Life History and Conservation of the Green Anaconda
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.
One question that always fascinated me was: how anacondas became anacondas?. What selection pressures lead to their evolution? IN the process fo exploring that I started looking at their molecular relationships and that the species in
the Northern part of the continent, the one I have been studying for 30 years,
is a different species than the one in the rest of South America. Morphologically speaking both species are nearly identical. However, upon further inspection males of the Southern
species (Eunectes murinus) reach larger sizes than males of the northern
species (Eunectes akayima). There
is no difference in size of the females that we could noticed but females E.
murinus seem to have relatively shorter tails than females E. akayima.
There is more of this in the tab on
Taxonomy.
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, their health status and diseases they may suffer, the biology of neonates, their mating system and reproduction; among many other aspects of the life of the 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
in green anacondas and in a related cousin that occurs in Bolivia. I have learned how often they are wounded by their own prey, the possibility that they can attack a hu man being. I have also learned several
tricks to work with them such as how to measure, how
to wire an anaconda both forced or by chirurgical implantation on them with minimun distribution of the behavior, among many others. Recently
I published the bulk of my findings in a book for Oxford
University Press. 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 processed
more than 900 animals in the last decades 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 my research has brought anacondas from one of the least known species
of snakes, to one of the best studied.
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).
At first I was surprised that anacondas seemed to possess the only polyandrous mating system among snakes, where many males mated with one female. At the time I started the study, it was believed that polyandry was constrained in snakes because of phylogenetic reasons (it was not present in the lineage to begin with). Yet, when I found out that anacondas were polyandrous, I started looking with more attention at the mating system of other snakes. To my surprised, I discovered that anacondas were not alone in being polyandrous. Furthermore, there were many more snakes in this same mating system. In fact, after looking at the data with my eyes wide open, I concluded that polyandry is, in fact the dominant mating system in snakes (click here for 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.
Using my knowledge of anaconda natural history, I was able to present data-based
speculations about the nature of Titanoboa cerrejonensis.
Because they are phylogenetically related and used similar habitat, it
is legitimate to assume they had similar ecological roles. Under this assumption I was able to postulate
that Titanoboa started breeding at 5 m in length. A full size Titanoboa would havd had neonates that measure between 1.8 and 2 meters. My data also shows that it grew at similar rate
than today’s anaconda, contradicting one of the central tenets of that paper
that described it that speculated that it grew a lot faster than today’s snakes
because the planet was warmer. Also, if
the size distribution of Titanoboa is similar to that found in anacondas it is
quite possible that the extinct giant would have surpassed 20 m in total
length.
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 (
Geographic
Magazine in January 1999. It was also featured on multiple
documentaries over the years.
Currently teach at New Mexico Highlands
University where I run the Vertebrate Ecology Lab. 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. My collaboration with colleagues from other
part of South America has resulted in a few meaningful projects with green as
well as the Yellow anaconda complex (Eunectes notaeus) in Brazil,
Bolivia and Argentina.
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 25 years now getting funds from different
sources. However, 25 years is probably only a fraction 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 been funding my field research by bringing people who want to get up-close and personal with anacondas to the field to work with me. These contributors provide a donation that I have used to continue the field work as well as to train students into the techniques and methods to study anacondas. People that come join me are not tourist. They may be up until the time they arrive to my field site. But as soon as they are with me, they become field assistants. In other words, I do not spend time an energy I need for the field work catering to tourist. Rather the contributors become active members of my team and do all the work that other biologists or students do, shuffle through the swamp, help in the processing of the animals until late hours when needed, just like we all do. They may be bitten by bugs, piranhas, or any of the other unpleasantries that the field work involves.
Below is a list of the articles published so far:
Rivas, J.A.; Terra, J.S.;
Roosen, M.; Champagne, P.S.; Leite-Pitman, R.; De La Quintana, P.; Mancuso, M.;
Pacheco, L.F.; Burghardt, G.M.; Vonk, F.J.; Garcia-Pérez, J.E., Fry, B.G.,
Corey-Rivas, S.. 2024. Description of the Northern Green Anaconda (Eunectes
akayima sp. nov. Serpentes; Boidae): What Is in a
Name? Diversity 2024, 16, 418. https://doi.org/10.3390/d16070418.
