The Yellow Anaconda Can Form Days-long 'Breeding Balls'

By: Desiree Bowie  | 
A thick-bodied snake with yellow scales and black spots, poking its head through its coils
The yellow anaconda (Eunectes notaeus) resides in mostly aquatic habitats, Paul Starosta / Getty Images

In South America's dense, waterlogged ecosystems, you'll find the yellow anaconda. These large, muscular snakes, known scientifically as Eunectes notaeus, are not just formidable predators; they also exhibit complex and unique mating rituals involving a competitive and communal dance known as the "breeding ball."

In 2023, scientists uncovered an "S-start" movement in the semi-aquatic species: a rapid, sidewinding motion the anaconda uses to escape threats. This particular behavior, observed and studied in laboratory settings, reveals an unexpected agility in these heavy-bodied snakes.


Let's take a closer look at some of the snake's eye-catching features.

Yellow Anaconda Size and Appearance

The yellow anaconda (Eunectes notaeus) has a distinctively shaped head, notably broader than its neck, with eyes and nostrils situated on the upper part of the head. This arrangement is beneficial for a semi-aquatic snake, as it allows the anaconda to see and breathe effectively while its body remains mostly submerged in water.

Complementing this adaptation is their robust, muscular build, underscoring their power as constrictors. This strength is crucial for their hunting technique and enhances their ability to swim and hunt efficiently.


How Big Do Yellow Anacondas Get?

A member of the mighty boa family, yellow anacondas are noted for their size and distinctive appearance. While smaller than the green anaconda, yellow anacondas still reach considerable sizes, typically measuring between 5 and 9 feet (1.5 and 2.7 meters) in length.

The females are generally larger than the males and can weigh anywhere from 5 to 12 pounds (2.7 to 5.4 kilograms). There is a noticeable sexual dimorphism in these species, with females typically being much larger and heavier than males. Some have grown as big as 110 pounds (49.9 kilograms).

Coloring and Camouflage

In line with its name, the anaconda's appearance is characterized by a yellowish or golden background color, accented with brown or blackish bands and rosette-like patterns atop them.

This coloration provides excellent camouflage in their marshy aquatic environments — their preferred place to hunt and dwell. Much like its relatives in the boa family, the skin of the yellow anaconda is smooth to the touch.


Geographic Range

Yellow anacondas are native to southern South America, primarily in the Pantanal, one of the world's largest tropical wetlands, which spans across Brazil, Bolivia and Paraguay. You can also find them in regions of southern Brazil, northeastern Argentina and eastern Bolivia.

In these regions, yellow anacondas inhabit a variety of wetland and riparian areas (a region adjacent to rivers and streams characterized by distinct vegetation and soil types influenced by water). The giant snakes are particularly adapted to life in slow-moving rivers, marshes and swamps, which provide the water they need for hunting and camouflage.


Those areas and their murky waters are also home to an array of their prey species, making them an ideal setting for the ambush-hunting tactics of the anacondas.

The humid, tropical climate of their natural habitats provides the necessary warmth for their metabolic processes as the yellow anaconda is ectothermic, relying on external heat sources to regulate its body temperature.

The species' preference for aquatic habitats also aids in their locomotion. While they can move on land, yellow anacondas are more agile and graceful in the water, where they can easily swim.

This aquatic proficiency is essential for capturing prey, escaping predators and, in the case of females, finding suitable locations to give birth.


Summertime Predation

Predation, the process in which one organism hunts and consumes another, plays a significant role in the life cycle of yellow anacondas, particularly in the context of their native South American wetlands.

Their hunting patterns are closely tied to seasonal environmental changes. Specifically, following the subsidence of heavy flooding typical of their habitats from June to November, there is a notable shift in the ecosystem dynamics.


This period, marked by reduced water levels, changes the availability and visibility of prey species, such as wading birds, which become more accessible to predators. For yellow anacondas, this time can result in increased predation activities, both as predators and as prey. (Though these creatures have very few natural predators.)

