. Such cooperation has also been posited as important in our own evolution Washburn and Lancaster In both Gombe and in the Tai forest in the Ivory Coast, there is a strong positive relationship between the number of hunters and the odds of a successful hunt Boesch and Boesch ; Stanford et al.
At Tai, Christophe Boesch has documented highly cooperative hunting behavior by the chimpanzees there, and meat-sharing behavior after a kill that rewards those chimps who participated in the hunt.
One of the main recent findings about hunting by chimpanzees was its seasonality Stanford et al. This is apparently a time of food shortage in the forest, since the chimpanzees' body weights do decline Wrangham Why would chimpanzees hunt more often in some months than in others?
This is an important question, because studies of early hominid diets have shown that meat-eating occurred most often in the dry season, at the same time that meat-eating peaks among Gombe chimpanzees Speth And the amount of meat eaten, even though it composed a small percentage of the chimpanzee diet, is substantial.
I estimate that in some years, the 45 chimpanzees of the main study community at Gombe kill and consume more than pounds of prey animals of all species. This is far more than most previous estimates of the weight of live animals eaten by chimpanzees.
A large proportion of this amount is eaten in the dry season months of August and September. In fact, during the peak dry season months, the estimated per capita meat intake is about 65 grams of meat per day for each adult chimpanzee. This approaches the meat intake by the members of some human foraging societies in the lean months of the year.
Chimpanzee dietary strategies may thus approximate those of human hunter-gatherers to a greater degree than we had imagined.
Several other aspects of hunting by Gombe chimpanzees are noteworthy. First, although most successful hunts result in a kill of a single colobus monkey, in some hunts from colobus may be killed. The likelihood of such a multiple kill is tied directly to the number of hunters in the hunting party.
Interestingly, the percentage of kills that are multiple kills has rose markedly in the late 's and early 's, which in turn meant that many more colobus overall were being eaten in the late 's compared to five years earlier Stanford et al. This is most likely due to changes in the age and sex composition of the chimpanzee community.
The number of adult and adolescent male chimpanzees in the study community rose from 5 to 12 over the 's, due to a large number of young males who were maturing and taking their places in hunting parties. One could therefore say that the fate of the Gombe red colobus monkeys is in the hands of the chimpanzee population; this is reflected in the colobus mortality rate in relation to the number of hunters available in a given era.
Throughout her years of research, Jane Goodall has noted that the Gombe chimpanzees tend to go on "hunting crazes," during which they would hunt almost daily and kill large numbers of monkeys and other prey Goodall The explanation for such binges has always been unclear.
My own research has focused on the causes for such spurts in hunting frequency, with unexpected results. The explanation for sudden changes in frequency seems to be related to whatever factors promote hunting itself; when such factors are present to a high degree or for an extended period of time, frequent hunting occurs.
For example, the most intense hunting binge we have seen occurred in the dry season of From late June through early September, a period of 68 days, the chimpanzees were observed to kill 71 colobus monkeys in 47 hunts. It is important to note that this is the observed total, and the actual total of kills that includes hunts at which no human observer was present may be one-third greater.
Primate Groups in Relation to Predation
To try to solve the binge question my colleagues and I examined the database of hunts recorded over the past decade to see what social or environmental factors coincided with hunting binges. Knowing that hunting was seasonal helped, in that I expected binges to occur mainly in the dry season, and this proved to be the case.
But other interesting correlations leapt out as well. Periods of intense hunting tended to be times when the size of chimpanzee foraging parties was very large; this corresponded to the direct relationship between party size and both hunting frequency and success rate. Additionally, hunting binges occurred especially when there were females chimpanzees with sexual swellings the large pink anogenital swellings that females exhibit during their periods of sexual receptivity, or estrus travelling with the hunting party.
When one or more swollen females was present, the odds of a hunt occurring were substantially greater, independent of other factors Stanford et al. This co-occurrence of party size, presence of swollen females and hunting frequency led me to ask the basic question, "why do chimpanzees hunt?
Among the great apes the gorilla,the orangutan,the bonobo, and the chimpanzee and ourselves, only humans and chimpanzees hunt and eat meat on a frequent basis.
Since neither humans or chimpanzees are truly carnivorous - most traditional human societies eat a diet made up mostly of plant foods - we are considered omnivores. The important decisions about what to eat and when to eat it should therefore be based on the nutritional costs and benefits of obtaining that food compared to the essential nutrients that the food provides. However, as I discussed above, there are social influences such as party size and composition that seem to play an important in meditating hunting behavior as well.
Understanding when and why chimpanzees should choose to undertake a hunt of colobus monkeys rather than simply continue to forage for fruits and leaves, even though the hunt involves risk of injury from colobus canine teeth and a substantial risk of failure to catch anything, has been a major goal of my research see also Stanford b, In his study of Gombe chimpanzee predatory behavior in the 's, Geza Teleki considered hunting to have a strong social basis Teleki Some early researchers had said that hunting by chimpanzees might be a form of social display, in which a male chimp tries to show his prowess to other members of the community Kortlandt In the 's, Richard Wrangham conducted the first systematic study of chimpanzee behavioral ecology at Gombe and concluded that predation by chimps was nutritionally based, but that some aspects of the behavior were not well explained by nutritional needs alone Wrangham Toshisada Nishida and his colleagues in the Mahale Mountains chimpanzee research project reported that the alpha there, Ntilogi, used captured meat as a political tool to withhold from rivals and dole out to allies Nishida et al.
And William McGrew has shown that those female Gombe chimps who receive generous shares of meat after a kill have more surviving offspring, indicating a reproductive benefit tied to meat-eating.
My own preconception was that hunting must be nutritionally based. After all, meat from monkeys and other prey would be a package of protein, fat and calories hard to equal from any plant food. Forty million years ago, simians from Africa migrated to South America, presumably by drifting on debris, and gave rise to the New World monkeys.
Twenty five million years ago the remaining Old World simians catarrhines split into apes and Old World monkeys. Common names for the simians are the Old World baboonsmacaquesgibbonsand great apes ; and the New World capuchinshowlers and squirrel monkeys.
Primates are characterized by large brains relative to body size compared to other mammals, as well as an increased reliance on visual acuity at the expense of the sense of smellwhich is the dominant sensory system in most mammals.
The Predatory Behavior and Ecology of Wild Chimpanzees
These features are more developed in monkeys and apes, and noticeably less so in lorises and lemurs. Trichromatismthree independent channels for conveying color information, is present in some primates.
Except for apes, primates have tails. Most primates also have opposable thumbs. Many species are sexually dimorphic ; differences may include muscle mass, fat distribution, pelvic width, canine tooth size, hair distribution, and coloration. Primates have slower rates of development than other similarly sized mammals, reach maturity later, and have longer lifespans.
Depending on the species, adults may live in solitude, in mated pairs, or in groups of up to hundreds of members. Some primates, including some great apes gorillas and humans and baboons, are primarily terrestrial rather than arboreal ; but all species possess adaptations for climbing trees. Arboreal locomotion techniques used include leaping from tree to tree, and swinging between branches of trees brachiation ; terresterial locomotion techniques include walking on two limbs bipedalism and modified walking on four limbs knuckle-walking.