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Anaconda: The Secret Life of the World's Largest Snake



Anaconda tells the unexpected story of the world's largest snake. Written by Jesús Rivas, the undisputed expert on the biology of anacondas, this is the first authoritative book on the biology of the green anaconda. In this book, Rivas describes his experiences over a quarter of a century, exploring the secret life of these fantastic snakes, including: their diet, movement patterns, life and tribulations, survival, behavior, and fascinating reproductive life. More than just presenting facts about anacondas, Rivas tells his story about studying them in the field. Anaconda presents a comprehensive treatment of the natural history of the elusive green anacondas. Drawing on twenty-five years of research on this reptile in the wild and in captivity, Rivas delves into the biology, behavior, demography, reproductive habits, and diet of the anaconda, as well as issues relating to its conservation. Rivas uses an ecological and evolutionary framework to present his research and supplements hard data with descriptions of his research methods, including how he tracked down the anaconda for observation and study in wild. The resulting book is a complete and engaging examination of the world's largest snake. The rich photographs provided, paired with Rivas' storytelling, makes this the perfect book for anyone looking to learn (or even learn more!) about this mysterious snake. This book is published by Oxford University press and there is no pdf available for distribution. The book can be acquired at
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e Secret Life ofthe Worlds Largest Snake
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Oxford University Press is a department of the University of Oxford. It furthers
the University’s objective of excellence in research, scholarship, and education
by publishing worldwide. Oxford is a registered trade mark of Oxford University
Press in the UK and certain other countries.
Published in the United States of America by Oxford University Press
198 Madison Avenue, NewYork, NY 10016, United States of America.
© Oxford University Press 2020
All rights reserved. No part of this publication may be reproduced, stored in
a retrieval system, or transmitted, in any form or by any means, without the
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address above.
You must not circulate this work in any other form
and you must impose this same condition on any acquirer.
Library of Congress Cataloging- in- Publication Data
Names:Rivas, Jesús Antonio, 1964– author.
Title:Anaconda:e Secret Life of the World’s Largest Snake/ by Jesús A.Rivas.
Description:NewYork :Oxford University Press, 2020. |
Includes bibliographical references and index.
Identiers:LCCN 2019052497 (print) | LCCN 2019052498 (ebook) |
ISBN 9780199732876 (hardback) | ISBN 9780199753369 (epub)
Classication:LCC QL666.O63 R58 2020 (print) | LCC QL666.O63 (ebook) |
DDC 597.96/ 7— dc23
LC record available at https:// 2019052497
LC ebook record available at https:// 2019052498
1 3 5 7 9 8 6 4 2
Printed by Sheridan Books, Inc., United States of America
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Foreword ix
Acknowledgments xv
1. Science, Love, and Anaconda Research 1
2. e Anaconda Challenge:Learning How to Learn 8
3. What Is a Good Real Estate for an Anaconda? 23
4. Anacondas’ Tales:Sickness and Health, Birth and Death in the
Life of a Giant Snake 49
5. How Much Is Supper? Predator Diet and Prey Retaliation 79
6. e Largess of Motherhood:Fecundity and Female
Reproductive Strategy 104
7. How Big Can a Giant Be? 166
8. “Hey, It’s Me, It’s Me!” Courtship and Confusion in the Male
Reproductive Strategy 180
9. e Origin of the Mystery:Adapting to Life in a Big Dam 222
10. Conservation of Anacondas and Beyond:e Interface of
Biology, Politics, and Economics 233
11. Epilogue:A Dierent Approach 286
Literature Cited 299
Index 311
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Few animals in nature conjure up the mystique, awe, and fear triggered by
mention of the anaconda, the most massive, and yet elusive, of serpents. Of
the more than 3,500 identied snake species in the world, anacondas have
received remarkably little study, especially in the eld. Even in captivity, their
size, dangerousness, and frequently pugnacious temperament have led to few
scientic studies. We especially know little about their behavior, ecology, and
conservation needs. In a world where humans seem intent on destroying bi-
odiversity, altering climate, and modifying all aspect of nature that interfere
with short- sighted human “needs” and growing populations, there remain,
fortunately, dedicated scientists trying to understand the amazing life forms
we still have on the planet. By doing so they aid in stemming the destructive
tides of anthropocentric conceit. ose risk- taking pioneers studying the
large animals most vulnerable to extinction are especially valuable in helping
us appreciate the lives of charismatic animals before it is too late.
