To say that primates in general—including humans—are fearful of snakes is a generalization that just might be provable through statistical analysis. Avoidance of snakes is a well-documented behavior in nonhuman primates, although, in seemingly perverse behavior, primates sometimes approach snakes out of sheer curiosity.4 Innate avoidance of snakes by primates would presuppose some evolutionary relationship with snakes, either as predators or as dangerous co-inhabitants of tropical and temperate territories.
Experiences of a personal nature always hit home more than any number of descriptions, and any up-close-and-personal encounters with large and dangerous snakes in the wild are especially memorable. While studying primates in the forest, you spend most of the time looking into trees and not watching where you put your feet. You get used to stepping on or tripping over exposed roots of large trees, fallen branches, rocks, and so on. One of us (RWS), while studying lemurs in Madagascar, had the shock that often lurks just around the corner for all field researchers. Stepping on a large branch, I felt it begin to move. Letting out a scream, I nearly jumped out of my shoes. I had stepped not on a branch, obviously, but on an extremely large Malagasy boa. Knowing the snake was not poisonous (there are no venomous snakes in Madagascar) was comforting, but it's still a major shock when the ground moves under your feet and you look down on a truly massive snake.
It's possibly even more shocking to inadvertently encounter a deadly snake, such as a viper, as it lies in ambush. When doing primate research in most forests of Africa, Asia, and the New World, where poisonous snakes are common, one must constantly be aware of their presence and be prepared to avoid attack. It's also necessary to have a precautionary plan in case someone is bitten. In most regions where primates are studied, poisonous snakes are the most dangerous things one is likely to encounter during research.
While doing a census of primates at the Costa Rican forest reserve of La Selva, a student and I were walking on one of the trails when we came face to face with a 10-foot-long fer-de-lance. These are among the most feared and dangerous snakes of the Neotropics and are much more inclined to bite than any other large viper. From a Western perspective, compared to other snakes, they also are extremely frightening looking. (While hating to fall into the anthropomorphic trap, it's hard to describe these flat-headed, snub-nosed snakes as anything but downright ugly.) The snake was coming toward us on the path. Needing to get past the snake and continue in the same direction, we finally decided that I would try going into the forest and walking around the fer-de-lance while the student monitored the snake's movements. After entering the dense underbrush and starting to make some progress around the snake, I heard shouting that the snake was moving. Luckily, after an initial adrenaline rush, the student added that the snake was moving away from me into the forest on the other side of the path. The incident was so disturbing that no one on the research team returned to the forest for the rest of the day.
The little vignette above could be interpreted in a couple of different ways: (1) those were truly timorous researchers to shut down operations for the rest of the day because of an encounter with a snake, or (2) we have stumbled onto a fascinating topic—is there an instinctive and/or learned fear of snakes in human and non-human primates?
There's scientific support for theories that avoidance of snakes is instinctive behavior in primates based upon studies of chacma baboons, squirrel monkeys, and several species of tamarins and marmosets. But, there's also research that supports the theory that fear and avoidance of snakes are learned behaviors; this side of the debate is based upon studies of squirrel monkeys, macaques, and lemurs. So far, neither side has proven their point conclusively.5
The controlled study of human fear or avoidance of snakes has not been a prolific area of research. In 1928 two psychologists, Harold and Mary Jones, published their research entitled "Maturation and Emotion: Fear of Snakes," in the journal Childhood Education. They found that children up to the age of 2 years had no fear of a harmless 6-foot-long North American snake nor of a small boa constrictor, but by the age of 3, children showed caution around the snakes, and definite fear was apparent after the age of 4. Another study in the 1960s asked children to identify their own fears. Fifty percent of the 467 children aged 5—12 years in this study picked animals as the most feared objects, and the most frequently cited animals were snakes.6 From this small sampling, we must conclude that no definitive answer regarding "nature versus nurture" is available in regard to human or non-human primate avoidance of snakes, but the very fact that any evidence exists for a comfortable curiosity in very young children exposed to large snakes makes a hardwired primate fear response highly implausible.