Rivas,
J. A., de La Quintana, P., Mancuso, M., Pacheco, L. F., Rivas, G. A., Mariotto,
S., Salazar-Valenzuela, D., Baihua, M. T., Baihua, P., Burghardt, G. M., Vonk, F. J., Hernandez, E.,
García-Pérez, J. E., Fry, B. G., & Corey-Rivas, S. 2024. Disentangling
the Anacondas: Revealing a New Green Species and Rethinking Yellows. Diversity,
16(2), 127. https://doi.org/10.3390/d16020127
Rivas, J. A. &
Jaremko-Wright, W. 2023. What
determines demographic growth in green Anacondas? Strong interactions among
vertebrates in a neotropical ecosystem. Frontiers in Ecology and Evolution. Volume 11 - 2023 | doi:
10.3389/fevo.2023.1184192
Rivas J.A. 2023. What can studying anacondas tell us
about Titanoboa cerrejonensis?
Exploring the life of an extinct giant snake using an extant pretty big snake. Herpetological
Journal 33: 68–75. https://doi.org/https://doi.org/10.33256/33.3.6875
.
Rivas, J. A. 2023.
Determining Breeding Status of Green Anacondas (Eunectes murinus): A condition Index Assuming Isometry. South American
Jouirnal of Herpetololgy., 28, 89-94. http://doi.org/10.2994/SAJH-D-22-00022.1
.
Smaniotto, N, Moreira, L. Rivas, J. A. and C. Strüssmann. 2020. Home range size, movement, and habitat use of
yellow anacondas, Eunectes notaeus. Salamandra 56: 159-167. Link
De
la Quintana, P. Rivas, J. A. Valdivia,
F. and L. F. Pacheco. 2018. Eunectes murinus (Green anaconda): Dry season Home Range. Herpetological Review. 49: 546-547 Link
De
la Quintana, P. Rivas, J. A. Valdivia,
F. and L. F. Pacheco. 2017. Home
range and habitat use of Beni
Anacondas (Eunectes beniensis) in
Bolivia. Amphibia-Reptilia. 38: 547-553 DOI:10.1163/15685381-00003124 .
Rivas, J. A., Molina, C. R., Corey-Rivas, S. J., and
G. M. Burghardt. 2016. Natural History of Neonatal Green Anacondas (Eunectes
murinus): A Chip off the Old Block. Copeia: 402-410. https://doi.org/10.1643/CE-15-238
Dunbar, J. P. Zarelli, M., Martin, S. A., Gandola, R. Kavanagh, K. A. Walsh, F. M.
and Rivas, J. A. 2015 Trunk vertebrae osteomyelitis in a spectacled
caiman (Caiman crocodilus). Herpetological Bulletin 134:
15-18. Link
De
La Quintana, P., Pacheco, L,. J. A. Rivas.
2011. Eunectes
beniensis: Cannibalism. Herpetological
Review 42:614. Link
Rivas, J. A.
2010. Is Wildlife Management
Business or Conservation: A Question of
Ideology. Reptiles and Amphibians 17: 112-115. Link
Rivas, J. A. & S. J. Corey. 2008 Eunectes
murinus (green anaconda). Longevity.
Herpetological Review. 39: 469. Link
Rivas,
J. A. 2008. Ticks (Amblyomma
spp.) on Black Iguanas (Ctenosaura similis) in Costa Rica.
Iguana. 15:
25-27. Link
Rivas, J.
A.; Ascanio R. & M. D Muñoz. 2008.
What is the length of a snake? Contemporary Herpetology. 2008(2): 1-3 Link
Rivas, J. A. 2007 Conservation of Anacondas:
How Tylenol Conservation and Macroeconomics Threaten the Survival of the
World’s Largest Snake. Iguana. 14:10-21. Link
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. DOI:
10.1037/0735-7036.119.4.447447
Rivas, J. A. 2004. Eunectes
murinus (green anaconda): Subduing behavior. Herpetological
Review 35(1): 66-67. Link
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. Link
Rivas, J. A. and Burghardt G. M.
2001 Sexual size dimorphism in snakes: wearing the snake’s shoes. Animal Behaviour.
62(3): F1-F6. Link
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. Link
Rivas, J. A., Owens R. Y. and P. P. Calle. 2001. Eunectes murinus: Juvenile
predation. Herpetological Review. 32 (2): 107-108. Link
Rivas, J. A. and R. Y. Owens. 2000. Eunectes
murinus (green anaconda): cannibalism.
Herpetological Review.
31(1):44-45. Link
Rivas, J. A., Thorbjarnarson, J. B.,
Owens, R. Y and M. C, Muñoz, 1999 Eunectes murinus: caiman predation. Herpetological Review. 30 (2): 101 Link
Rivas, J. A. 1998. Predatory attack of a green anaconda (Eunectes murinus) on an adult human. Herpetological Natural History Vol. 6(2): 157-159. Link
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. Link
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. Link
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. Journal of Zoo and
Wildlife Medicine. 25: 53-62. Link