The decreased flooding allows larger predators and birds of prey to navigate areas that might have been inaccessible during the wetter months, potentially leading to a rise in encounters between yellow anacondas and their predators.


Feeding Habits and Diet

Yellow anacondas' diet is diverse and opportunistic, reflecting their habitat and size. As semi-aquatic snakes, they primarily hunt in and around water bodies. Young yellow anacondas usually start with smaller prey like birds, rodents and amphibians.

As they grow, their diet expands to include bigger and more varied prey. They have been known to feed on a variety of animals, such as fish, larger rodents and even small caimans. Yellow anacondas also prey on bird eggs, which provide an easy nutritional source.


Interestingly, their diet can include more unusual prey like crab-eating foxes (Cerdocyon thous), a small carnivorous mammal found in the same regions. Tegu lizards, another potential prey, are large lizards that share the anaconda's habitat and can be hunted, especially by younger snakes.

Yellow Anaconda Mating Behaviors

During the mating season, which occurs in April and May, females attract mates through a combination of chemical and behavioral signals. The primary method is through the release of pheromones, which are chemical signals that can be detected by multiple males over considerable distances.

These pheromones signal the female's reproductive status and readiness to mate. This chemical communication is crucial in the dense, often visually obstructed marshes and swamps where yellow anacondas often live.


In addition to chemical cues, the female may exhibit certain behaviors or movements that indicate her receptiveness to mating, though this aspect is less understood than chemical signaling.

A Prolonged Breeding Ball

Once the males locate the pheromone-emitting female, they converge on her location and form a breeding ball. This entangled mass of snakes occurs when males coil around each other — and the female — creating a complex knot of bodies.

The size of the breeding ball can vary, ranging from a small cluster to a large, intertwined mass, sometimes involving a dozen or more snakes.

This dynamic tangle of snakes stays in constant motion, with each male striving to get closer to the female. They wrestle and vie for position, attempting to maneuver themselves into a spot favorable for mating.

The male anacondas often form breeding balls in or near water, and this convergence can last anywhere from several hours to days. During this period, the snakes are intensely focused on mating, often paying less attention to their surroundings.

Eventually, the breeding ball disbands, typically after a dominant male successfully mates with the female. Once mating is concluded, their situationship is over, as these solitary animals do not form lasting bonds.

Gestational Periods

Post-breeding, the female undergoes a gestation period. Unlike some other snake species, yellow anacondas give birth to live young, a process known as ovoviviparity. The gestation period lasts several months, often culminating in the birth of the young during the late rainy season or early dry season.

The number of offspring can vary, but a female yellow anaconda can give birth to a substantial number of young, sometimes ranging from 20 to 40 or more.

The newborn snakes are independent from birth and receive no parental care. They are born fully formed and capable of swimming and hunting, immediately embarking on their solitary lives.


A New Adaptation for Protection

Researchers have recently discovered that yellow anacondas are capable of an unusual movement, similar to sidewinding, to escape danger. This movement, observed in laboratory conditions, involves the snake contorting into an S-shape and quickly jumping sideways.

Named the "S-start," this action is performed by keeping the middle segment of the snake's body flat on the ground while raising and sliding the curved parts of its body along the top and bottom of the S-shape.


This discovery was made by Nicholas Charles and his team at Harvard University, who studied 10 yellow anacondas of varying ages and sizes. They found that only some snakes, particularly the younger and more muscular ones, could perform the S-start. The oldest and largest anacondas in the study did not exhibit this movement.

Through computer simulations, the researchers learned that a specific ratio of the snake’s weight to the strength of its muscles is necessary for the S-start to be executed effectively. Snakes that were too heavy or light could not perform the movement correctly.

The simulations suggested that as anacondas age, their increasing bone mass and decreasing muscle relative to their size make it more difficult to execute the S-start.

This article was written in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.