Jesús Rivas is a scientic adventurer and passionate student of reptiles,
especially snakes and, in particular, anacondas. Gigantic anacondas (al-
ways females) have undoubtedly intrigued native peoples for millennia and
westerners since the time of the conquistadors. Although frequently kept in
zoos, their laid- back and largely inactive life there makes them seem dull,
except for when they are on the attack, as in the laughably uninformed lm
Anaconda. Yet, outside of mating, described in detail in this book, perhaps
the most dramatic behavior of anacondas is, in fact, capturing, killing, and
swallowing prey. Yet most zoos do not show anacondas feeding on natural
prey, letalone live prey of any type. So much for zoos being committed to
real, not bowdlerized, education, although they do have to navigate complex
public sensibility and ethical terrain.
is fascinating book gives us deep glimpses into how these animals live in
nature. It combines detailed studies of the behavioral ecology and ethology of
anacondas with the personal story of how Jesús developed as a scientist and
eld biologist along with his oen provocative ethical and political views on
research and conservation. However, the snakes, and Rivas’s love and respect
for them, are always apparent. Tasks necessary for his work such as locating,
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x 
capturing, measuring, and sexing snakes; implanting transmitters; and nav-
igating remote and oen inhospitable environments were scientic keys to
the project’s success. Yet Jesús and colleagues also had to cope with injuries,
come to terms with causing stress and pain to animals, and deal with the
emotions of losing animals. Viewing and treating the anacondas as individ-
uals with distinct personalities are no less appropriate for snakes than they
were for Jane Goodall’s pioneering approach to the Gombe chimpanzees.
e detailed stories of Mamacita, Olivia, Lina, Marion, Penelope, and many
others line up the scientic ndings with the individual, much as how we ex-
pect medical doctors to view their patients.
Additional work beyond the individual is also necessary for successful
eld biology and natural history. How many animals live in an area, and
where do they hang out as seasons change? What is the role of temperature,
so important to ectothermic species? How does one nd out what the ani-
mals are eating and how diet may change across size and season? What are the
causes of death? How does the mating system inuence the myriad aspects
of their lives? What is behind the great dierences in size between males and
females? Anacondas have the largest female- biased size dimorphism of any
terrestrial vertebrate. Is it only by chance they haunt the Amazon?
In addition to carrying out and supervising extensive eldwork to gather
basic quantitative and descriptive information, scientists need to systematize
results, relate them to other studies, and, where possible, construct models.
is is done in several areas of the book, for instance, in predicting the max-
imum size a female may reach, determining growth rates, understanding
why some prey types are preferred over others based on size and nutrition,
and comparing anacondas to other species. e section on the implications
of the recent discovery of the extinct, and even more gigantic, Titanoboa in
South America is riveting. Rivas also tackles issues rare in herpetological
eld studies and rarely discussed with any non- human species. What is it like
to be the animal? Can we obtain some glimpse into their personal worlds,
their Umwelt, as advocated by the Estonian biologist Jakob von Uexküll
over 100years ago but only now playing out in modern science? In short, do
anacondas have a life worth living as sentient beings apart from any ecolog-
ical role their existence serves?
In addition to these fascinating questions, research such as Jesús under-
took entails numerous political, funding, and bureaucratic problems en-
demic to all long- term projects but more pronounced in remote eldwork
crossing national and cultural boundaries. As a Venezuelan, Rivas had a
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 xi
good grasp of governmental contacts, universities, and the Spanish language.
Still, he had to deal with the urban– rural divide in the ranching culture of
the llanos. His eectiveness in confronting and solving such challenges may
have contributed to the success of the project as much as his scientic know-
ledge and empathy with the snakes. Indeed, dealing with people of numerous
backgrounds and persuasions was necessary. us, there are numerous
anecdotes of the many colleagues, supporters, students, and others who
aided the work in Venezuela and labs around the world. Jesús is generous in
acknowledging his many debts to numerous folks and institutions.
