Abstract

Giant garter snakes (Thamnophis gigas) are endemic to wetlands of California’s Central Valley, and once ranged from the northern Sacramento Valley to the southern San Joaquin Valley.³ With over 95% of California’s wetlands destroyed due to agricultural and urban development,¹ T. gigas has been extirpated from much of its former range, and remaining populations are fragmented, which has resulted in the federal and state listing of T. gigas as a threatened species. Furthermore, with much of its native habitat destroyed, giant garter snakes have been forced into artificial and suboptimal habitats, such as irrigation and flood control canals and ditches, rice fields, and sloughs in agricultural fields and urban areas.² In order to formulate a conservation plan for this species, the US Geological Survey initiated a study of the life history and habitat use of the giant garter snake in 1995. This on-going study involves capture of free-living snakes for biologic data collection (weight, length, sex, overall health, pregnancy status, genetic sampling, etc.), for mark and recapture data to estimate population size, and for radiotelemetry determinations of snake movements and habitat use.

Integral to the telemetry study has been collaboration with the UC Davis School of Veterinary Medicine for surgical implantation of radio-telemetry devices (Holohill System^TM SI 2T transmitters, 7.5 g, 3.5x1.0 cm). Since 1995, 142 snakes have been implanted, and 107 of these snakes have been recaptured for surgical removal of radio transmitters. Briefly: the snakes are mask induced, intubated, and maintained with isoflurane inhalation anesthesia for surgery. The length of the snake from the insertion point of the implant to the exit point of the antenna-placement catheter is prepared in a standard sterile surgical manner. Entry into the coelom is through vertical skin incisions with blunt dissection intercostally for implant placement. Implants are anchored to a rib with a single suture to prevent migration within the coelom. The antennas are placed in the caudal coelom by using an 8–10 French polypropylene catheter as a placement guide: the catheter is manually guided caudally within the coelom to a point 2–3 cm distal to the length of the antenna, and then the antenna is threaded into the lumen of the catheter. The catheter tip is then exteriorized through a small stab incision, grasped at its distal tip and pulled caudally for removal, leaving the antenna within the coelom. The primary incision closure both at the implant insertion site and at the catheter exit point is achieved with a continuous suture pattern incorporating two adjacent ribs using 4-0 PDS. Skin closure technique initially involved experimentation with several different suture patterns to achieve a water-tight seal, to prevent some of the post-operative complications encountered—a modified subcuticular pattern proved most effective. Transmitter removal is achieved by locating the transmitter in the coelomic cavity by palpation, and performing a coeliotomy as previously described. Post-operatively, snakes are maintained in 80°F incubators for 24 hr, then transferred back to the USGS field station, where they are maintained in aquaria at ambient room temperatures of 78°F. Snakes are held in dry aquaria for the first 72 hr post-operatively, and then moved to aquaria with shallow water pans for soaking and feeding, for an additional seven to 10 days following surgery. Snakes are held for a maximum of 2 wk if no post-operative complications are noted prior to release. Ancillary diagnostics included blood collection from the ventral coccygeal vein for PCV/total solids prior to surgery, ultrasound for pregnancy detection, and collection of cutaneous larval parasites opportunistically.

Issues that were taken into consideration in establishing this surgical protocol revolved around three key elements: 1) giant garter snakes are more aquatic than most other garter snakes, and most published references on implant surgery in snakes have been on terrestrial species; 2) the slower healing time of skin in reptiles, and achieving a seal in the incision line before release of the snake to its aquatic habitat, was a significant concern because exposure to water could cause infection if the incision was not completely healed; and 3) snakes had to be released within a relatively short time following implantation because they were predominantly pregnant females, and it was important that their clutches be delivered in the wild and not in captivity. Significant complications encountered to date have been necrosis and/or dehiscence at the surgical site, with presumed leakage of water and subsequent bacterial contamination through incompletely sealed surgical sites into the coelom around the implant and/or antenna. Infection manifests either as dehiscence of the suture line detected at some point after surgery (recapture and observation of the surgery sites was opportunistic due to the behavior of the snakes after release). Typically, dehiscence at the suture line occurred during the first shedding after surgery. Miscellaneous medical findings included single to multiple subcutaneous and/or intracoelomic abscesses, resulting in various levels of morbidity.

Literature Cited

1.  Frayer WE, DD Peters, HR Powell. 1989. Wetlands of the California Central Valley: Status and Trends, 1939–Mid-1980s. US Fish and Wildlife Service, Portland, OR. 29 pp.

2.  Hansen GE. 1988. Review of the status of the giant garter snake (Thamnophis couchii gigas) and its supporting habitat during 1986–1987. California Department of Fish and Game Contract no. C-2060. Final Report. Unpublished. 31 pp.

3.  Hansen GE, JM Brode. 1980. Status of the Giant Garter Snake Thamnophis couchii gigas (Fitch). California Department of Fish and Game. Inland Fisheries Endangered Species Program Special Publication. 80–5. 14 pp.