Natural History of Neonatal Green Anacondas ( Eunectes murinus ): A Chip Off the Old Block

Abstract

Our knowledge of the biology of neonatal snakes has lagged behind that of adult animals, mostly due to the difficulty of finding and studying neonatal snakes in the wild. Traditional approaches view neonatal reptiles as miniature replicates of their adult counterparts. In this contribution, we present data on the natural history of neonatal Green Anacondas from opportunistic captures in the wild over a 17-year period, as well as from a brief study on captive-born radio-tagged individuals. Both approaches converge in presenting a picture of the ecology of neonatal anacondas showing many similarities between their natural history and that of adult anacondas in spite of the great size difference. The neonates' biology resembles that of adults, especially males, in their preference for birds in their diet, the relative prey size they choose, slow growth rates they experience, low feeding frequency, little mobility, and preference for similar habitats of stagnant, shallow water covered by aquatic vegetation. The conventional wisdom that neonatal reptiles are replicates of their adult counterparts seems to be largely on target in Green Anacondas. © 2016 by the American Society of Ichthyologists and Herpetologists.

Full text

Natural History of Neonatal Green Anacondas (Eunectes murinus): A Chip Off
the Old Block
Jes´
us A. Rivas
1
, Cesar R. Molina
2
, Sarah J. Corey
1
, and G. M. Burghardt
3
Our knowledge of the biology of neonatal snakes has lagged behind that of adult animals, mostly due to the difficulty
of finding and studying neonatal snakes in the wild. Traditional approaches view neonatal reptiles as miniature
replicates of their adult counterparts. In this contribution, we present data on the natural history of neonatal Green
Anacondas from opportunistic captures in the wild over a 17-year period, as well as from a brief study on captive-born
radio-tagged individuals. Both approaches converge in presenting a picture of the ecology of neonatal anacondas
showing many similarities between their natural history and that of adult anacondas in spite of the great size
difference. The neonates’ biology resembles that of adults, especially males, in their preference for birds in their diet,
the relative prey size they choose, slow growth rates they experience, low feeding frequency, little mobility, and
preference for similar habitats of stagnant, shallow water covered by aquatic vegetation. The conventional wisdom
that neonatal reptiles are replicates of their adult counterparts seems to be largely on target in Green Anacondas.
Nuestro conocimiento sobre la biolog´
ıa de serpientes neonatales se ha quedado atra
´s en comparaci ´
on con el
conocimiento de serpientes adultas, mayormente debido a la dificultad de encontrar y estudiar serpientes reci ´
en
nacidas en el campo. El entendimiento tradicional sobre la biolog´
ıa de reptiles reza que los neonatos son replicas en
miniatura de los adultos. En esta contribuci ´
on presentamos datos producto del estudio oportunista a largo plazo de la
biohistoria de anacondas reci´
en nacidas as´
ı como los resultados de un estudio breve de telemetr´
ıa de anacondas
neonatales nacidas en cautiverio. Ambos m´
etodos convergen en presentar un cuadro de la biohistoria de anacondas
neonatales consistente que muestra similitudes entre su biohistoria con la de anacondas adultas. La biolog´
ıa de los
neonatos asemeja a las adultas en el tipo de presas que prefieren, tama˜
no relativo de las presas que consumen, una lenta
tasa de crecimiento, poca movilidad y preferencia por ha
´bitats con agua estancada, poco profunda y cubierta con
vegetaci´
on acua
´tica. La sabidur´
ıa convencional sobre biolog´
ıa de reptiles neonatos que propone que estos son r ´
eplicas
en miniatura de los adultos, parece dar en el clavo con las anacondas verdes.
REPTILIAN life history studies have advanced largely
based on comprehensive field and laboratory studies.
However, understanding the biology of most reptiles
has been challenged by the difficulty of acquiring field data
from neonates due to their small size and secretive behavior
(Dunham et al., 1994; Morafka, 1994; Morafka et al., 2000;
Pike et al., 2008; Willson et al., 2008; Pizzatto et al., 2009).
