Content uploaded by Jesus Rivas
Author content
All content in this area was uploaded by Jesus Rivas on Nov 24, 2020
Content may be subject to copyright.
-···-··'"-·'-~
-~·-·-'-·
1,;...,,·-~
......
._..,·
..
•
...
·,
...
,..
..
;;,.'.Mo
..
<•'-'""''"''"'·~~•:.0..."--'·-
"'
-
._
..
·,_,
------bL
.....
'"°*-~~-
--
-
~•
•-•-•
••
-
........._
..
---....
..
~.'
Vol. 8(1), 2001 Herpeto/ogical Natural History, 8(1), 2001, pages 95-97.
©2001
by
La Sierra University
95
FEASIBILITY AND EFFICIENCY OF TRANSMITTER
FORCE-FEEDING IN STUDYING THE REPRODUCTIVE
BIOLOGY OF LARGE SNAKES
Jesus A. Rivas•
Department
of
Ecology and Evolutionary Biology, University
of
Tennessee,
Knoxville, Tennessee 37996-0900, USA
The
secretive
nature
of
the
snakes imposes a
serious challenge for field studies. Snake mating sys-
tems, for instance. are hard to study unless they occur
in
exceptionally large aggregations (Gardner 1955,
1957). These exceptional events may bias the obser-
vations
in
particular situations such that they do not
necessarily reflect the typical mating system
of
the
species. Radiotelemetry has been used in studies of
snake biology, particularly
in
research about home
range and habitat use (Reinert 1992) and therrnoreg-
ulation (Peterson et
al.
1993). Telemetry has also
been used
to
study mating systems (Duvall and
Schuett 1997; Duvall et al. 1992). However, implant-
ing
radiotransmitters surgically requires a higher
degree
of
invasive manipulation than
is
desirable if
the
natural behavior
of
the animals
is
not
to
be dis-
turbed. Force-feeding transmitters to snakes can
be
done much faster and with an abbreviated handling
time. This technique
has
been used
in
the past with
Vipera
ber11s
(Madsen and Shine I 994), although it
is
not
known
what
the
longevity of
the
transmitters
was
or how effective they were
in
studying the biology
of
the animals.
In
this paper I document the efficiency
of
force-feeding radio t.ansmitters
to
study the mat-
ing
system
in
anacondas
(Eunectes
1nurinus).
MATERIALS
AND
METHODS
Transmitters used (ATS, model 15A2) con-
tained
an
antenna
coiled
up
inside
the
unit
and
cov-
ered
up
with a waterproof resin. The units were
cylinders with dimensions
15
x 2 cm and weighing
91
g,
including two 3.6 V batteries. The frequency
range was 164-165 MHz and units were set to last
for 8 mo. I lubricated the transmitter with vegetable
cooking oil and. holding snakes vertically
by
the
head. transmitters were gently pushed down the
!Current
address
for
correspondence:
Venezuela
Nature
Tours.
17126
Lawson
Valley
Road.
Jamul.
California
91935.
USA.
Email: anaconda@prodigy.net
--
..
--··
"•"·-··.-·-
digestive tract
of
the animal by palpating them into
the stomach, or as far down as possible (Fig. I). In
larger females, muscles tended to prevent palpation
too far posteriorly, but I was always able to push the
transmitter down far enough to prevent the animal
from regurgitating it. In males, due to their small
size (Rivas 1999), I could push the transmitter all
the way to the stomach and even palpate it out sub-
sequently to recover the transmitter,
if
needed to
implant into another animal.
Over a 4-yr period, I gathered males actively
searching for females and females involved
in
breeding aggregations before and during the mating
season (Rivas 1999). I equipped
16
males and
15
females with transmitters and monitored their
behavior during the mating season and throughout
pregnancy. On several instances when a male found
a female, I removed the transmitter from the male by
palpating it out. I also palpated out the transmitter
of
all the males at the end
of
the breeding season
of
each year to recover the transmitters for future use.
Retrieving the transmitter from females was not
possible due to their more muscular body that pre-
vented me from feeling
or
pushing the
il"llnsmitter.
Figure t. Force-feeding a
transmitter
to
an
adult
male
anaconda
(Eunectes murinus).
The
unit
is
oiled
and
pushed gently down the snake's throat Photo
by
Phillip
Bourseiller.
/
/
96
Herpetological Natural History, Vol. 8(1), 2001
RESULTS AND
DISCUSSION
The method
of
force-feeding transmitters
proved to be an effective means
of
studying the
mating system
of
anacondas.
No
animal died or
showed any
ill
effect as a consequence
of
either the
force-feeding or the extraction
of
the transmitter. In
fact, all
of
the females continued with mating, and
all
of
the males continued their trailing activities. In
no instance was the transmitter regurgitated after
implantation, and all animals were followed for at
least a week. The transmitters comprised 0.3%
of
average female weight and l .3%
of
average male
weight. Perhaps due to its small size transmitters
were not perceived as a meal
or
as obstacles to the
animal's
movements.
