Natural History of the green anaconda (Eunectes murinus)
By
Jesús A. Rivas Ph.D.
Introduction.
The green anaconda, Eunectes murinus (Boidae)
is among the largest snakes in the world. While the title of the longest may be
contested with the reticulated python anacondas are by far the heaviest.
Due to their durable skin pattern they have been commercially hunted, mostly in
the local hand crafted work but 
also
on the commercial industrial trade. Despite their obvious appeal of
charismatic mega fauna and their potential for skin trade and pet trade little
was known of their biology before I began to study them in 1992. To the
present my study has produced numerous publication in scientific journals
(Rivas 2000; Rivas & Burghardt 2001; Rivas et al. 2007a; Rivas
et al. 2007b), that along with several documentaries on
anaconda biology have brought the anacondas from the shadows where they
were to the status of one of the best studied snakes in the world.
In the following sections I will summarize some of the major aspects of what I
have learned studying them since 1992.
Distribution
Anacondas may be found in all the low lands of Tropical South America always
associated to aquatic bodies with precipitation higher than 1500 mm a
year. It inhabits wetlands, lakes rivers and
swamps. It prefers stagnant water with little depths and with vegetal
coverage. It may be found in or near water bodies where it ambush its prey. It spends most of their time under
the water, or 
in
caves at the river banks. They are seldom in clear or running water it
rather lives under floating vegetation (e.i. Eichhornia ssp, Salvinia sp, Pistia
sp).
Even though anacondas are distributed throughout tropical South America and in the Amazon basin they are very difficult to find in the deep forest. Most of the knowledge of anacondas comes from my study site in the Venezuelan llanos that is to the present the only field study of this species. The llanos is located on the northern edge of the distribution range of the anaconda The mayor difference between the llanos and the rest of the Amazon basin (and perhaps the reason I was successful studying them) is that fact that in the llanos there is a strong dry season when the water is reduced to small areas and the anacondas concentrate on the few water bodies that have water. In the dry season it is also possible to find them buried under the mud once the water body dries out. Anacondas may spend under the mud weeks or months until the wet season resumes. (Click here for a complete description of the llanos; PDF file)
Diet:
Anacondas are ambush hunters that blend with the environment thanks to their
cryptic coloration. It waits in ambush until the right prey appears at
the right time to strike it with their deadly speed. Despite their bulk
and large size anacondas can move incredible fast to attack their prey.
Once captured the prey is subdued by a combination of strength that immobilizes
it, immersion in water (most prey are terrestrial vertebrates) a and a maneuver
that consist of pulling and twisting the spine which very quickly breaks the
spine and renders the prey unable to escape or to fight (Rivas
2004).
Despite their aquatic habits fishes are surprisingly uncommon in their diet, perhaps because they are so difficult to catch under the water. Anacondas prey regularly on just about any other vertebrate. On the diet of anacondas we can find reptiles (25.8%) such as spectacled caimans (Caiman crocodilus), yellow headed side neck turtle (Podocnemis vogli), green iguana (Iguana iguana), and tegu (Tupinambis teguixin) among others. About half of the diet (51.6%) are birds of different sizes including several species of storks, herons, ducks, ibises and many other wading birds. The remnant 22.6% is comprised by mammals from rodents and marsupials on the early ages to Capybaras (Hydrochaeris hydrochaeris) and white tail deer (Odocoileus virginianus). However this distribution does not represent the diet of any given snake. Young anacondas and males (that are always smaller) tend to feed mostly on birds while the larger females virtually drop birds from their diet and feed almost exclusively on reptiles and mammals.
Anacondas may take prey from a broad range of sizes ranging from 10% of their body weight all the way to 146% (Rivas 1998). This feat seem hard to believe and the reason anacondas can do is because a series of morphological and behavioral adaptation that allow them to swallow larger prey. Other than the streptostylic jaw (with mobile joins to swallow large prey) anacondas also live in the water so having a very large prey does not hinder their movement as much as it would if they were terrestrial. Swallowing a large prey, say a capybara or a deer, may take as much as 6 or 8 hours and the frequency of feeding may be surprisingly low.
One striking fact about anacondas diet is regarding
the diet of larger animals. To take a very large prey involves great risks for
the female that may be wounded, or even killed by the prey during the attack (Rivas 2004; Rivas et al. 2007b).
The evolution of the streptostylic jaw that allows macrostomata (more advanced snakes) consume
such large prey came with some glitches. The jaw of most snakes (with the
exception of some blind snakes) is better viewed as a high tech device.
It is intended to accomplish a very specific function which is does formidably
well but as any high tech it has its limitations: it is very fragile and it 
has
its limitations. The level of mobility of a snake skull that allows the
snake to swallow a very large prey comes with a price tag attached to it: lack
of crushing power and a very vulnerable skull 
that
does not give much protection to the central nervous system. The system
of hinges and levers that occurs in, say, a lizard head requires rigid bones
and solid structure for the muscle to pull against them. On the snake
head many of those solid joins have been replaced by mobile ones. The
result is that the bite of a large anaconda is not comparable with the bite of
a reptile of its size. Crocs and large lizards can relay
on their bite to kill the prey and avoid being injured by them while at the
same time the count with a solid skull that protect them from harm.