However, here we present some of the available studies on
the biology and behavior of neonatal, hatchling, and
juvenile reptiles beyond issues related to parental investment
and incubation conditions. For example, there are several
studies on social behavior of hatchling iguanas (Burghardt,
1977; Burghardt et al., 1977; Greene et al., 1978; Werner et
al., 1987; Rivas and Levin, 2002), dispersal and orientation of
neonatal turtles (Congdon et al., 2015; Iba ˜
nez and Vogt,
2015; Putman and Mansfield, 2015), and the field ecology of
neonatal crocodilians (White and Rivas, 2003; Balaguera-
Reina et al., 2015). The paucity of data is particularly
noteworthy with snakes, where data from field studies are
particularly lacking. There have been some studies on
antipredator behavior (Herzog and Burghardt, 1986; Placyk
and Burghardt, 2005; Gregory et al., 2007; Mori and
Burghardt, 2008), snakes’ innate dietary preferences (Burg-
hardt, 1992; Burghardt and Krause, 1999; Burghardt et al.,
2000), their ontogenetic switches (Mushinsky and Lotz,
1980; Savitzky and Burghardt, 2000), the ontogenetic
changes in vipers’ venom (Mackessy et al., 2003; Wray et
al., 2015), and neonatal social behavior (Burghardt, 1983;
Greene et al., 2002; Holycross and Fawcett, 2002; Cobb et al.,
2005; Jellen and Kowalski, 2007; Reiserer et al., 2008; Howze
et al., 2012). Only one field study seems to have addressed
the natural history of neonatal basal snakes (Pizzatto et al.,
2009).
As noted, a major reason for the paucity of data on
neonatal snakes is that their small size makes them difficult
to find and observe. Even with the latest technology, most
neonates are too small to equip with radio transmitters fitted
with long-lasting batteries and survival may be low. However,
there are exceptions, such as Green Anacondas, which due to
their large size at birth are good candidates for studying
neonatal behavior and ecology. At more than 220 g and 75
cm at birth (Rivas, 2000, 2008), neonatal anacondas are
similar to, or larger than, adults of many other snake species
and therefore can safely be fitted with radio transmitters.
Our understanding of the biology of adult Green Anacon-
das has improved in recent years. There have been compre-
hensive studies of their mating system (Rivas and Burghardt,
2001; Rivas et al., 2007a), general natural history (Rivas,
2000; Rivas et al., 2007b), conservation and sustainable use
(Rivas, 2007, 2010), predation (Rivas et al., 1999, 2001; Rivas
and Owens, 2000), diseases (Calle et al., 1994, 2001), foraging
(Rivas, 1998, 2004), and demography (Rivas and Corey,
2008), along with notes on field techniques (Rivas et al.,
1995; Raphael et al., 1996; Rivas, 2008). Adult anacondas live
in shallow, stagnant water that is often covered by aquatic
vegetation (Rivas, 2000; Rivas et al., 2007b). They are ambush
hunters that may go for long periods without a meal,
although when they do eat they can take quite large meals.
1
New Mexico Highlands University, Department of Biology, 1005 Diamond Avenue, Las Vegas, New Mexico 87701; Email: (JAR) rivas@nmhu.
edu; and (SJC) sjcorey@nmhu.edu. Send reprint requests to JAR.
2
Universidad Central de Venezuela, Instituto de Zoolog´
ıa Tropical, Apartado 1041, Caracas, Venezuela. Deceased.
3
Departments of Psychology and Ecology and Evolutionary Biology, University of Tennessee, Knoxville, Tennessee 37996; Email: gburghar@
utk.edu.
Submitted: 25 January 2015. Accepted: 1 July 2015. Associate Editor: M. J. Lannoo.
Ó2016 by the American Society of Ichthyologists and Herpetologists DOI: 10.1643/CE-15-238 Published online: 3 June 2016
Copeia 104, No. 2, 2016, 402–410

Smaller anacondas prey mostly on birds. This is true for both
juvenile females and most males throughout their lives, but
as soon as females reach reproductive size, which is larger
than the maximum size obtained by most males, they switch
to feeding on mammals and reptiles (Rivas, 2000). Growth
rates decline in adulthood; there is a documented case of an
adult free-ranging anaconda taking 13 years to grow little
more than half a meter (Rivas and Corey, 2008). Adult
anacondas may be found in aggregations during the dry
season where they apparently gather in cave-like depressions
that are exposed in the river banks when the water level
drops, or the few ponds that hold water until the wet season
returns. During this time the snakes are essentially stationary
and appear tolerant to close proximity of conspecifics (Rivas,
2000).
