Yellow Anaconda

The yellow anaconda lives in the wetlands of south central South America, in the basins of the Paraguay and Paraná rivers. Its range covers the Pantanal of Brazil, Bolivia and Paraguay, the humid Chaco, and northeastern Argentina, from Formosa and Chaco to Corrientes, Misiones, Santa Fe and Entre Ríos, with the southern tip of its range in Rio Grande do Sul. It needs shallow water with plenty of vegetation: marshes, floating islands of plants, oxbow lakes, flooded grasslands and the edges of slow rivers. It is the anaconda that tolerates the coolest climate, and in winter it becomes much less active. The easiest places to see one are the Pantanal and the Iberá marshes, where it basks on floating vegetation on sunny mornings. It only reaches Uruguay as an occasional wanderer.

The yellow anaconda is smaller than its famous cousin, but it is still one of the largest snakes in South America. Most adults measure between two and a half and three and a half meters and weigh 15 to 35 kilograms. Females are clearly bigger and heavier than males, as in all anacondas, and the largest reliable records are females of about four point four meters. Longer snakes are sometimes reported, but none has been properly measured. Its body is thick and muscular, with a yellow, tan or greenish yellow background covered in black blotches and rings, a pattern that is unique to each snake and works as camouflage among the reeds and floating plants. Babies are born at about 55 to 60 centimeters.

The yellow anaconda and the green anaconda belong to the same genus, Eunectes, and look alike at first sight, but they are different species. The green anaconda is much bigger, reaching five meters and close to 100 kilograms, and has an olive body with round black spots. The yellow anaconda is smaller and has a yellow background with irregular black blotches. They also live in different places: the green anaconda is a snake of the Amazon and Orinoco, while the yellow anaconda lives further south, in the Paraguay and Paraná basins, and handles colder winters. The two only overlap in the Pantanal. Its other names reflect where it lives: Paraguayan anaconda in English, curiyú in Argentina and Paraguay, a word from Guaraní, and sucuri amarela in Brazil.

The yellow anaconda is a generalist carnivore that takes whatever prey is available in its wetland habitat. Fish, waterbirds and their eggs and chicks, small and medium mammals, caimans, turtles, lizards and other snakes all feature in its diet. Young capybaras, the largest rodents in the world and very common in the same marshes, are among its most remarkable prey, while adult capybaras are usually too big. It hunts by ambush, lying motionless in shallow water or at the water's edge with only its eyes and nostrils above the surface, and it also searches actively through the vegetation, following prey by smell with its tongue. When it strikes it holds on with its teeth, wraps its coils around the animal and squeezes until the blood flow stops. Then it swallows the prey whole.

The yellow anaconda spends much of its time in the water, with its eyes and nostrils on top of its head kept just above the surface. It is solitary outside the breeding season and can be active by day or by night, basking on riverbanks and floating vegetation to warm up and slipping into the water when disturbed. It is not aggressive toward people: its first reaction is always to hide or flee. If it is grabbed or cornered it hisses, strikes and bites, and the bite of a large one can be painful, though it is not venomous. There are no confirmed records of a yellow anaconda preying on a person. Despite its size it is a secretive animal, rarely seen even in areas where it is common.

The yellow anaconda breeds between April and May. During this period, several males may gather around a single female in what researchers call a breeding ball, competing for the chance to mate. After a gestation of around six months, females give birth to fully developed live young, like all boas. Litter sizes vary widely, from four to more than 80, though most fall between ten and forty, and bigger females have bigger litters. Newborns measure around 55 to 60 centimeters and are completely independent from the moment they are born, with no parental care. Many are eaten in their first months by birds, caimans and other predators, which is why females need to produce so many young.

The yellow anaconda is listed as Least Concern by the IUCN and is included in Appendix II of CITES, which regulates international trade. Between the mid 1940s and the late 1980s, between 10,000 and 60,000 skins were exported every year from Argentina, mostly to the United States and Europe. Argentina banned the hunting in 1986, and since 2002 the province of Formosa has run the Programa Curiyú, which allows local hunters a limited harvest under quotas and constant monitoring of the population. It is studied as a model of sustainable use of a wild reptile, because it gives local people a reason to protect the wetlands. Over the long term, the main threat is the loss of wetlands to agriculture, drainage, dams and the fires that have hit the Pantanal in recent dry years.