With or without innate fear . . . with or without learned fear . . . the relevant question we want to address remains: Does any evidence exist that snakes were predators on early hominids?
Because snakes swallow their prey whole, we need only consider snakes that are large enough (that is, with a large enough jaw gape) to accommodate a meal the size of one of our ancestral hominids. Clifford Pope, one of the first herpetologists to write factually about the giant snakes for the general public, described the prey of the pythons and boas: "Almost any not too formidable creature weighing less than 125 pounds is a potential victim, horns, armor, and spines notwithstanding."7 An extinct hominid the size of "Lucy," the famous australopithecine fossil—estimated when alive to have stood 3 and one-half feet tall and to have weighed approximately 60 pounds—would have posed no problem for the ancient relatives of today's so-called giant snakes to consume.
Six species, to be exact, earn the gigantic designation: the African python, the Indian python, the Asian reticulated python, the amethystine python of Australia, the South American boa constrictor, and the South American anaconda. These are labeled "giant snakes" for obvious
reasons. Measuring a maximum of 18.5 feet, the boa constrictor is the peewee of the group. The other five species are truly large—up to a maximum of 33 feet—the longest terrestrial animals alive today.
The weights of giant snakes are less known than their lengths due to the logistical challenge of getting an extremely long snake to cooperate by lying coiled on a scale. (This is why photographs you see of mammoth anacondas or pythons are posed with five or six people holding up different sections of the snake.) Two dependable estimated weights found in the published literature are pretty impressive—200 pounds for a 19-foot Indian python and 305 pounds for a 25-foot reticulated python.8
Depending on their sense of smell and, to some degree, heat sensitivity, the giant snakes are able to hunt prey both nocturnally and during the day. They are also just as at home in water as they are on land. They seek mammalian food, and they are able to swallow prey as large as their own weight (one huge meal may equal 400 times their daily energy needs). One of the most common misunderstandings is that giant snakes crush their victims. Not true! Clifford Pope explained: "After seizing its prey in vise-like jaws, the serpent coils its body about its opponent and squeezes. The muscular pressure is sufficient to keep the chest [of the prey] from expanding and thus halt the ability to breathe. Bone-breaking exertion, therefore, is unnecessary."9 The mouth—indeed, the whole digestive system—of a giant snake is intricately modified to facilitate consumption of colossal food items. Approximately 50 needle-sharp curved teeth flare backward on both the upper and lower jaws, and curved teeth even appear on the palate. A well-aimed strike is as hard as a blow from a hammer; the force of the strike allows the curved teeth to penetrate deeply, resulting in attenuated, jagged lacerations.10 A python or boa's jaw hinge is situated at the farthest point on the back of the skull, which allows immense flexibility for its gigantic gape; a gape that allows prey much larger than the snake's head to be swallowed whole. Once swallowed, an extendible, spacious gut receives the huge meal.