The view that neonatal reptiles are miniature replicas of
adults and undergo few ontogenetic behavioral changes
outside of reproduction that are not mere consequences of
changes in body size has been challenged by evidence of
significant ontogenetic changes in diet (Mushinsky and Lotz,
1980) and other behavioral changes associated with learning
in early stages of development (Gove and Burghardt, 1975;
Burghardt and Krause, 1999; Burghardt et al., 2000; Waters
and Burghardt, 2013). Neonatal animals are expected to have
higher energetic requirements than adults (Nagy, 2000) and
different metabolic requirements due to the scaling of
metabolic processes (Calder, 1996). Thus, we should expect
differences between the life history of neonates and adults of
the same species, such as neonates having higher activity,
foraging more often, and showing more mobility than
adults.
Here we present data from opportunistic observations of
neonatal wild-caught anacondas on diet, mobility, behavior,
growth rate, and habitat use collected over a 17-year period,
as well as from a brief radio-telemetry study done with
captive-born neonatal anacondas. We compare the results of
both approaches with what is known of adult anacondas
from previous studies.
MATERIALS AND METHODS
Study animals.—An obvious initial problem we faced was
accurately defining a neonate. In our long-term study of
Green Anacondas, we found neonate-sized individuals
several months after the parturition season. Newborn
anacondas have umbilical scars, but these can still be found
four months after birth in some individuals, while other
individuals, of presumably the same age, did not show them.
Because the birth season is from September to November
(Rivas, 2000), all animals found at neonatal size in the
March–April season are presumably between 3 to 7 months
old. Since growth is strongly influenced by food intake, an
older neonate-sized individual may face the same ecological
challenges as a true neonate in terms of temperature
regulation, predators, and prey availability. However, being
older, it may have different cognitive and experiential
abilities than a newborn animal. Not wanting to set an
arbitrary limit, we graphed the cumulative frequency of
anaconda sizes from 788 animals caught over 17 non-
consecutive years. There was a generally continuous distri-
bution of sizes except for a gap in the distribution in the early
classes (Fig. 1). We believe that the break in the distribution
corresponds to the most recent cohort of the population. We
chose 92 cm as the cut off for defining a neonate, as it is near
the midpoint between the largest of the neonate class (84.6
cm) and the smallest of the larger class (98.9 cm). Larger
animals are almost certainly older than one year of age.
Study site.—We have been studying the natural history of
anacondas since 1992 in the Venezuelan llanos at Hato El
Cedral, a 54,000 ha cattle ranch located in the Mu˜
noz district
(78300N and 698180W) of Apure State (Rivas, 2000). The
sampling area was approximately 6 km
2
surrounding the
station. During March and April, which comprise the driest
part of the dry season, we looked for animals in the swamp
and other aquatic habitats. The predominant vegetation
associated with this hyper-seasonal savanna involves few
trees, grasses, and shrubs that may be dense near water. A
series of human-made dikes have created more permanently
flooded habitats (‘‘m´
odulos’’) where the impact of the dry
season is diminished. The gates of the dikes are closed at the
end of the wet season to hold water for pastures and cattle.
Construction of elevated roads required dirt taken from
surrounding areas, which produced a series of artificial ponds
or borrow pits. Although artificial in origin, the borrow pits
resemble natural depressions that occur in the landscape.
They often are covered by aquatic vegetation, mostly water
hyacinths (Eichhornia crassipes,E. azurea) and an aquatic fern
(Salvinia sp.). Borrow pits are used by different animals that
either live in or near them, or that visit them to drink (for a
complete description of the habitat, see Rivas et al., 2002).
Survival.—Despite their size, anacondas are anything but
conspicuous. The dry season (in particular March through
May) provides the best time for finding wild anacondas
because all of the snakes that live in the flooded savanna
gather in the few depressions that hold water. We systemat-
ically searched all the bodies of water in the study area with
water depths less than 50 cm. Search parties were anywhere
between two to six people wading through the vegetation
looking for snakes by feeling under the water and probing
with sticks (see Rivas et al., 2007b for a complete description
of search and capture). During the first six years (1992–1997),
we sampled for about six hours a day for 55 days per season.
We were not able to collect data every year after 1997, and
during the years we collected data (2002, 2003, 2007, 2009,
Fig. 1. Cumulative distribution of sizes of 788 Green Anacondas
(Eunectes murinus) from a 17-year study in the Venezuelan llanos. We
used the natural break in the distribution of the population for the cut
off between neonates and the rest of the population (cut off, 91.75 cm,
indicated by arrow).
