Trailing the mythical anaconda.
Many of my fondest memories of Venezuela have anacondas in them. I can vividly recall my first encounter with a sizable anaconda. It was in the afternoon of a clear, hot day. I was walking along a raised dike road through a flooded savanna with a group of biologists, including my future wife, Ximena. Peering between the trees that lined the road, we spotted a large, coiled serpent basking on a small island, a raised knoll about 160 feet from the edge of the dike. As good biologists, we decided to catch the animal and measure it. Four of us waded out through knee-deep water and, as we got closer and saw just how large the snake was, we began to question the wisdom of our decision. Nevertheless, we persisted and soon were wrestling with a large, writhing snake on what turned out to be a rotten log full of stinging fire ants. The ants, ironically, became a much greater problem than the snake and kept stinging us long after we had the anaconda under control. Triumphantly, we carried our prize back to shore where the rest of the group awaited. It was the first anaconda any of us had ever gotten a good look at, but after a few minutes of close inspection, I realized that we had nothing with which to measure the snake. Following a short discussion, I took off my shoe and measured the snake at thirteen-and-a-half sock lengths, which, after careful calibration of my sock, turned out to be about sixteen feet. I think Ximena remained forever impressed by my ingenuity on that occasion.
For the most part small and secretive, snakes as a group have been spectacularly successful. With the exception of some of the world's colder climes, they can be found almost anywhere, from tropical seas to desert sagebrush. Yet there are not a great many people who would find such an encounter as ours pleasurable, as there are few other creatures that inspire such fear and loathing as these legless reptiles. The vast majority of snakes, however, are harmless to people, and many are even beneficial because of their predilection for eating rodents. Contrary to popular belief, snakes are not slimy, and keep to themselves as much as possible. In many ways, though, we are also drawn to snakes; they fascinate us in a way that few other animals do. For example, certain kinds of snakes - venomous ones such as rattlesnakes and cobras - quickly come to mind because of the potential threat, however small, they represent; but it is the giant snakes of the world - the pythons, boa constrictors, and anacondas - that seem to enthrall us the most.
A large anaconda is indeed a striking creature to behold. Its head, with a prominent red stripe, is dwarfed in comparison to the improbable bulk of the body, with its glossy skin and bold, black markings on an olive-yellow background. The anaconda's sheer size and bizarre proportions invite incredulous stares when seen behind the glass cages of zoos or wildlife parks, and it is not hard to understand why some encounters with free-living anacondas in the remote backwaters of South America have led to stories of animals of mythical proportions.
Perhaps more than any other snake, stories about anacondas have been subject to wild exaggeration and hyperbole. Padre Gumilla, a Jesuit priest who lived in the llanos of Venezuela alluring the seventeenth century, wrote that the anaconda hypnotized and captured its prey using invisible poisonous vapors that issued from its mouth. More recently, Dr. James Oliver, former curator of reptiles at the New York Zoological Society, related an account of a 1948 newspaper story about a 156-feet-long anaconda battling army soldiers and even knocking over buildings!
There are actually two different species of anacondas. The northern species, referred to as the green anaconda (Eunectes murinus) is widely distributed throughout the lowland regions of northern and central South America in the Amazon and the Orinoco river basins. The smaller yellow anaconda (Eunectes notaeus) is found in the Paraguay River drainage of northern Argentina, southern Brazil, and Paraguay. Known by many names, culebra de agua or madre de agua in Venezuela, buhio in Colombia, camudi in Guyana, and sucuri in Brazil, the name anaconda has an uncertain origin, but may have come from a Sinhalese word, henakandaya, used for pythons in Ceylon (now Sri Lanka) and brought to the New World by Portuguese traders. Although considerable debate rages over their maximum size, green anacondas are generally considered to be the world's largest snake, with reported lengths in excess of thirty feet. The only other snake to reach such lengths is the Asian reticulated python, but the anaconda is a much more heavy-bodied, massive animal. Nevertheless, snakes of this size are few and far between. In the early part of this century, Theodore Roosevelt offered a sizable reward for anyone who could produce a live snake over thirty feet long, a reward that to this day has gone uncollected. It is hard to speak of certainties when discussing the anaconda, however, for although it has attracted considerable renown for its size, it has seldom been the subject of scientific study. Aside from a few observations on anacondas in captivity, and an abundance of unconfirmed anecdotes about their natural history, almost nothing is known about their biology, and no attempts had ever been made to study the anaconda in its natural habitat.
As a herpetologist, I had always been drawn to the image of anacondas as secretive creatures, denizens of the murky rivers and streams of South America. While researching crocodiles in Venezuela, I had always kept one eye open for anacondas, but only rarely did I ever see one. Shy, retiring creatures, anacondas seldom emerged from the opaque waters long enough to be seen. However, I soon began to hear stories about some areas in the low llanos, or flooded plains, of central Venezuela, where anacondas could be found in large numbers. One area in particular, a cattle ranch called Hato El Cedral, was mentioned by a number of people. When I returned to Venezuela after finishing my graduate degree in 1990, I began thinking seriously about working with anacondas, and I began talking with the Venezuelan Wildlife Department (Profauna) about undertaking a joint project. In 1992, with funding from the Swiss-based Convention on International Trade in Endangered Species of Flora and Fauna (CITES) and the Wildlife Conservation Society, we initiated the first study of anacondas in their natural habitat.