Pythons, boas, and anacondas rely on a combination of sensory inputs to locate their food. These include vision, vibration, heat, and odor.11 Many other reptilian predators hunt by speculation, wandering around until they come within range of suitable prey; but among snakes, in particular, success in catching prey involves inconspicuously lying in wait for it to approach.12
Because they eat mostly active animals, giant snakes hunt mainly by sit-and-wait ambush, although at night reticulated pythons will search actively for prey. One boa constrictor under study in Panama entered a different medium-sized mammal burrow every 3 or 4 days, waiting up to 96 hours for prey to approach within striking distance. When prey is encountered, these snakes will usually retract their head and neck, then rapidly strike, immediately immobilizing the victim by constriction, followed by swallowing. Giant snakes eat a wide variety of vertebrates and, generally, this leads them to be very opportunistic in their choice of prey, taking advantage of almost any potential prey species of an appropriate size. Adult Asian, African, and Australian pythons, and anacondas and boas of the Neotropics, are restricted to terrestrial or aquatic habitats because their heavy weight precludes arboreality.13
All of the giant snakes have an especially frightening mystique, and all are reputed to be possibly dangerous, but better documentation concerning current and historical human predation exists for the super-sized reticulated python found in Southeast Asia, a giant snake that most authorities credit with being the longest and either the heaviest or at least the second-heaviest in the world. This species in adulthood reaches lengths exceeding 25—30 feet (females are larger than males, and adults continue to grow throughout their lives), and a 15-foot python is considered small.14
During the early twentieth century, on the Moluccan Islands off Indonesia, a 14-year-old boy was killed and swallowed by one of the smaller reticulated pythons. In the same area, in 1926, an adult man was seized by a python and squeezed to death. The snake was killed while still coiled around its meal, although it had not yet ingested the man. On the island of Sumatra, again in 1926, a 15-foot python dragged its victim— an adult man—into the forest by keeping its tail wrapped firmly around the reputedly shattered skull. And in Singapore, in 1937, 15 men were required to release a man from the clutches of a python measuring 22.5 feet in length.15
More recent reports tend to smack of tabloid sensationalism—as, of course, might the stories above. Two internet websites—Man-Eating Snakes I and Man-Eating Snakes II—actually exist to discredit tabloid stories by debunking doctored photographs, such as one showing a super-tremendous bulge in a snake's middle (a very obese human?), and another showing a human corpse supposedly liberated from a snake's stomach (still wearing—considering the situation—relatively unwrinkled clothing). Another "new" photo that hit the front pages in newspapers all over the world showed a "recent" incident of a man swallowed by a python; the photo had actually been snapped when Japanese soldiers were invading Southeast Asia during WWII.16
Man-Eating Snakes I website does document eight confirmed cases of human deaths in recent years caused by African, Burmese, and reticulated pythons in captivity. In none of these cases did the snake attempt to eat the human it had smothered by constriction.
Also substantiated are several incidents that occurred in the past two decades, incidents investigated by credible sources who collected copious details. On Thursday, 22 November 1979, in Northern Transvaal, South Africa, a teenage cattle herder was grabbed and killed by an African python lying in ambush in long grass. The snake was entwined around the corpse when a hail of stones from tribal elders caused the python to release the body. Dr. Bill Branch, one of South Africa's most respected herpetologists, determined the following about this attack:
The case reported here ... is almost certainly a true feeding attack, and indeed there is every indication that the snake would have continued swallowing the boy had it not been disturbed. Pythons are known to catch antelope, etc., by lying in ambush by the sides of game paths. The close proximity of the cattle being herded by the victim, and his sudden arrival as he ran along the path, probably initiated an instinctive feeding reflex in the python.17
Dr. Branch and his associates reviewed several other events that seemed to constitute human predation by African pythons. One occurred in 1973 in Angola; a photograph documents the human body encased in the digestive tract of a python. There is no doubt that ingestion occurred, but whether the adult male human was alive or dead at the time his body was swallowed remains in question, since there was a bloody civil war going on and a battle casualty might have been scavenged by a hungry snake.18
So far we've been discussing giant-snake predation on adult humans, but as one university professor from Brazil reminded us, there are frequent stories about anacondas taking human babies from hammocks in rural areas.19 An incident reported from northern Australia involved a traumatizing event; a mother reached into the crib for her baby and found a python (likely amethystine) beginning to encase the infant's head with its jaws.20 Stories of reticulated pythons in Southeast Asia consuming infants and children are almost too numerous to recount individually. Suffice it to say that "giant snake eats baby" is either true or one of the most frequent urban (in this case substitute rural) myths throughout the tropics.