Rivas et al.—Neonate natural history 403

and 2011), the sampling effort was limited to two weeks. In a
parallel study of reproductive biology, we marked 278
captive-born anacondas in 1995 and 1996 and released them
at the study site (where the mothers were collected) to assess
neonatal survival in the wild.
Diet.—Some animals were found in the process of constrict-
ing prey or having recently eaten a meal. If the animal had
recently eaten, we kept it in a burlap bag and waited until the
snake defecated before it was returned to the place of capture.
The feces were then preserved in 70% ethanol, and prey
remains were identified in the lab or with a reference
collection by specialists. Prey capture rate is low in wild
anacondas (Rivas et al., 2007b). To avoid negatively affecting
their growth and survival, we therefore refrained from
removing prey from stomachs. We did not use the post-
defecation mass for the snakes for calculations of ‘‘net snake
mass’’ because we could not be sure that remnants of the prey
were not increasing the mass of the snake. When we could
not weigh birds, we obtained the mass reported in the
literature (Hilty, 2003) since the mass of birds varies little
after they fledge. With mammals or birds that we could not
identify, we calculated the expected mass of the snake based
on the following empirical regression: Log(mass) ¼2.9565 3
Log(svl)3.0902 (n¼770; R
2
¼0.95) obtained for Green
Anacondas in the same area (Rivas, unpubl. data). The prey
mass was calculated as the difference of the recorded mass of
the anaconda and prey minus the calculated mass for the
anaconda. Relative prey size was calculated using the snake
mass excluding the prey mass, so the prey mass was not used
twice in the same calculation.
Habitat use.—Two pregnant female anacondas were collected
in the dry season (April) 1995 and kept in captivity until they
gave birth in November. Neonates from both clutches were
mixed and maintained in enclosures until January, when five
individuals were selected for implantation of radio transmit-
ters. Table 1 shows the data from neonates used in the
telemetry study. The neonates were not fed in captivity to
prevent biasing of prey preferences in the na¨
ıve animals
(Burghardt, 1992). Field observations indicate that this level
of fasting is not uncommon among neonatal anacondas
(Rivas, 2000; Rivas et al., 2007b). Transmitters (AVM, model
SM1-H) were 37 mm long, 9.5 mm wide, 6 mm thick, and
weighed 4 g with a 20 cm long antenna and a four-month
expected life span. Transmitter-implanted neonates had an
average mass of 202 g, so the transmitters represented about
2% of the neonatal mass. Transmitters were implanted in two
male and two female neonates. Subcutaneous infiltration of
0.03% lidocaine was used for local anesthesia, and the
transmitters were implanted subcutaneously (Raphael et al.,
1996).
We released the neonates, with their mothers, 24 h after
implantation in the borrow pits closest to where the mothers
had been caught in the wet season. We determined locations
of the neonates by radio telemetry approximately every other
day. We recorded data on their position, water depth,
vegetation, habitat, and movements. Two of the individuals
were killed by predators within a month (Rivas et al., 2001).
The transmitter of one of these animals was then re-
implanted in an animal from one of the same clutches (see
Angelica on Table 1) that had been kept in captivity for such
an eventuality. We plotted the locations on a map and used
the 100% Maximum Convex Polygon method (White and
Garrot, 1990) to estimate the home range of each animal
using Arcmap (ESRI, 2006, ArcMap Version 9.2. Environ-
mental Systems Research Institute, Redlands, CA).
RESULTS
Survival.—Of a sample of 696 wild-caught anacondas from
the study area, only 25 (3.6%) were classified as neonates. Of
these 25 animals, only two (8%) were recaptured in a later
year. Of the 278 neonates released from pregnant females,
none were caught in any later year, at any size. Of the 25
wild-caught neonates for which data were available, 23 were
found in shallow, stagnant water under plant cover either in
a borrow pit (13) or along the edges of a m ´
odulo (10) with
only two found in a savanna creek. Eight (32%) wild
neonates were moving on the road, five (20%) were
foraging/catching prey, six (24%) were basking, three (12%)
were found buried under the mud, and three (4%) were
swimming on the surface in open water or under aquatic
vegetation. The two recaptured, wild-caught neonates grew
at rates of 0.068 and 0.074 mm/day, respectively, over an 11-
month period. The one with the lower growth rate was
missing an eye at the time of the first capture. Surprisingly, it
was still alive the next year despite this apparent handicap.