Part of Jesúss story is how he got to the United States and navigated the
scientic and academic scene he found there, which oen contrasted with
the world he knew in Venezuela. is is told in the last chapter, along with
briey telling the story of how he became my Ph.D.student at the University
of Tennessee. Ihad been working for many years in Panama and Venezuela
on green iguana behavior and ecology. Indeed, some of our studies were in
the llanos on ranches near those where Jesús worked. Surprisingly, while
seeing much reptilian wildlife in the llanos, Ihad never come across an ana-
conda! is conrms that these gigantic animals are indeed not oen visible
in normal circumstances.
I was most fortunate to have worked with Jesús and immediately realized
how creative, thoughtful, intense, and dedicated he was; soon his great po-
tential as a scientist became apparent, along with his iconoclastic approach
to phenomena that Ihave also pursued in my career. We bonded closely,
although he was a voluble, outgoing Venezuelan and Iderived from more
taciturn northern Europeans. We both shared a lifelong passion for reptiles,
especially snakes, from early childhood on. Jesús had also done important
work on the behavior and ecology of green iguanas that Iand my students,
along with A.Stanley Rand and others, had carried out with the Smithsonian
Tropical Research Institute in Panama as well as in the llanos of Venezuela.
His discovery of a simple method to sex hatchling iguanas would have been
invaluable in our work on hatchlings, where we did not have a nonintrusive
sexing method.
Jesús points out that although his parents were educated professors, they
were not happy when he gave up the prospects of a safe and respectable career
as an engineer to study biology and especially the behavior of reptiles. “You
won’t be able to get any decent job.” is resonated with me because decades
earlier, in 1961, when Itold my mother, excitedly, that although a chemistry
major, my university had just established a biopsychology program that
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xii 
would allow me to study snakes and their behavior, the animals Ihad loved
since early childhood, her rst words, aer a silence, were “How are you
going to support a family?” Iwas crushed, and that conversation has never
le me; and similar ones are indelible for Jesús as well. Ironically, this sim-
ilarity in our lives Ionly learned about in this book! Fortunately, both fam-
ilies accepted our decisions; a memorable event in my career was meeting
Jesús’s mother when she came to Knoxville when Jesús received his Ph.D.and
stayed at our cabin on Clinch Mountain.
e nal chapters of the book provide a glimpse of the geological and cli-
matic history of South America, especially the realm of anacondas. is helps
us to understand their evolution and current distribution. is step back to
look at the larger ecological and historical context of anacondas as they live
today is informative. e narrative then shis to current human pressures on
anacondas and their habitats, political conicts, and issues of wildlife man-
agement and conservation, including misplaced schemes that may do more
harm than good as well as thoughtful suggestions for practices that may ac-
tually work. e criticism of economics and capitalism, especially in poorer
countries, is severe. e stakes are great, and the goal is nothing less, as he
ends this section, than a “peaceable economy.
is book is a rare treasure for anyone interested in the lives of large, poten-
tially dangerous animals and the scientists who study them. Acomplement
to this book is the recent one by Gay A.Bradshaw, Carnivore Minds:Who
ese Fearsome Animals Really Are (Yale University Press, 2017). She dives
deeply and with great sensitivity into the world of other large, fear- inducing
predators such as great white sharks, killer whales, grizzly bears, mountain
lions, and crocodiles as well as some of the scientists who study them. With
this book by Jesús Rivas, giant anacondas are added to the list of species with
remarkable, if somewhat alien, minds and an equal right to persist on this
planet as do apes, elephants, whales, bears, and sharks. Humans were respon-
sible for the demise of many such animals in the past. ese include the moa,
dodo, auroch, elephant bird, great auk, cave bear, Steller’s sea cow, passenger
pigeon, Carolina parakeet, Tasmanian tiger, and giant monitor of Australia
(which makes the Komodo dragon appear to be a dwarf).
e fate of large predatory animals in the world writ large is comparable
to the tiny canary in the mine. If they succumb, are we far behind? We view
our own species as mentally and morally vastly superior to other animals.