The fossil record for snakes is scarce and, unfortunately, there's not much that can be inferred from it about the relationship between any of the large snakes as predators and early hominids as prey. One related fact we can state unequivocally is that snakes—the latest of the reptilian groups—began radiating into many ecological niches at the same time that mammals diversified. The evolution of snakes is seen by paleontologists as closely tied to a predator—prey relationship with mammals. Early members of the suborder Serpentes were relatively large terrestrial carnivores in the process of evolving the present adaptation of jaw mobility that enables them to swallow their prey whole.21
While snakes are the most recent of the reptilian groups to evolve, the boid family—the pythons, boas, and anacondas—are most similar to the primitive snakes. The oldest known snakes are from the Eocene epoch (approximately 35—55 million years ago). These early specimens were large and stout; boas and pythons seem to be their little-modified descendants.22
John Murphy and Robert Henderson in Tales of Giant Snakes discuss the tantalizing and oft-touted idea of ancient "supersnakes," but they conclude that if prehistoric supersnakes existed, paleontologists have yet to find any evidence. In reality the long and bulky ancient snakes that have been found in the fossil record from the Paleocene to the Pleistocene epochs are smaller (measuring a maximum of 27 feet) than some living giant snakes of today!
Another window to the relationship between giant snakes and early hominids is the predator—prey relationship between these reptiles and living non-human primates. Our research into predation on primates found that reptiles are recorded in low numbers of unsuccessful attacks (2.5% of the total), verified predations (4%), and suspected predations (about 13%). It's our opinion, nonetheless, based on discussions with herpetologists, that these low percentages are more attributable to a lack of fieldwork on snakes in Africa, Madagascar, Asia, and the Neotropics than on any tendency for snakes to avoid consuming primates. Despite the paucity of research, we could still identify both the African python and the reticulated python as "primate specialists," meaning these two giant-snake species appeared repeatedly in our data sets when we looked at both anecdotal evidence and quantitative data.23
While the field studies on all tropical snakes are few and far between, making for sparse data on the interactions between primates and these predators, nevertheless, the record of snake predation is replete with descriptive anecdotes. If we start at the beginning of the written accounts of primates as prey to snakes, we must go back to the late nineteenth century for the first published record. A short report by Mr. O. Channer, entitled "The Food of the Python," appeared in the Journal of Bombay Natural History Society in 1895. Illustrated by a masterful woodcut, the entry briefly describes the capture and strangulation of a Hanuman lan-gur (the sacred monkey of India) by an Indian python.
Baboons captured by African pythons were also observed and re-ported.24 One field naturalist in the 1960s recorded the entire sequence of just such a dramatic life and death struggle:
I witnessed a python catching a half-grown baboon. I was attracted to the scene by the noise of the baboon troop and arrived shortly after the snake had wrapped its coils round its prey. The other members of the troop crowded round the scene of the tragedy, the more adventurous amongst them occasionally darting forward and nipping the coils of the snake in a hopeless effort to frighten it and make it discard its catch.25
A 5-year study of red colobus monkeys in Abuko Nature Reserve, The Gambia, carried out by Erica Starin, revealed that predation by reptiles (African pythons and Nile crocodiles) was a major cause of mortality in adult monkeys. Based on her direct observation of kills and the examination of carcasses, 40% of known deaths were attributable to pythons and crocodiles. Subsequently, she suspected an additional 13 red colobus in the study group that had disappeared without a trace were also python victims—snake predation leaves no evidence unless the event is witnessed or the snake's stomach contents are examined. Pythons were surprisingly ubiquitous in the Abuko Nature Reserve— entering tourist photohides, eating captive primates in the reserve's zoo, and allowing close approaches by humans. Starin and an associate found that the population density of pythons over 6 feet long (considered adult at this length) was approximately 20—25 snakes in an 83-acre area. The researchers surmised that pythons at this density have considerable impact on small- and medium-sized mammals, such as primates.26
Giant snakes also consume small, nocturnal, arboreal primates as two German scientists found out in Indonesia a few years ago.27 During a study of slow loris (a nocturnal prosimian of unhurried, deliberate movements), weak signals from a radio-collared loris were traced to a patch of dense ferns on the forest floor. When these signals continued over a 3-day period from such an unlikely location for an arboreal primate, the field researchers investigated and found a reticulated python. They confirmed that the signals were being emitted from the interior of the python that had, undoubtedly, swallowed the slow loris. Using this unexpected opportunity, they radio-tracked the python for a week longer until it excreted the radio collar. However, there was no trace of the slow loris in the feces. There's a certain whimsical lesson involved in this exercise: primate researchers must be patient—it's not exactly like waiting around for paint to dry, but waiting for a python to defecate can be a leisurely operation. The other revelation is that snake predation involving small primates is nearly impossible to observe even indirectly. The predator—prey connection is revealed only if researchers are on the scene.