The average prey size of neonatal anacondas was 26.3% of
the snake’s pre-meal mass. Eight of the 25 (32%) wild-caught
neonates found were either consuming or had recently
ingested a meal. Seven of the nine prey items obtained from
neonates were birds and two were mammals (Table 2). We did
not find any fish, reptile, or amphibian remains.
Habitat use.—Radio-tagged animals moved little (Table 1),
resulting in a considerable overlap of individuals’ home ranges
(Fig. 2). About a month into the study, the transmitters failed
as they stopped putting out a regular beep (three months
ahead of schedule). We also noted that at this time the
incision for the implantation had not closed in three out of
five neonates.
Table 1. Measurements and activity of neonate Green Anacondas implanted with radio transmitters. TL ¼total length, SVL ¼snout–vent length, TNL
¼total number of localizations, Time ¼number of days that the animal was under study, WD ¼water depth, PV ¼percentage of times the animal was
visible, HR ¼home range is the surface area, in hectares, used by the animal calculated by the maximum convex polygon. ADM ¼average distance per
move (in meters) and ADD ¼average daily distance moved per day. An asterisk (*) indicates snakes that were killed by predators during the study.
Name TL SVL Mass Sex TNL Time WD PV HR ADM ADD
Cassie* 84.7 76.1 210 F 14 22 19.5 0 0.464 37.83 10.01
The Beast 79 67.5 200 M 10 22 17.5 33.3 0.013 6.42 2.47
Ingrid* 86 74.6 190 F 15 22 30 0 0.008 11.24 3.28
Pedro 83.7 72.8 180 M 13 17 11.33 0 0.008 4.08 2.4
Angelica 88.3 76.3 230 F 20 28 17.45 52.6 0.017 7.02 4.09
Average 84.3 73.5 202 14.4 19 19.16 17.2 0.102 13.32 4.45
404 Copeia 104, No. 2, 2016

Telemetered neonates were always found in shallow water
and rarely in water deeper than 20 cm. They were concealed
by their cryptic coloration and coiled within the bulbs and
roots of E. crassipes (73%), in a combination of E. crassipes
and Salvinia sp. (20%), in grasses with Salvinia sp. (3%), in
Salvinia sp. only (3%), or in open water (1%). When located,
the snakes often had their heads above the surface, facing the
shore. All but one of the locations (98.6%) were in the same
borrow pit where the snakes had been released; the
remaining one was in a nearby creek.
The only instance in which an individual was found in any
place other than a borrow pit was Cassie, a neonate observed
Table 2. Neonate anacondas in the Venezuelan llanos found with evidence of a recent meal. We used the reported mass of the prey (Hilty, 2003)
and subtracted it from the mass of snake and prey to calculate the snake net mass. When the prey mass was unknown, we estimated the mass of the
prey by subtracting the estimated mass of the anaconda based on its SVL. These cases are indicated with an asterisk (*). RPS ¼relative prey size. In
this table we also include a neonate that was born in a naturalistic enclosure and that caught a bird before it was found in the birth cage.
ID SVL (cm) Mass (g) Prey item Prey size (g) RPS %
E247C 73 262.25* Unknown mammal 112.75 42.99
E498C 81.5 336.19* Unknown mammal 96.81 26.65
E1008C 58.03 180 Jacana jacana 40 22.22
E1009C 82 335 Crotophaga ani 101.55 30.31
E1012C 75.43 440 Jacana jacana 80 18.18
E738 73.8 315 Crotophaga ani 100 31.75
E861C 76.5 301.19* Passerinae 51.18 16.31
E588 81.8 460 Jacana jacana 70 15.22
Captive 73 195 Phacellodomus rufifrons 21.5 11.03
Fig. 2. Home ranges of radio-tagged neonatal Green Anacondas in the Hato El Cedral (Apure State, Venezuela), with water boundaries outlined in
gray. Point A indicates where the neonates were released. Aerial map inset: Point A release location, B the last place where Cassie was seen alive, and
C the place where the radio of Ingrid was found, presumably after a predation event. To the west was a m´
odulo that held a large area of permanent
water and to the north was a flowing river.
Rivas et al.—Neonate natural history 405

in a small creek that connects the borrow pits with a main
river, 224 m northeast of the location where she was released.