Hato is the term used in Venezuela for the large cattle ranches that historically were the vanguard of European settlement in the seasonal llanos region. The llanos, which stretches south into Colombia, is a huge expanse of mixed savanna and forest dissected by myriad tributaries of the Orinoco River. Comprising over 130,000 acres, ten Manhattan Islands could fit within the boundaries of El Cedral ranch. Located about 140 miles west of the town of San Fernando de Apure, the ranch is in a part of the llanos characterized by vast treeless expanses broken only by thin strips of riverine forest. On El Cedral, seasons fluctuate in the extreme. Frequent downpours flood the savannas from May through October, and during this wet season parts of the ranch can look almost like an inland sea. But beginning in November and December the rains taper off, and the plains are quickly metamorphosed into a dry, parched landscape with occasional muddy streams or drying pools of water. It is these conditions that favor finding anacondas.
In January 1992 an eclectic group assembled to embark on what would prove to be a fruitful and fascinating research program. Included were two Venezuelan students, Jesus Rivas and Maria Munoz, and, from the Wildlife Conservation Society (WCS), Bill Holmstrom, Paul Calle, and me. Holmstrom, with a lifelong interest in anacondas, is the collections manager of the Reptile Department at the Bronx Zoo, and one of the few people who has ever published observations on anaconda behavior. Calle is on the staff of WCS's Animal Health Department and had traveled to Venezuela as part of the field veterinary program that assists WCS's wildlife conservation projects.
The ranch supports a spectacular assortment of wildlife. During the wet season, many of the animals are thinly scattered across the savannas, but as the rains begin to dissipate and the waters recede, most animals concentrate in and around the drying pools. Although I have visited many wild areas in South America, on my first visit to the ranch I was taken aback by the sheer abundance of birds, mammals, and reptiles that can be seen around El Cedral's dry-season waterholes. Multicolored blankets of wading birds stretched across the still-moist pockets of savanna, while herds of capybaras, the world's largest rodents, were so numerous and tame that driving along the ranch's few roads was like being on an obstacle course. Crocodile-like caimans concentrated in dry-season pools almost gave credence to the old tales of being able to walk from one shore to the other by stepping on their backs. Fishing became as simple as dangling a baited hook in the water for a few seconds until it was gobbled up by piranhas.
In the mud and muck around these pools is where we find anacondas. But even big anacondas are surprisingly cryptic, and it frequently takes a practiced eye to spot one as it lies buried in mud, or hidden under grass or floating vegetation. Sometimes the only hint of the serpent's presence is a small raised ridge in the poolside mire, or the telltale tip of its nostrils protruding through the mud or vegetation. Other times, especially when they move overland between pools of water, anacondas can be seen from afar as we cruise the roads with spotters sitting atop our Land Cruiser.
The study's success depended on our finding and capturing a large number of snakes. By marking and releasing snakes, we could learn a lot about the size and the composition of the anaconda population based on the frequency with which we recaptured marked individuals. Our hunting technique is not very sophisticated; most of the time looking for anacondas is hot, tiring work - sloshing through the water and mud of drying pools in the hopes of spotting one, stepping on one, or poking one with the poles we carried. Anaconda hunting is often a boom or bust enterprise, sometimes with days between captures. Most of our days consisted of long intervals of trudging punctuated by brief, frenzied bursts of activity. At times we found so many snakes we did not have enough hands to deal with them all.
Because the ponds we search are also full of piranhas, electric eels, caimans, and freshwater stingrays, anaconda hunting is never dull. An atmosphere of tense expectancy pervades the simple act of walking through knee-to-waist-deep water. However, the danger that these animals represent is not very substantial, with the possible exception of the stingray - which can inflict injury with its poison-tipped tail barb - and we have managed to avoid serious incident. In fact, our worst experience has been an encounter with an unknown substance in certain pools that causes an uncontrollable itching of the feet, forcing us to bound from the water, tear off our shoes, and scratch our skin raw.
During the last four years, anaconda catching on El Cedral has become an annual dry-season affair. Although many study participants are snake enthusiasts, anaconda hunting is invariably a unique experience for all. We work in groups of at least two people, being careful never to be out of shouting distance of the others. Small snakes, less than ten feet long, are easily caught by one person. The cry "snake!" usually means someone has found a larger animal and needs help. If the snake is buried in the mud or hidden underwater, we always try to locate the head (the end that bites). Some of the larger snakes are dealt with by pulling them tail first out into the open, occasionally resulting in a slightly eccentric tug of war. In captivity, anacondas have a reputation for being irascible and prone to biting, but on El Cedral they usually do not become aggressive until they are grasped by the neck. Snakes may be processed and released at the point of capture, or, if we want to continue hunting, they may be placed in a large bag or barrel, loaded into our vehicle, and later evaluated at our house, which doubles as our research laboratory.
