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.
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 is 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 an 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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