The next time this snake’s radio signal was followed, Cassie’s
transmitter was found chewed up at the base of a tree that
held a Crested Caracara (Polyborus plancus) nest (Rivas et al.,
2001). Cassie was also one of the neonates that had moved
most frequently (Fig. 2; Table 1). The other individual that
showed more mobility, Ingrid, was also a victim of predation,
which we documented by finding the transmitter in a bush
away from the water with a chewed-up antenna (Rivas et al.,
2001).
On day 10 after release, one of the neonates (see Pedro on
Table 1) was observed with evidence of a recent meal. Of the
five animals followed for approximately one month each,
only Pedro showed evidence of having eaten. Although other
habitats were available, neonates remained in a set of
connected borrow pits heavily covered by water hyacinth
at a mean water depth of 17 cm. Fifteen meters to the west, a
large body of water confined by the road (labeled m ´
odulo in
Fig. 2) had much deeper water, and approximately 50 m to
the north was a permanent river. While the route to the
m´
odulo involved travel on dry land and consequent
exposure to predators, the route to the river was through
other borrow pits that had very similar habitats to the ones
the neonates were utilizing, yet most neonates did not use it
(Fig. 2).
DISCUSSION
Survival.—The neonate recapture rate of 8% was very low in
comparison with the 24.7% found in adult anacondas (Rivas,
unpubl. data). This difference could be due to at least two
non-mutually exclusive reasons. First, neonates suffer higher
mortality than adults, and second, neonates are more
difficult to find due to their small size and crypsis. Since we
rely on feeling them under the vegetation, the bodies of
smaller animals would be less likely to be detected. We
believe that low detection in neonates is a major reason for
the low recapture rate. However, given that we have surveyed
this population between 1992 and 2011, it would be
remarkable if the paucity of recaptures was due to crypsis
alone. We would presumably have recaptured the marked
neonates as larger individuals in later years. Rather, the lack
of recaptures suggests that juvenile mortality is high.
Although recent simulation models suggest that neonatal
mortality in reptiles is not as high as formerly believed (Pike
et al., 2008), our data indicate that high neonatal mortality is
likely the case in anacondas. That two out of five radio-
tagged animals were preyed upon during the first month of
life strongly supports this assumption (Rivas et al., 2001).
Growth rate.—Thegrowthratefoundamongneonates
appears low for animals that are expected to be in their peak
growing period. This growth rate is also low when compared
to the growth rate of captive neonatal anacondas fed
regularly. We estimated growth rate from three neonates
raised at the San Diego Zoo using their mass when they were
acquired to estimate their length (length data were not
available). Growth in the first 504 days is estimated to be 0.14
mm/day, twice that found in wild animals. Reported growth
rate from other captive anacondas shows a neonatal growth
rate as high as 2.13 mm/day in the first 445 days (Lamonica
et al., 2007). The difference between the growth rate of
captive animals is unclear. Likely it is due to different feeding
regimes. What is certain is that neonatal anacondas have the
capacity to grow much faster than they do in the wild.
The slow growth of wild animals may be due to limited
food consumption. Even when wild animals have plenty of
natural prey available, foraging may result in high exposure
to predators. It is likely that the food intake of neonates is
limited to the potential prey that moves within reach of the
neonate in their retreats where the neonates both avoid
predation and stalk prey. In fact, both of the recaptured
animals mentioned above were re-caught the following year
in exactly the same place they were found the first time,
consistent with the notion of low mobility. This strategy
might work well in the hyperseasonal savanna: neonates that
venture out of the safe aquatic refugia during the pro-
nounced dry season will be exposed to predators; on the
other hand, staying in a shrinking body of water would be
beneficial because it increases encounters with small birds
(prey) that come to drink. Unfortunately no comparable
study addresses other neonatal Boidae, so we lack a frame of
reference to compare the growth rate of neonatal anacondas
with snakes of comparable size. Perhaps the only baseline
information that are comparable to anacondas in their
ecology and phylogenetic position are aquatic pythons
(Liasis fuscus). They are tropical constrictors that inhabit
aquatic environments; however, with a body mass between
16 and 35 g at birth (Pizzatto et al., 2009), they are a fraction
of the size of neonatal anacondas. In a long-term study on
their ecology, Madsen and Shine (2000) found growth rates
between 1 and 1.9 mm/day, far higher than that found in
neonatal anacondas. Adult anacondas do have a very slow
growth rate, even slower than that of neonates (Rivas and
Corey, 2008).