If we do not heed the ndings of those among us who provide tools and
insights for solving problems facing the planet, our arrogance will continue
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 xiii
to result in tragedy. Today many people despair that humanity, especially po-
litical leaders, will wake up in time, especially across a collection of dispa-
rate cultures stymied by conict, ignorance, and misinformation but needing
to work together. Nonetheless, our only hope is hope. In the meantime, the
work of creative eld biologists, brilliantly exhibited by Rivas, provides us
access to the lives of remarkable animals and the tools to save them from
I thus invite you to read this book, not supercially or quickly but taking
time to absorb the many messages and the depths of data, logic, reasoning,
empathy, interpretation, and wide- ranging information that you will en-
counter. Jesús points out that he wrote this book “with a thorough scientic
knowledge of the anacondas resulting from years of rigorous scientic obser-
vation and inquiry” but also “with an unapologetic and very partisan point
of view of somebody who loves the snakes and is honored to have had the
chance to gaze into the secret world of the anacondas.” Indeed, only through
understanding the lives of others can we understand ourselves.
Gordon M.Burghardt
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Written by Jesús Rivas, the undisputed expert on the biology of anacondas, this is
the rst authoritative book on the biology of the green anaconda, the world’s larg-
est snake. Rivas describes his experiences over a quarter of a century exploring the
secret life of these fantastic snakes, including their diet, movement patterns, life and
tribulations, survival, behavior, and fascinating reproductive life. But more than just
presenting facts about anacondas, Rivas tells his story about studying them in the
eld. Ultimately, his love for anacondas and his unorthodox approach give his voice a
unique accent that makes this book stand out among other books of its kind. e rich
photography and its storytelling approach make this an enjoyable and thoroughly
readable book that can sit as comfortably on a coee table as in the bookshelves of
advanced scholars.
green anaconda
movement pattern
reproductive life
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Chapter 1 Science, Love, and Anaconda
is introductory chapter provides an overview of anaconda research. It describes
the author’s experience growing up as a herpetologist in Venezuela, which has both
advantages and disadvantages. e advantage is the large number and ubiquity of
snakes. e disadvantages are the lack of guidance and lack of opportunities to learn.
Aer the discovery of an illegal trade of anacondas throughout the continent, the
author decided to advocate for anacondas, using a grant from the Convention for the
International Trade of Endangered Species to study the most fascinating snake in the
world. e chapter dierentiates between two kinds of biologists: hypothesis-driven
ones and organism-driven ones. Hypothesis-driven biologists seek an organism that
ts their question, but organism-driven biologists nd the organism they love and let
the organism indicate what has to be studied about it.
anaconda research
illegal trade
hypothesis-driven biologist
organism-driven biologist
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Chapter 2 e Anaconda Challenge
is chapter focuses on the llanos, Venezuelas natural oodplain, where the author
decided to start his study of anacondas. e llanos is a atland that comprises about
a third of both Venezuela and Colombia. It is composed of an extensive system of
natural, seasonally ooded grasslands. e llanos is located to the north and west of
the Orinoco River and sits on the northern borders of the Amazon basin. Because of
this, most of the wildlife of the Amazon can be found in the llanos, where it is easier
to observe animals in the open vegetation of the savanna. e extreme seasonality of
the llanos made all the dierence in the success of the author’s anaconda research.
Anacondas, being aquatic, concentrate in the few water bodies that hold water during
the dry season. During this time, the chance of nding anacondas was much higher.
e chapter then explores the physical build of snakes. Most snakes have adaptations
of the skull and jaws involving mobile hinges, and a whole arrangement of joints and
muscles evolved for swallowing large prey. e extra mobility of the snakes jaws is
obtained by giving up solid skull sutures that the ancestral lizards had, rendering the
snake’s head more vulnerable to damage.
