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.
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.
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 first hand knowledge of
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. 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 (
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
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, 18 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.
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
Below is a list of the articles published so far:
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.
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. 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.
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.