Being on the scene is mostly just plain luck—a case of being in the right place at the right time. Researchers in the Tampolo forest of Madagascar were alerted to an incident of snake predation by the calls of birds and lemurs. They found a bamboo lemur in the coils of a large Malagasy tree boa. The process of suffocation through constriction took 60 minutes.28
The first quantitative study of large tropical snake diets was not published until 1998. Giant snakes may not have attracted much scientific attention, but they have been, and continue to be, exploited in huge numbers by the exotic leather industry. Wild and captive pythons are routinely slaughtered at factories where their skins are tanned into leather. After the pythons were gutted in factories in the city of Palembang in southern Sumatra, Indonesia, Australian herpetologist Rick Shine examined the contents of stomachs and intestines for recognizable prey items. Of the
417 identifiable remains of food in the python alimentary tracts, less than 4% consisted of primates (11-pound long-tailed macaques and two species of leaf monkeys that weigh approximately 15 pounds).29
Any relationship between ancestral hominids and giant snakes would by virtue of geographic origin involve the Asian and African pythons rather than the New World boa constrictors and anacondas. Even today's Old World pythons have a wider variety of large terrestrial mammals— such as primates—to choose from than do anacondas and boas, since fewer large, ground-dwelling mammalian species have evolved in the Neotropics.30 Thus, the range of species and absolute numbers of primates and other medium to large mammals in the diets of Asian and African pythons can be expected to be greater than those found in the diets of the Neotropical giant snakes.
Given that fact, there are still quite a few instances of snake predation on New World primates witnessed by field personnel. Eckhard Heymann of the Deutsches Primatenzentrum in Germany witnessed an anaconda capture a moustached tamarin in northeastern Peru.31 The tamarin family under study by Heymann regularly used fallen tree trunks to cross a narrow lake. On one of these excursions the adult female was seized by the giant snake, and three coils were instantaneously thrown around the monkey. Heymann conjectured that the anaconda must have hidden directly beneath the water surface and was further obscured by aquatic vegetation. Boa constrictors are one of the predators of capuchin mon-keys,32 but these active, tool-using little Neotropical monkeys have plenty of anti-predator behaviors up their sleeves to foil snakes. They mob boas quite viciously, and one monkey was observed clubbing a venomous snake with a branch.33
We feel compelled to say just a few words, before we leave the subject of snakes, about other serpents besides the six giant species. Vipers and pit vipers, cobras and mambas, mussuranas and rat snakes have different feeding patterns than the giant constricting boid snakes. Many inject venom, an adaptive strategy that allows them to subdue and ingest very large prey.34 After cobras strike, for example, they retain an initial grip until struggling ceases; most vipers, however, bite rapidly, then release their prey, and relocate the prey after it has died. Poisonous snakes were a significant source of mortality to sleeping rhesus macaques at a research site in India.35 Again, there is a striking similarity to our primate cousins since modern humans experience 3 million snake bites each year. Snakes and primates; snakes and hominid ancestors; snakes and humans—a long-term relationship.
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