The
green anaconda
is the largest snake in the world. Although famous, very little is
known
about its life history. Until I began my research, no field studies had
been carried out on the species. Due to the skin trade and habitat
degradation
its numbers have declined in places 
where they are not protected. In an effort to protect the species
I, with a team of friends and colleages, began the Anaconda
Project in 1992, with the aim to learn the basic aspects of the
anaconda’s
biology in order to create guidelines for its protection and possible
management.
At first, to work with anacondas seemed like a formidable challenge that I could not overcome. However, I chose to work in the llanos where the strong dry season makes the animals much easier to find and catch. Later I learned how to find, catch, and restrain them in the field. The areas of my research are: population dynamics, habitat use and mobility, diet preferences, predation of adults and juveniles, mating system and reproduction; however I have collected information on a large number of other fields of the life of the animals.
Among the many aspects that I have learned in a casual way from the
snake,
just by following them for so long is the presence of cannibalism,
how often they are wounded by their own prey, the posibility that they
can attack a human being. I have also
learned
several tricks to work with them such as how to measure,
how to implant a transmitter on them with
777minimun
disturbation of the behavior, among many others. My ultimate goal is to
learn all the secrets of the life history of the animal and get a more
first hand knowledge of it.
I have
gathered
eleven years worth of data catching and processing
more than 900 animals and with more than 170 recaptures. I have
followed
with radio transmitter more than 38 animals, collected more than 100
diet
samples, I have also found 51 breeding
aggregations and studied the mating, pregnancy and delivery
of more than 47 females. With the information 
gathered I hope to develop a management plan for the conservation of
the
species and the area in general.
I discovered that
anacondas
make breeding aggregations of one female and
several
males. Despite the uneven sex ratio, no conflicts, fights, or
agonistic
interactions occur between the males other than perhaps pushing each
other
away from the mating position. I also found that anacondas have an
amazing
Sexual Size Dimorphism (SSD) where the
female is much 
larger than the males. Indeed anacondas show the largest SSD found in
any
tetrapod. This SSD is surprising due to the high likelihood that males
compete physically for the females (by pushing each other). Physical
competition
would produce selection pressure for large size in males, however this
is obviously not seen in anacondas. One of the aims of my research is
to
explain why we find this SSD 
despite
the physical competition among males. So far I have found that
males
seem to rely on tactile cues in order to identify who the female is in
the breeding ball. I have found evidence that
larger
males are mistaken for females and courted by smaller males.
Courted
males, as well as courting males, will be in disadvantage. Thus,
there seems to be an optimal male size where it can outcompete other
males
but it is not too large to be mistaken (click here
to read the whole article).
By
studying
anacondas in a integratred way I learned not only several aspects of
their
secret life but also that they can be excellent models for the study of
relevant issues in the ecology of snakes and vertebrates in general.
Having
the largest Sexual Size Dimorphism of 
any
species I can hardly think of any animal that would be a better model
for
SSD research. Anacondas also present a surprising ontogenetic change in
biomass from birth to adulthood, with a 500-fold increase it is much
higher
than the 
increase we find in any other species of snakes. This makes anacondas
an
excellent model to study the ontogenetic changes and develop
predictions
regarding this issue. Due to their large size, anacondas offer
advantages
for study that are not found in other snakes, including easy extraction
of blood samples sufficient for study of physiological processes and
genetic
analyses, and possibility of implanting radio transmitters for long
term
studies.
Due to
the
charismatic nature of my study animal, my research has received wide
attention
from printed media. It was featured in New York Times, Smithsonian
magazine, BBC Wildlife, Das Tier (Germany), in National 
Geographic
Magazine in January
1999. It was also featured on National
Geographic Explorer plus an two articles in the National Geographic
web site (Press
release, NG
Archive) .
Currently I am
planning
and ambitions project to study the ecology of anacondas in other
habitats
of its distribution in order to 
compare
with my findings on the llanos. By conducting a comprehensive
field
research project about the life history of this magnificent animal I
came
to realize the importance of the often forgotten naturalist’s approach
to research that can teach us the whole dimension of nature.
One of the main challenges that one faces to really understand the life
cycle and private life of long lived animals is to obtain funding to
study
the population long enough to get the complete picture. Most financial
institutions would grant research for one year or two relatively easy.
They could extend the funds for three, four, at the most, for five
years
in the best case scenario. I feel quite lucky that I have managed to
keep
this study running for 11 years now getting funds from different
sources.
However, 11 years is not even half of what I estimate the life span of
anacondas is and I am yet a long way to understand the life of these
animals
the way I would like to. 
With the help of a German TV network, Pro 7, I have started a program
to
adopt a snake where people who want to contribute with the long term
study
of anacondas can adopt an animal. The funds obtained from the
project
are used in the conservation and research of the species and the person
who makes the contribution get pictures of the animals as well as
updates
from 
the life of the animal in the field. To join this program and
adopt
a snake click here
Below is a list of the articles published so far:
Go to: Jesus' home pageCalle, P. P.. Rivas J. A. Muñoz, M. C. Thorbjarnarson, J. B. Holmstrom, W. and W. B. Karesh. 2001. Infectious Disease serologic survey in free-ranging Venezuelan anacondas (Eunectes murinus). Journal of Zoo and Wildlife Medicine 32(3): 320-323. (PDF file)Rivas, J. A. and Burghardt G. M. 2001 Sexual size dimorphism in snakes: wearing the snake’s shoes. Animal Behaviour. 62(3): F1-F6. (PDF file)
Rivas, J. A. 2001. Feasibility and efficiency of transmitter force-feeding in studying the reproductive biology of large snakes. Herpetological Natural History. 8(1): 93-95. (PDF version)
Rivas, J. A. 1998. Predatory attack of a green anaconda (Eunectes murinus) on an adult human. Herpetological .Natural History Vol. 6(2): 157-159. (PDF version)
Rivas, J. A. and R. Y. Owens. Eunectes murinus (green anaconda): cannibalism. Herpetological Review. (PDF version)
Rivas, J. A., Thorbjarnason, J. B. Munoz, M, C, and R. Y Owens. (1999) Eunectes murinus (green anaconda). caiman predation . Herpetological Review. Herpetological Review. 30 (2): 101 (PDF version)
Calle, P., Rivas, J., Muñoz M., Thorbjarnarson, J., Dierenfeld, E., Holmstrom, W. Braselton, E., And Caresh W. 1994. Health assesment of free-ranging anacondas (Eunectes murinus) in Venezuela. Jour. Zoo. Wildl. Med. 25: 53-62.
Anaconda Links
Link to: Slogging through the haunt of the anacondas (a brief synthesis of my research)
UTK Researcher Studies Anacondas
Retour accueil Nat. Geo. (French)