anaconda research
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Chapter 3 What Is a Good Real Estate for an
is chapter discusses the process of tracking and tagging the anacondas. Trying to
follow anacondas throughout the year gives one a new perspective about the dier-
ences between humans and snakes. Anacondas like caves. In the rivers with forested
banks, the roots of the trees oen hold the bank together and prevent it from being
eroded away. e caves under the roots of the trees tend to be great hiding places for
aquatic organisms like anacondas, caimans, iguanas, tegus, and toads. Both in dry and
wet seasons, anacondas use the caves. Indeed, the presence of caves seems to be the
reason that there are fewer anacondas in the parts of the creek, Caño Guaratarito, that
has no trees. Interestingly, the proportion of males and females found in Guaratarito
was strongly biased toward females. It seems to be a place where large snakes come to
feed but does not seem to be a good place to nd many males.
forested bank
aquatic organism
Caño Guaratarito
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Chapter 4 Anacondas’ Tales
is chapter examines the lives and tribulations of anacondas. Anacondas, like
humans, produce the same number of males and females because the sex ratio is
determined by sexual chromosomes. If the sexes start out equally abundant, why
would there be so many more males out there in the wild? In some vertebrates one
sex or the other disperses away from the area they were born. However, the author
does not believe migration plays a signicant role with anacondas. For starters, males
are the sex with the wanderlust, not females. As such, this would not account for the
lower number of females. Moreover, migration should not be very high due to the
fact that the study site is the only region that holds water, so animals might move
out in the wet season, but they would likely come back in the dry season, when the
author did most of the sampling. If migration is not the answer for the uneven sex
ratio, one is le with mortality of females as the only explanation. If females suered
higher death rates, that could explain the male bias in the population that the model
estimated. Looking further into the results of the population model, the author found
that females have a lower estimated survival than males. e chapter then considers
the reasons females have higher mortality. e large expenditure in reproduction
females incur might result in death not only as a result of predation but also during
or aer feeding due to weakening.
male anaconda
female anaconda
sex ratio
death rate
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Chapter 5 How Much Is Supper? Predator
Diet and Prey Retaliation
is chapter focuses on the anaconda’s diet and the role it plays in the snake’s biol-
ogy. Clearly, food intake is a critical aspect of any animal’s ecology for it determines
how the animal obtains energy needed for survival and reproduction. Anacondas,
with their strong build, are not fast enough to pursue prey on land. Although fast
swimmers, an open mouth moving forward is not very hydrodynamic for catching
shes, which are oen quite fast. However, anacondas can launch an extraordinarily
fast attack on a prey out of the water when needed. In fact, the predatory strike of an
anaconda can be so fast that it beats the eye. e anacondas preferred strategy is to
wait in a place under the vegetation, with which they blend wonderfully, and wait for
the right prey to come within range. is way, anacondas avoid being detected mov-
ing around looking for prey and save in locomotion energy. As reptiles, anacondas
have a slow metabolism, which means they consume very little energy for metabolic
maintenance. e chapter then looks at how anacondas kill their prey.
anaconda diet
metabolic maintenance
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Chapter 6 e Largess of Motherhood
is chapter assesses the reasons and limitations for large size in female anacondas.
Considering how large females are—nearly ve times the size of males—it is obvious
that the evolutionary pressures for large size act more strongly on females than males.
One aspect in which natural selection denitely favors large size in females has to do
with reproductive output. e larger a female is, the more babies can develop in her
body and the larger the reproductive output. Reproductive value, or lifetime repro-
ductive success, is the number of potential ospring that an individual can leave in
the population over its lifetime. ere are costs animals must face when they make
reproductive decisions. Some of these costs are dependent on fecundity and some of
them are not, such as the risk of being preyed upon during mating or pregnancy. A
young adult female that has just reached maturity is under two opposite pressures:
one is to breed right away and secure a few babies into the next generation, and
the other is to skip reproduction, grow larger, and make more babies in a later year.