I removed the transmitter from
13
males. In two
cases, after
21
and
23
d, the transmitter had to be
palpated out through the cloaca. In the I I remaining
cases.
the
transmitter
was
still
in
the
stomach,
even
after more than 30 d, and was extracted through the
mouth. In three males I allowed the transmitter to
pass naturally which took 21, 43. and 45
d.
It
is
notable that the variance
in
the time a transmitter
remained within the animals was high. Even though
I extracted most transmitters before they passed nat-
urally,
it
is
imponant that the transmitters stayed
in
the male's digestive tract long enough to track them
throughout counship and mating. Most females
(9
of
13)
kept the units until delivery as they do not
feed during pregnancy (Fig. 2
).
Only four females
passed the transmitter before panurition
in
12, 14,
24, and 36
d.
The extreme difference between these
females and the others suggests that they might have
250
~200
0
0
150
~
100
E
"
z
50
•
•
••
•
•
• • •
•
,.
Females
Males
0
+--~~+-~~+-~~+--·~·~..---~---.
150
200
250
300 350
snout-vent
length
(cm)
400
Figure 2.
Time
that
rhe
transmitter
lasted
within
the
stud-
ied
animals.
All
females
passed
the
rransmitter
naturally.
Most
males
had
the
transmitter
removed
artificially
at
the
end
of
the
breeding
season
of
when
rhey
found
a
female.
Thus
duration
time
for
males
is
a
minimum
estimate.
had food in their digestive tracts at the time
of
the
procedure so the transmitter might have passed
along with the stomach contents. Two other females
were not captured after the mating and I could not
record the exact length
of
time that they kept the
transmitters. These females had not defecated the
transmitters after
61
and
68
d when the rainy season
staned. Presumably they kept them until the
panuri-
tion since they
do
not fit the pattern
of
the animals
that defecated.
The
retention times found in females
are
not conspicuously different from those
of
males
(Fig. 2). There does not seem to be a correlation
between passage time and animal size. The variation
in retention time
in
females seems to be strongly
influenced by the effect
of
pregnancy on feeding.
Thus, the time that the transmitter is retained is
highly variable, and perhaps
it
is most related to
whether the animals were digesting a prior meal
when the transmitter was fed
or
not. Breeding
females do not eat during pregnancy or breeding
(Rivas 1999) and couning males seem not to eat
either during the mating season, judging by the long
time that most transmitters lasted in many animals.
We
implanted transmitters into
16
males
of
which eight found breeding females (Rivas 1999).
This is not necessarily an accurate reflection
of
male success in finding females, because in three
cases I removed the transmitter before the end
of
the season. Thus 50% might be a minimum estimate
of
the actual success rate
of
males finding females.
Due to their panicular feeding morphology, it
is
easy to force-feed a transmitter to a snake to
study its biology. This technique proved to be reli-
able for shon-term follow-ups, since none
of
the
individuals thus implanted regurgitated the trans-
mitter. The procedure did not seem to interfere with
the animals'
nat~ml
behavior, as suggested by the
large number
of
males that found females, and by
all the females whose mating was studied, This
technique can be used quite successfully for studies
of
mating systems,
or
even reproductive biology,
if
care is taken in not implanting the transmitters in
animals that have recently fed. I believe this
method can be used successfully with other species.
However, it might be less effective in smaller
species with shorter passage times and higher feed-
ing frequency.
.
·.
,.
. .
-·
".,
. ' .
Notes
LITERATURE CITED
Duvall, D.G., Arnold, S.J., and G.W. Schuett. 1992.
Pitviper mating system: ecological potential, sexual
selection, and microevolution. In: J.A. Campbell
and E.D. Brodie
Jr.
(eds.) Biology
of
Pitvipers, pp.
321-336. Selva, Tyler, Texas.
Duvall,
D.
and G.W. Schuett 1997. Straight-line move-
ment and competitive mate searching in prairie rat-
tlesnakes, Crotalus viridis viridis. Anim. Behav.
54:329-334.
Gardner, J.B. 1955. A ball
of
garter snakes. Copeia
1955:3!0.
Gardner, J.B. 1957. Agarter snake "ball." Copeia 1957:48.
Madsen,
T.
and R.G. Shine. 1994. Costs
of
reproduction
influence the evolution
of
sexual size dimorphism in
snakes. Evolution 48: 1389-1397.
Peterson, C.R, Gibson, A.R., and M.E. Dorcas. 1993.
Snake thermal ecology: the causes and conse-
quences
of
body-temperature variation.
In:
R.A.
Seigel and J.T. Collins (eds.), Snakes: Ecology
and
Behavior, pp. 241-314. McGraw-Hill, New York.
Reinen,
H.K.
1992.
Radiotelemetry field studies
of
pitvipers: data acquisition and analysis. In: J.A.
Campbell and
E.
D.
Brodie (eds.), Biology
of
Pitvipers. pp.185-197. Selva, Tyler, Texas.
Rivas. J.A. 1999. The life history
of
the green anaconda
(Eunectes murinus), with emphasis
on
its reproduc-
tive biology. Unpubl. Ph.D. Thesis, Department
of
Ecology and Evolutionary Biology, University
of
Tennessee,
Knoxville, Tennessee.
,~
.,, . .,, -
,·
97