Anacondas on the other hand need to use their body to subdue their prey and
while doing this they can be very vulnerable to wounds. This is not
different that what happens with any 
other
constrictor. What is different about anacondas is that when anacondas go after
a large prey their prey are things like a spectacled caiman or a capybara that
has a much larger potential to wound them than if they preyed
in smaller or less dangerous prey. To have an idea of what a capybara is
in this sense imagine a rat the size of a Rottweiler. Imagine now the
teeth of a regular rat and scale them up to the size of the large dog but keep
the sharpness of the rat's teeth. That is what an anaconda faces when she
eats a capybara. Most other constrictors do not face such a large risk
when they try to take a meal. The results are that anacondas think it well
before they go for dinner. This may be the reason that wild females eat
surprisingly seldom in the wild. Preliminary data indicate that a large
anaconda may eat only two or three times in a year and perhaps the reason that
pregnant females do not feed at all during the 7 months that the gestation of
the babies lasts (Rivas 2000).
Home range:
Anacondas do not move much if they can help it. They use a relatively small home range during the dry season (mean 25,2 ha) and they basically keep warm and moist as needed. When the rainy season begins anacondas disperse seeking shallower water since the places that hold water during the dry season are bound to be too be deep during the flooding season. They perform some sort of a seasonal migration (mean 1.3 Km) towards shallower areas. During the wet season the home range is slightly larger than in the dry season (mean 37,4 ha). At the end of the rainy season animals come back to their original dry season turf, showing a high level of phylopatry. Pregnant females do not migrate during the wet season, they rather find high lands where they bask frequently until the time they give birth..
Reproductive
Biology:
Anacondas have an amazing Sexual-Size Dimorphism with much smaller males (mean
6.9 Kg.) than females (mean 32,6 Kg.) (Rivas & Burghardt 2001).
Individuals live by themselves until the reproductive season (mid February to
mid May) when several males gather around one 
females. Mating aggregations have from one to 13 males
(mean 3.8) and last about 4 weeks (Rivas
et al. 2007a). After the female's attractive period is over males
disperse back to their original home ranges while females stay pretty much in
the same areas. In the breeding aggregation the females mate several
times, likely with several males, while there is no evidence that males mate
with several females in the same season. Likely for the problems of
finding and courting several females during the short mating season (Rivas
2000). Males would probably court other females after they are done in
one mating but the odds of that happening are low since the females are
dispersed among the landscape and finding several females may be
unlikely Thus, a de facto polyandry mating system
exists in the species.
Females incubate the eggs on their bodies for 7 months after which they give birth to live babies. The average clutch size is 29 babies but it is tightly linked to size of the female. Smaller animals have small clutches (8 to 15) of small babies (mean 150 g) while larger females may have larger clutches (85) of much larger (300 g) babies. During the time she is pregnant, the female does not feed. This is a considerable investment of energy since she is undergoing quite a strong metabolic activity developing the babies. It is possible that the reason that females do not feed is to prevent the risk of being injured by her prey. Clearly if the female were to be wounded then she and all her clutch could be in jeopardy. Animals that us this strategy are known as "capital breeders" since the animals relay on the stored fat and reserves in order to breed and not on the income that it may gather during the pregnancy.
Female anacondas do not breed every year because the investment done in every mating event is so large (about 35% of her body weight) that the female cannot recover her condition in the short time between parturition (September to December) and the new mating season (February to May). In average a little more than one third of the females breed in any given year. Smaller females breed every other year while larger animals need even longer to recover from their reproductive events.
Demography
The demographic information is perhaps the most difficult information to extract from a wild population certainly from long-lived animals. One thing that stands out looking at the population structure is the relative lack of overlap between males and females. Males rarely pass the threshold of 3 meters (only less than 5% of the males do) while females represent just about all the animals beyond 3 meters. Adult anacondas have virtually no overlap in size between males and females making it possible to determine de sex based only on the size, provided that the individual is an adult. (Rivas & Burghardt 2001). Female and male anacondas are the same size at birth but males slow down growing significantly when the reach reproductive ages while females continue to grow. Individual growth rate decreases with size. Preliminary data indicate an average of 11 cm/year in juveniles while adult animals may have a much slower growth rate. I have found that very large animals (more than 5 meters long) did not grow at all in as many as 5 years. However, the growth rate of 11 cm/year should not be applied across the board to all the juveniles. Differences in feeding strategies may determine broad variation in growth rate of juveniles.
Anacondas inhabit a world very different than ours not only physically but also cognitively. Physically they live in stagnant water, in swamps covered by aquatic vegetation, the last kind of places we would like to hangout. The internal world of an anaconda is also years light away from ours. Eating three times a year, taking 6 hours to gulp down a meal, mating once a year and for four weeks in a row are only some of the many aspects in which anacondas are different from us. These difference hinder our ability to understand their life and their ecology and ultimately hinder our potential to protect and preserve the species. As human encroachment on their habitat becomes ever more intense we should expect that there may be conflict between humans and snakes. Only doing our best to understand their life on their terms can be learn enough to protect them and to ultimately live in harmony with this wonderful reptile.
References
Rivas, J. A. 2004 Eunectes murinus (green anaconda): Subduing behavior. Herpetological Review 35(1):
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