A female that is going to breed faces another decision: how to invest her breeding
resources. She can produce a large number of neonates of small size or a few ospring
of large size.
female anaconda
anaconda size
natural selection
reproductive output
reproductive value
lifetime reproductive success
anaconda breeding
anaconda reproduction
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Chapter 7 How Big Can a Giant Be?
is chapter determines the maximum size that anacondas can reach, which has been
the subject of long-standing debate among herpetologists. ere are many accounts
of snakes around 9 to 11 meters (29.5 to 36 feet). A lot of the controversy concerns
the credibility of the records, the confusion created by the fact that the skins stretch
when the snakes are skinned, and the use of estimates or unreliable methods of mea-
surement. e largest snake that the author has caught, out of nearly 1,000 animals,
was only a little more than 5 meters (16.4 feet) long. What is the reason for such a
dierence? Due to their slow growth rate, anacondas require a long time to reach a
large size. e author’s study area is a cattle ranch where the protection of wildlife is a
relatively recent practice. Presumably, the really large animals might have been killed
o earlier and the animals that exist now in the areas where the author studied might
not have had enough time to grow to really large sizes. However, this is not all that
there is to it. e chapter then considers the constraints of large body size in anacon-
das, especially female ones. It also describes the discovery of the fossilized remains of
a snake of formidable proportions in eastern Colombia, Titanoboa cerrejonensis, and
makes predictions about its size and natural history.
anaconda size
growth rate
female anaconda
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Chapter 8 “Hey, It’s Me, Its Me!” Courtship
and Confusion in the Male Reproductive
is chapter addresses why male anacondas are so much smaller than females. ere
are a number of advantages for animals to be small. Small animals have lower meta-
bolic costs since they need to maintain only a small body that requires less food.
Animals that need less food have potentially higher survival than those that have a
large body to maintain. is is particularly important in droughts or times of food
shortages. A small body is also an asset because it lowers the cost of locomotion.
Hauling around a large body incurs larger energetic expenditures than if the body is
small. is is particularly important for males that move much more than females in
the mating season to track receptive females for mating. Being small also oers other
benets, such as being inconspicuous, not being a very coveted meal, and hiding eas-
ily from potential predators. Of course, there are also some disadvantages of being
small. Small animals cannot ght o predators as well as large ones and are limited by
their gape to what kind of prey they can eat. Furthermore, larger males may produce
more sperm, which would give them an advantage in siring more ospring, particu-
larly in those cases where the female mates with several males.
male anaconda
anaconda size
sperm production
oso-9780199732876_AK.indd 9 16-Jun-20 6:44:27 PM
Chapter 9 e Origin of the Mystery
is chapter traces the paleo-history of South America to tackle evolutionary ques-
tions about anacondas. Going back in history 150 million years ago, the current con-
tinents of South America and Africa were joined in a single mega-continent that
also included current Australia and Antarctic. In the northern part of this continent
(current South America and Africa) was a large river that started roughly where the
current Congo River starts and drained the continent out of what is currently western
Ecuador. Approximately 110 million years ago, South America separated from Africa
and dried west. e continent was drained by the paleo-Amazon. As South America
dried west, it collided with the Nazca plate in the eastern Pacic. As the two land-
masses moved against each other, the Nazca plate subsided under South America,
pushing up the western border of the latter, giving rise to the Andes. e creation of
the Andes would result in the eventual closing of the drainage of the paleo-Amazon
into the Pacic Ocean. e chapter looks at the signicance of this paleo-history to
the evolution of anacondas. It seems like the conditions in the paleo-history of the
continent of constant ooding were not all that dierent from the conditions that
anacondas encounter currently in the llanos.
South America
Congo River
Nazca plate
oso-9780199732876_AK.indd 10 16-Jun-20 6:44:27 PM
Chapter 10 Conservation of Anacondas and
is chapter focuses on environmental conservation, a relatively new branch of biol-
ogy that has one foot rmly set in science and the other rmly set in economics and
politics. It discusses the technical aspects about anaconda management that can be
used for conservation of anacondas and conservation of biodiversity in general. e
most common methods of extractive wildlife management are farming, harvesting,
or a combination of both. In a farming model, animals are kept in captivity, with
all their needs provided by the keepers. Farming anacondas in a closed system is
unlikely to be successful. However, the possibility of an open farm system exists;
this has more potential of being used as a conservation tool as it requires a natural
environment where the animals live. On the other end of the spectrum is harvesting
or cropping. In a cropping system, animals are harvested from the wild; thus, a direct
link exists between the economic activity and the conservation of the species and its
habitats. e economic incentives the locals receive are directly linked to the habitat,
producing clear reasons for them to protect and not overexploit natural areas. us,
cropping has real potential to be used as a conservation tool, but like open farming,
it is not conservation by itself. e author then addresses holistically the problem of
conservation in Latin America and beyond.
