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
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
several tricks to work with them such as how to measure,
how to implant a transmitter on them with
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).
anacondas in a integratred way I learned not only several aspects of
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.
the largest Sexual Size Dimorphism of any
species I can hardly think of any animal that would be a better model
SSD research. Anacondas also present a surprising ontogenetic change in
biomass from birth to adulthood, with a 500-fold increase it is much
increase we find in any other species of snakes. This makes anacondas
excellent model to study the ontogenetic changes and develop
regarding this issue. Due to their large size, anacondas offer
for study that are not found in other snakes, including easy extraction
of blood samples sufficient for study of physiological processes and
analyses, and possibility of implanting radio transmitters for long
charismatic nature of my study animal, my research has received wide
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
Currently I am
and ambitions project to study the ecology of anacondas in other
of its distribution in order to compare
with my findings on the llanos. By conducting a comprehensive
research project about the life history of this magnificent animal I
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
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
in the best case scenario. I feel quite lucky that I have managed to
this study running for 11 years now getting funds from different
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
the way I would like to.
With the help of a German TV network, Pro 7, I have started a program
adopt a snake where people who want to contribute with the long term
of anacondas can adopt an animal. The funds obtained from the
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
the life of the animal in the field. To join this program and
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., Ascanio, R. E. and R. Y. Owens. What is the length of a snake? Contemporary Herpetology
Rivas, J. A., Owens R. Y. and P. P. Calle. 2001. Eunectes murinus: Juvenile predation. Herpetological Review. 32 (2): 107-108.
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)
Rivas, J. A., Muñoz M del C., Thorbjarnarson, J. B., Holmstrom, W. and Calle P. 1995. A safe method for handling large snakes in the field. Herp Review. 26: 138-139.
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.
Link to: Slogging through the haunt of the anacondas (a brief synthesis of my research)
National Geographic Explorer
UTK Researcher Studies Anacondas
Premio a la Excelencia
Retour accueil Nat. Geo. (French)
Retour accueil Nat. Geo. (French) II
Green anaconda ( By Paula Boylard)