Feasibility and efficiency of transmitter force-feeding in studying the reproductive biology of large snakes (click here for a pdf version)


                The secretive nature of the snakes imposes a serious challenge for field studies. Snakeís mating systems, for instance have been hard to study unless in exceptionally large aggregations (Gardner 1955, 1957). These exceptional events may biases the observations to particular situations that do not necessarily reflect the typical mating system of the species. Radiotelemetry has been used in studies of snake biology particularly oriented for the research about home range, and habitat use (Reinert 1992) and thermoregulation (Peterson et al. 1993). Telemetry also has been used to study mating system (Duvall et al 1992; Duvall and Schuett 1997). However, to implant surgically radiotransmitters requires a higher degree of invasive manipulation than is desirable if we do not want to perturb the natural behavior of the animals. Force-feeding transmitters to the snakes to be studied can be done much faster and with a shorter perturbation. It has bee used in the past (Madsen and Shine 1994); however, we do not know details of the duration of the transmitters and how effective it was to study the biology of the animals. In this chapter I document the efficiency of force-feeding radio transmitters to study the mating system in mating system in anacondas (Eunectes murinus).


            The transmitters used were (model 15A2) built by ATS contained the antenna coiled up in the unit, and covered up with a waterproof resin. The dimensions of the units were 15cm long by 2 cm radius and 91 g; with two batteries 3.6 volts each. The frequency of these units was in the range of 164-165 MHz range. These units were set to last for 8 months. I lubricated transmitter with vegetable cooking oil and holding the snake vertically by the head, forced it down the digestive track of the animal by palping it down to the stomach, or as far down as possible (Figure 1).   In larger females, the muscles of individuals tended to prevent the maneuver too far down, but I always could push it far enough to prevent the animal from regurgitating it. In males due to their small size (Rivas 2000) I could push it all the way to the stomach or even palp it out and recover the transmitter if I needed to implant on another animal.

            Over a four years period, I gathered males before and during the mating season that are actively searching for females; or females that were involved in breeding aggregations (Rivas 2000). I equipped 16 males and 15 females 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 palping it out in a similar way it was fed to it initially. I also palped 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 prevented me from feeling or pushing the transmitter by palping.

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 neither the force-feeding or the extraction of the transmitter. In fact all the females continued with the mating and all the males continued with their trailing activities. In no instances the transmitter was regurgitated after the implantation and all animals were followed for at least a week. The transmitters was 0.3% of the average females size and 1.3% the average size of the; perhaps due to its small size, the transmitter that was not perceived as a meal or an obstacle for the animalís movements.

            I removed the transmitter from 13 males. In two cases after 21 and 23 days, the transmitter had to be palped out through the cloaca, In the eleven remaining cases the transmitter was still in stomach, even after more then 30 days, and was extracted through the mouth. In three males I allowed the transmitter go pass naturally which took 21, 43 and 45 days. Notice however the large variance in the time that the transmitter remained in the animals. Even though I extracted most radios before they came out, it must be noticed that the transmitters did stay in the maleís tract for long enough to follow them during the courtship and mating. Most females (9 of 13) kept the units until delivery as they do not feed during pregnancy (Figure 2). Only four females defecated the transmitter before parturition in 12, 14, 24, and 36 days. The extreme difference between this females and the other suggest that they might have 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 time that they kept the transmitters. These animals had not defecated the transmitter after 61 and 68 days when the rainy season started; and presumably they kept it until the parturition since they do not fit the pattern of the animals that defecated soon. The retention times found in females are not conspicuously different from those of males (Figure 2). There does not seem to be a correlation between the passage time and the size of the animals. 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 or not. Breeding females do not eat during pregnancy or breeding (Rivas 2000) and courting males seem not to eat either during the mating season, judging for the long time that most transmitters lasted in many animals.

            I implanted transmitters in 16 males of which 8 found breeding females (Rivas 2000). 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 the males finding females.

            Due to their particular feeding morphology, it is easy to force feed a transmitter to a snake to study its biology. This technique proved to be reliable for short-term follow-ups, since none of individuals implanted regurgitated the transmitter. The procedure did not seem to interfere with the animalís natural behavior, as suggested by the large number of males that found females and 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 that this method can be used successfully with other species, however, it might be less effective in smaller species with shorter passages times and higher feeding frequency.

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 movement and competitive mate searching in prairie rattlesnakes, Crotalus viridis viridis. Animal Behaviour 54: 329-334.

Gardner, J. B. 1955. A ball of garter snakes. Copeia 1955: 310.

Gardner, J. B 1957. A garter 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 consequences of body-temperature variation. In: R. A. Seigel and J. T. Collins (eds.) Snakes Ecology and Behavior, pp. 241-314. McGraw-Hill, Inc., New York.

Reitner, 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. 2000. The life history of the green anaconda (Eunectes murinus), with emphasis on its reproductive biology. Unpublished dissertation. Department of Ecology and Evolutionary Biology. University of Tennessee, Knoxville. USA.


Figure 1. Force feeding the transmitter to an adult male anaconda. The unit is oiled and pushed gently down the snakeís throat. Photo Phillip Bourseiller


Figure 2. Time that the transmitter lasted in the studied animals. All the females passed the transmitter naturally. Most males had the transmitter removed artificially at the end of the breeding season of when they found a female thus the duration time for males is a minimum estimate that the transmitter last on them.