environmental conservation
anaconda management
anaconda conservation
biodiversity conservation
wildlife management
anaconda farming
anaconda harvesting
oso-9780199732876_AK.indd 11 16-Jun-20 6:44:27 PM
Chapter 11 Epilogue
is concluding chapter highlights the unconventional approach of the author in
discussing anacondas. A conventional style would be antiseptic regarding personal
views, and all opinions would be carefully removed as non-scientic. Instead, the
author shared his ideas, mixed with scientic facts, opinions, and interpretations. He
has done this on purpose because that is how actual humans think. Trying to remove
the human from the science renders the science stereotypical and hinders its ability
to address the diversity of problems because it does not have a diversity of views.
Ultimately, diverse backgrounds and diverse experiences will allow for diverse ques-
tions and answers that will allow scientic thinking to evolve and move forward into
new horizons. e author also considers the problem of consumerism and its eects
on environmental conservation.
environmental conservation
scientic thinking
oso-9780199732876_AK.indd 12 16-Jun-20 6:44:27 PM
... Our two study species, Burmese pythons ( Python molorus bivittatus, Pythonidae) and brown treesnakes ( Boiga irregularis , Colubridae, Caenophidia), are phylogenetically distant, and these generalist predators have highly invasive populations in south Florida, and Guam, respectively ( Rodda et al. 1999b ;Taillie et al. 2021 ). Burmese pythons attain huge sizes and are one of only four species of extant snakes with maximal mass > 100 kg ( Barker et al. 2012 ;Rivas 2020 ), nearly 1000 times greater than hatchling mass ( Secor 1995 ;Pittman and Bartoszek 2021 ). Brown treesnakes (maximal total length ∼ 3 m) are also reasonably large for a Fig. 1 Range of sizes used to determine maximal gape. The inset shows brown treesnakes (SVL = 40,177 cm; gape = 1.2, 5.7 cm), whereas the Burmese pythons (SVL = 61,397 cm; gape = 2.8, 22 cm) are in the background. ...
Full-text available
Synopsis Snakes are a phylogenetically diverse (> 3500 species) clade of gape-limited predators that consume diverse prey and have considerable ontogenetic and interspecific variation in size, but empirical data on maximal gape are very limited. To test how overall size predicts gape, we quantified the scaling relationships between maximal gape, overall size, and several cranial dimensions for a wide range of sizes (mass 8–64,100 g) for two large, invasive snake species: Burmese pythons (Python molorus bivittatus) and brown treesnakes (Boiga irregularis). Although skull size scaled with negative allometry relative to overall size, isometry and positive allometry commonly occurred for other measurements. For similar snout-vent lengths (SVL), the maximal gape areas of Burmese pythons were approximately 4–6 times greater than those of brown treesnakes, mainly as a result of having a significantly larger relative contribution to gape by the intermandibular soft tissues (43% vs. 17%). In both snake species and for all types of prey, the scaling relationships predicted that relative prey mass (RPM) at maximal gape decreased precipitously with increased overall snake size. For a given SVL or mass, the predicted maximal values of RPM of the Burmese pythons exceeded those of brown treesnakes for all prey types, and predicted values of RPM were usually least for chickens, greatest for limbed reptiles and intermediate for mammals. The pythons we studied are noteworthy for having large overall size and gape that is large even after correcting for overall size, both of which could facilitate some large individuals (SVL = 5 m) exploiting very large vertebrate prey (e.g., deer > 50 kg). Although brown treesnakes had longer quadrate bones, Burmese pythons had larger absolute and larger relative gape as a combined result of larger overall size, larger relative head size, and most importantly, greater stretch of the soft tissues.
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