During the dry season, many anacondas can be found in holes or burrows along the banks of streams and rivers, and we spend a lot of time peering into dark recesses. On El Cedral, anacondas were also fond of culverts, where they can be found hiding under slabs of concrete. On one occasion while poking my head into a culvert I was greeted by a cloud of enraged Africanized bees that chased us for more than a half mile.
In the four field seasons we have been working on El Cedral, we have captured and marked over three hundred anacondas. At our cramped, tin-roofed house we can collect anacondas' feces and determine what they are eating before releasing them in a few days at the point of capture. During successful anaconda hunting forays, the house becomes a chaotic assemblage of field gear, muddy clothes, swinging hammocks - and snakes, each in a bag or barrel marked with the date and time of capture. For each snake we fill out a data sheet, recording everything from length and sex to the nature and location of their injuries. The anacondas are marked by clipping a combination of their scales near the base of the tail, and also by noting the unique pattern of light and dark scales on the tail's undersurface, which acts like a snake "fingerprint"; no two are alike. During this entire process we keep the snake calm by placing a sock over its head so it cannot see, or bite. During our first year, Calle surgically implanted small radio transmitters in twelve of the snakes we caught. Using a special radio receiver and a small, handheld antenna, Rivas and Munoz spent a year on El Cedral following these radio-tagged snakes by car, on foot, horse, or boat. They plotted each snake's movements and noted what kinds of areas anacondas preferred and which they avoided. By locating the snakes at frequent intervals, they quickly learned a lot about anaconda behavior. Rivas and Munoz were hard put to keep up with all the snakes, particularly once the rains began and the snakes spread out into the newly flooded savannas.
However, their persistence paid off and revealed a gold mine of information about the daily activities of anacondas. Much to our surprise, the anacondas were quite mobile and had well-defined "home ranges," that is, areas with which they were intimately familiar. The snakes would move around these home ranges, with certain areas being preferred during certain times of the year, mostly depending on water level. During the dry season, many of the males become especially mobile as they search for receptive females.
Like many snakes, anacondas can go for long periods without eating, but at times large snakes can be seen literally bulging with a freshly swallowed meal. Although we have found no evidence of anacondas using the poisonous vapors attributed to them by Padre Gumilla, anacondas do have a varied diet, consuming small mammals, birds, turtles, and even large prey such as caiman and deer.
We are also learning that anacondas have a fascinating mating system. Many longtime residents of the llanos had told me about finding large groups of anacondas together. Although I was skeptical of these reports, in March of our first year we began finding some of the large females grouped with several smaller males. Males appear to seek out females during the late dry season, and, when more than one male finds the same female, these "mating balls" can ensue. In subsequent years we have found numerous groups of snakes intertwined in gorgonlike masses, usually while partially buried in mud or under moist mats or grass around the edges of drying pools. These mating balls of anacondas can be quite impressive; one photographed female was completely enwrapped by seven males.
Soon after the mating period ends the rains return to transform the dry plains first into muddy fields, and then into a vast marsh. When water levels reach their peak in August and September, the only snakes seen are usually very large, fat anacondas basking on the few islands of dry habitat. We suspected that these were gravid females, raising their body temperatures to speed the growth of their developing embryos. (Like most reptiles, the majority of snakes lay eggs. However, a small number of snakes, including the poisonous pit vipers and the New World boas, retain their eggs inside their bodies and give birth to live young. To ensure that their embryos develop properly inside their bodies, females will often bask in the sun, raising their body temperatures and speeding embryonic growth.)
To confirm this, Rivas and Munoz began catching these snakes and taking them to the town of San Fernando, where they had enlisted the services of an adventuresome obstetrician to conduct ultrasound examination of our snakes - much to the consternation of the expectant human mothers in the waiting room. Indeed, these basking snakes were found to be gravid females, and in some cases it was even possible to count the number of embryos - up to seventy in large females.
Although we have just completed our fourth year of research at El Cedral, we are only beginning to shed some light on the biology of anacondas. Many aspects of their behavior and ecology still remain hidden in the mud and vegetation that conceals them so well. Our work will continue to focus on aspects of the biology about which we still understand little. For instance, what is actually going on in the mating balls of snakes? Is just one male mating, or are a female's brood the product of multiple fathers? Also, what happens to the newborn anacondas? Once they are born they seem to disappear, and we do not see them again until they are four to five feet long. Rivas and Munoz will try to answer this question by placing tiny radio transmitters in some of the newborn. As we return to the Venezuelan llanos each dry season, we will pursue the answers to these and other questions in the hopes of uncovering more mysteries about these enigmatic giants.
John Thorbjarnarson is the Wildlife Conservation Society's assistant director for Latin America.
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|Publication:||Americas (English Edition)|
|Date:||Jul 1, 1995|
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