Diet and prey size.—The diet of neonatal anacondas seems to
be comprised mostly of birds, which is consistent with the
diet of small adult anacondas (Rivas, 2000). Although we did
not find evidence for predation by neonatal anacondas on
hatchling spectacled caimans (Caiman crocodilus) or savanna
sideneck turtles (Podocnemis vogli), these animals occupy the
same habitat and are small enough to be potential prey. We
were surprised at the absence of fishes and amphibians in the
diets of neonatal anacondas, particularly in light of their
abundance in shared habitats. Amphibians do not have
keratinous structures that resist digestion and thus we would
not have found them in feces. So, the lack of amphibian
remnants does not necessarily indicate that neonatal ana-
condas avoid them. On the other hand, we would have
found scales or some remnants of fishes if they were present
in the diet. Our methods of sampling had another limitation:
small prey items that did not make a noticeable lump would
have gone unnoticed and we would not have obtained such
feces for analysis. Amphibians and fishes are also missing in
the diet of adult anacondas (Rivas, 2000).
The proportion of neonates that were found in the act of
obtaining a meal seems high compared with that of adult
anacondas (Fig. 3). This may be related to the need of young
animals to gather energy to fuel growth. Nagy (2000)
predicted that neonates would have higher metabolic activity
and spend more time foraging and less time engaged in other
activities such as thermoregulation and socialization. How-
ever, the activity pattern found in neonates might not
accurately reflect the actual activity pattern of neonatal
anacondas. Rather, certain activities make them more
vulnerable to detection by both scientists and potential
predators. In fact, 71% of the observations were from animals
406 Copeia 104, No. 2, 2016

engaged in activities that made them more detectable such as
moving, eating, and basking (Fig. 3).
The notion that opportunistic observations of neonates
misrepresents their activity is supported by the telemetry
observations, which suggest far higher frequency of being
under vegetation and a considerably lower feeding frequen-
cy. Our telemetry study suggests that the odds of detecting an
animal are lower than 20%, even if we knew where the
animal was (Table 1). Low feeding frequency was also found
by Pizzatto et al. (2009) with L. fuscus. They found only two
feeding events while following ten neonates for seven
months. The true feeding frequency in neonates probably
falls somewhere between the high rate suggested by animals
found in the act of constricting prey, which likely overesti-
mate the actual feeding rate, and the low rate found by
telemetry data that likely underestimate it. A low feeding
frequency is also supported by the low growth rate found in
wild-caught animals.
Average prey size observed in neonates is on par with the
expected prey size found in adult snakes (Greene, 1992;
Sazima, 1992), and it is not uncommon for large constrictors
(Branch and Haacke, 1980; Rivas, 1998, 2000; Shine et al.,
1998). Andreadis and Burghardt (2005) reported that neona-
tal Northern Water Snakes (Nerodia sipedon), given the choice,
chose meal sizes close to 25% of their body mass. This is very
close to the relative prey size found in wild neonatal
anacondas. We do find a high variance in their diet with
some neonates taking prey as large as 42% of their body
(Table 2). This is perhaps due to their sit-and-wait strategy
that probably deems them dependent on whatever prey
comes within range.
Mobility and habitat use.—Some of the neonates’ transmitter
implantation incisions had not healed at the end of the study
period, so the possibility exists that this may have influenced
the observed behavior. However, we do not believe that the
superficial wounds had a significant influence on the
behavior of neonates or data collected for several reasons:
none of the wounds showed any sign of infection; wild
anacondas regularly show very large wounds as a conse-
quence of predation attempts, from which they recover well
(Rivas et al., 2007b); and the behavior found in radio-tagged
anacondas (hiding regularly in underwater vegetation, using
borrow pits with shallow water, etc.) did not differ from that
found in wild-caught neonates. Anacondas with open
wounds are often found basking long hours (Rivas, pers.
obs.). Radio-tagged neonates did not show any of these
behaviors.
Neonatal anacondas were frequently (54%) found under
plant cover in borrow pits with shallow, stagnant water. This
is comparable with the 60% frequency with which adult
anacondas use the same kind of habitat (Rivas et al., 2007b).
However, we believe that these data underestimate the actual
frequency with which both adults and neonates use these
habitats due to detection biases. In fact, telemetry data
suggest a much higher frequency.
