Life history and conservation of the green anaconda (Eunectes murinus)

By Jesús A. Rivas

        The green anaconda is the largest snake in the world. Although famous, very little is known about its life history.  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 colleagues, 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.  People interested in helping with anaconda research and conservation can make tax deductible donations following this link.

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 and conservation, habitat use and mobility, diet preferences, predation of adults and juveniles, mating system and reproduction.  I am in the process of writing a book for Oxford University Press with a compendium of almost 2 decades of research on anacondas.

        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 possibility 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 minimum influencing of the behavior, a peek on how long they live, among many others. My ultimate goal is to learn all the secrets of the life history of the animal and get a more firsthand 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 IMG_2242between 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 integrated 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.   I have put together a short description of anacondas natural history for those people interested on it. 

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, 20 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. 

          Lately I have funded the operation with the contributions of people that want to participate hands-on on the research and contribute economically to the whole undertaken.   

Below is a list of the articles published so far:

Book Chapters

Rivas, J. Muñoz, M, Burghardt, G. and J Thorbjarnarson 2007.  Sexual size dimorphism and mating system of the Green Anaconda (Eunectes murinus). pp: 312-325  In:  R. W. Henderson, R. and Powell, G. W. (eds.), Biology of Boas, Pythons, and Related Taxa. Eagle Mountain Publishing Company, Eagle Mountain.


Rivas, J. A., M. d. C. Muñoz, J. B. Thorbjarnarson, G. M. Burghardt, W. Holmstrom, and P. Calle. 2007. Natural History of the green anacondas (Eunectes murinus) in the Venezuelan llanos.  pp: 128-138-415. In:  R. W. Henderson, and R. Powell, (eds.), Biology of Boas, Pythons, and Related Taxa. Eagle Mountain Publishing Company, Eagle Mountain.


Published Articles:

De La Quintana, P., Pacheco, L,. J. A. Rivas.  2011.  Eunectes beniensis:  Cannibalism. Herpetological Review 42:614.


Rivas, J. A.  2010.  Is Wildlife Management Business of Conservation:  A Question of Ideology.  Reptiles and Amphibians  17: 112-115.


Rivas, J. A. & S. J. Corey.  2008 Eunectes murinus (green anaconda). Longevity.  Herpetological Review


Rivas, J. A.; Ascanio R. & M. D Muñoz.  2008.  What is the length of a snake?  Contemporary Herpetology. 2008 (2): 1-3


Rivas, J. A. 2007  Conservation of Anacondas: How Tylenol conservation and macroeconomics threaten the Survival of the world's largest snake.  Iguana  10-21 unabridged version


Rivas J. A. and G. M. Burghardt 2005. Snake mating systems, behavior, and evolution: The revisionary implications of recent findings.  Journal of Comparative Psychology 119: 447-454

Rivas, J. A. 2004 Eunectes murinus (green anaconda): Subduing behavior. Herpetological Review 35(1): 66-67


Calle, 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.


Rivas, J. A. and Burghardt G. M. 2001 Sexual size dimorphism in snakes: wearing the snake’s shoes. Animal Behaviour. 62(3): F1-F6.


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. Herpetological Natural History. 8(1): 93-95.


Rivas, J. A., Owens R. Y. and P. P. Calle. 2001. Eunectes murinus: Juvenile predation. Herpetological Review. 32 (2): 107-108.


Rivas, J. A. and R. Y. Owens. 2000. Eunectes murinus (green anaconda): cannibalism. Herpetological Review. 31(1):44-45. PDF


Rivas, J. A., Thorbjarnarson, J. B., Owens, R. Y and M. C, Muñoz, 1999 Eunectes murinus: caiman predation. Herpetological Review. 30 (2): 101


Rivas, J. A. 1998. Predatory attack of a green anaconda (Eunectes murinus) on an adult human. Herpetological .Natural History Vol. 6(2): 157-159.


Raphael, B.L., Calle, P.P.; Karesh, W., Rivas, J. A., and D. Lawson. 1996. Technique for surgical implantation of transmitters in snakes. Proceedings of the Wildlife Disease Association. 1996: 82.


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. Herpetological Review. 26: 138-139


Calle, P., Rivas, J., Muñoz M., Thorbjarnarson, J., Dierenfeld, E. Holmstrom, W. Braselton, E. and Karesh W. 1994. Health assessment of free-ranging anacondas (Eunectes murinus) in Venezuela. Jour. Zoo. Wildl. Med. 25: 53-62.

Unpublished monograph and technical reports:

Rivas, J. A. 2000. Life history of the green anacondas with emphasis on its conservation Biology. Unpublished Ph.D. dissertation at the University of Tennessee, Knoxville. USA.

Thorbjarnarson, J., Rivas, J. and Muñoz M. 1995. Ecology and behavior of the anacondas in Venezuela. Technical Report to the National Geographic Society Committee for Research and Exploration.

Muñoz M. and Rivas J. 1994. Ecologia y Conservación de la anaconda (Eunectes murinus) en Venezuela, segundo Informe. Technical report to CITES

Rivas J. and Muñoz M. 1992 Ecologia y Conservación de la anaconda (Eunectes murinus) en Venezuela. Technical report to CITES. 78p.








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