Ultimate Explorer Snake Hunter Stalks Constrictors

Brian Handwerk
for National Geographic Ultimate Explorer

August 1, 2003

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For the past 11 years, National Geographic Ultimate Explorer guest correspondent Jesús Rivas has studied constrictors. This reptile family includes such notable snakes as anacondas, pythons, and boa constrictors, all big animals notorious for their ability to tackle prey even larger than themselves.

In Ultimate Snake Rivas, of the Universidad Central de Venezuela, journeys around the globe to seek out fascinating constrictors and the scientists studying them.

In Puerto Rico, boas have taken to the trees where they lurk near the mouths of caves. There, night-vision cameras capture them preying upon the many bats who emerge each evening—hunting them effectively even in pitch darkness.

Australia's woma pythons hunt underground in tunnels away from the searing heat of the surface. There the hardy reptiles employ an adaptive technique—using their bodies to crush their prey against the walls of the narrow tunnel.

And in South Africa, Graham Alexande of the University of the Witwatersrand has made discoveries about southern African python parenting. His findings challenge the commonly held belief that newborn pythons are left to fend for themselves, and show python mothers in a more nurturing light.

At home in Venezuela, Rivas continues his ongoing research on the green anaconda. During a unique long-term study funded in part by the National Geographic Society's Committee for Research and Exploration, he's become the world's foremost expert on these little-known but fascinating giants.

Anaconda Research a 'Hands-On' Proposition

Rivas' work is hands-on in the most literal sense. To study these reclusive animals, he wades barefoot through the swamps of Venezuela's llanos wetlands ecosystem in search of his water-dwelling subjects. Once found, he wrestles them into exhaustion with his bare hands.

The dangerous work is necessary, because the snakes don't lend themselves to other kinds of study. "With snakes in general, they are so secretive that if you get to see them long enough to catch them you're lucky, let alone long enough to do naturalistic observations," Rivas explained. "Some animals can be studied with hands-off observation but these snakes are hard to deal with in that sense."

Despite the challenges, Rivas and his team have captured and released more than 900 animals over the past 11 years, including some 170 recaptures. They've also studied the breeding cycle of 47 females, providing an unprecedented look at these "ultimate snakes."

Green anacondas are reported to be the largest snakes in the world. They reach lengths of nearly 30 feet (9 meters), can weigh 1,200 pounds (550 kilograms), and some tales tell of even bigger snakes.

Typically, the animals are about 12 feet (4 meters) long, a foot (30 centimeters) in diameter, and weigh several hundred pounds (75 to 150 kilograms). While other snakes can grow longer, they cannot match the anaconda's length and bulk.

Killing With Stealth and Strength

The massive snakes may eat only a few times a year, but when in the mood for a meal they are a formidable animal to be respected by anything or anyone in their area.

Constrictors hunt by ambush, as do many other snakes. It's their method of killing and consuming their prey that has made them the stuff of legend.

"What she does is try to squeeze the heart of the animal so tight that it can't fill up with blood," Rivas explained. "Then there's a total circulatory arrest in the animal." The snakes also twist while constricting, in order to break the backs of their unfortunate prey.

"Most snakes just eat their prey alive," Rivas continued. "But constrictors eat prey that is too big, animals that could do the snake harm. So they have to kill it first. These animals developed a way to kill without claws, or cutting teeth."

While the anaconda can deliver a painful bite, the animal's head is quite flexible and has mobile hinging jaws that are unable to deliver a really crushing bite. The adapted skull, however, serves another purpose: it gives the snake the ability to swallow prey whole—including animals of human size or even larger.

Anacondas have long been known as "man-eaters." Although such an event is a terrifying thought, few deaths have been recorded. "The reason that they don't eat people is that people don't come into contact with these snakes very much," said Rivas. "But even a medium-size snake could kill a full-size person."

Birds are much more common victims. Snakes smaller than 10 feet (3 meters) dine primarily on birds. Bigger anacondas tackle other prey like caiman, turtles, white-tailed deer, and the capybara, a pig-sized rodent that Rivas describes as "kind of a hybrid between a hippo and a chipmunk."

While a decade of wrestling snakes represents a lot of work, Rivas explained that his study is far from finished and that science still has much to learn about the constrictors.

"In general the biology, reproduction, diet, and lifecycle of these animals in the wild are very poorly understood," he said. "These animals live for probably 20 years or maybe even 40 years—so studying them for 10 years is not going to get you the whole picture. The animals I caught in 1992, well, I caught them again this year, so they are still alive. They might outlive me for all I know. It's a challenge. Studying natural history in the wild takes time. When you are studying an animal that lives for 30 years you need to follow it for at least that long."