Radio-tagged neonates did not move much during the time
they were followed, resulting in a considerable overlap of
individual home ranges. This low mobility is consistent with
mark–recapture observations, where the only two recaptured
neonates were found in exactly the same place as the original
capture. Neonatal anacondas fall within the size classes of
many adult snakes of other species, yet the average distance
traveled in one day and the home range area are both much
smaller than reported for other snakes. Larger home ranges
than those found in neonatal anacondas occur with other
neonatal species such as the rather sedentary Eastern
Massasauga Rattlesnake (Sistrurus catenatus catenatus; Jellen
and Kowalski, 2007). This is remarkable because home range
is associated with body size (Schoener, 1981), and neonatal
anacondas, exceeding 200 g, are more than one order of
magnitude larger than neonatal Massasaugas (at 9.6 g).
Northern Water Snakes (N. sipedon), with a comparable
aquatic ecology, also show much larger home ranges than
neonatal anacondas (Tiebout and Cary, 1987; Brown and
Weatherhead, 1999a, 1999b; Pattishall and Cundall, 2009).
The home range found is also very small compared with
other large-sized tropical constrictors such as Carpet Pythons
(Morelia spilota; Pearson et al., 2005), Puerto Rican Boas
(Epicrates inornatus; Puente-Rol ´
on and Bird-Pic ´
o, 2004), and
Boa Constrictors (Boa constrictor; Holtzman et al., 2007).
Neonatal anacondas also have small home ranges compared
with other tropical, more closely related taxa that also live
primary in aquatic ecosystems such as Water Pythons
(Pizzatto et al., 2009). This low mobility, however, is
consistent with the behavior of adult anacondas (Rivas,
2000) and with the emerging picture of neonatal anacondas
suffering very high predation.
The predation of two of the radio-tagged neonates (see
Cassie and Ingrid on Table 2) could be associated with a high
risk of mortality associated with dispersal (Bonnet et al.,
1999). We speculate that female anacondas might hedge
their bets in these unpredictable habitats by producing
animals with different strategies. If the conditions are
favorable for more active individuals (e.g., fewer predators),
active animals might be more successful, while more
sedentary animals will do better in areas with strong
predation. Selection for one or the other in the first months
of life will influence what proportion of adult individuals will
have a given strategy in a particular habitat. We believe that
selection in this habitat favors neonates having less active
foraging modes.
Social behavior and aggregations.—Wild anacondas frequent-
ly aggregate in a few ponds or cave-like depressions, but
whether these are actual social aggregations or just aggrega-
tions around a limiting physical resource by animals that
tolerate each other is uncertain. In our studies of the wild
population at the peak of the dry season, we once found 34
anacondas smaller than 2 m SVL in a drying pond that was
Fig. 3. Comparision of activities between neonates (n¼21) and
anacondas of all sizes (n¼514) from a long-term study on their natural
history (Rivas et al., 2007b). Main differences are the higher percentage
of neonates found eating and the obvious lack of mating by neonates.
Rivas et al.—Neonate natural history 407

barely 400 m
2
(Rivas, unpubl. data.). Studies are currently
underway to determine the level of relatedness between these
individuals. However, the variance in size suggests that they
are not likely to be from the same clutch. Neonatal snakes are
known to aggregate and appear to show a preference to
congregating with conspecifics (Burghardt, 1983; Gilling-
ham, 1987). Reiserer et al. (2008) reported thermal benefits of
aggregations for neonatal Sidewinder Rattlesnakes (C. ceras-
tes).
As the first study on the neonatal biology of any South
American boid, and only the second on neonates of any large
snake, we have barely broken ground in the understanding of
the neonatology of Green Anacondas. What we have found
supports the conclusion that neonates appear to closely
resemble adults in several aspects of their ecology, including
habitat preferences, mobility, prey size, and even growth rate.
ACKNOWLEDGMENTS
We thank The Wildlife Conservation Society, The Wildlife
Conservation Society Field Veterinary Program the Depart-
ment of Clinical Care, The National Geographic Society, the
Doue de le Fountain Zoological Park, CITGO corporation,
and Miami Metro Zoo for financial and logistic support. We
also thank COVEGAN, Estaci ´
on Biol´
ogica Hato El Fr´
ıo, for
logistic assistance and for permission to perform this study
on their property. We thank E. Lamarca for identification of
bird remnants in the feces. We also thank B. Baldwin for
unpublished data access to animals at San Diego Zoo. We are
in debt to M. Quero and P. Azuaje for their advice and
cooperation in the development of this research as well as
assistance during the field work. We are in special debt to the
late J. Thorbjarnarson for all his help and mentorship in this
study and over the years, and we dedicate this paper to his
memory.
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