Tubal Ligation as a Means of Controlling a Population of Urban White-Tailed Deer: Preliminary Results
Urban white-tailed deer (Odocoileus virginianus) overabundance continues to be problematic. This study utilizes permanent surgical sterilization of females as a means to reduce local urban deer populations. The study area is the city of Highland Park, IL, and the does are separated geographically into control and treatment groups. Control does receive tissue sampling, radiocollars, and ear tags, and the treatment does additionally receive tubal ligations under gas anesthesia. All bucks are sampled and ear-tagged. Preliminary results and comments on procedures are presented. A ventral midline laparotomy is preferred over a right flank laparoscopy for the tubal ligation.
Overabundance of white-tailed deer continues to challenge urban wildlife managers and local governments.23 Different management approaches to control white-tailed deer population densities have been tried, including sharp-shooting, trap and relocate, and immunocontraception. While lethal culling is by far the most effective control technique, it is not always an option in urban settings due to logistic or political considerations. Trap and relocation programs are expensive and time-consuming and may result in the death of up to 90% of translocated deer.9 Immunocontraception using porcine zona pellucida has been tested extensively in white-tailed deer and free-ranging equids.11,19,21 The need for continued re-injection of females makes this a less viable option for long-term population control in large areas due to logistic difficulties and costs.19
Many hormonal contraceptive agents have been tested in deer, but have yet to be proven practical for population control.4,7,14,17,24 Chemical vasectomies performed on male white-tailed deer caused azoospermic ejaculates and were cost-effective.1 However, male-based contraception is not a reasonable option for free-ranging white-tailed deer since all resident males must be rendered sterile, an effort that is not practical based on the dispersal behavior of bucks. Surgical sterilization of females is effective in preventing pregnancies but has not been attempted at the population level.6,22 A pilot study using surgical sterilization of white-tailed deer at the Milwaukee County Zoo was successful in reducing the deer population.6
Localized management of white-tailed deer can be successful due to their social behavior.12,16,18 Numerous studies indicate that urban female deer use home ranges of under 190 acres (0.3 miles2). Females establish their home ranges overlapping and slightly extending the range of their mother13 and tend to use traditional fawning areas each year. Females are highly philopatric, and matrilines form within a population. Previous studies of urban deer indicate that dispersal rates of adult females ranges from only 5–7%.5,10 Immigrants rarely, if ever, succeed in joining a resident social group and do not succeed in becoming members of the breeding population.9
Hobbs et al. present a stage-structured model to represent variation in the duration of the effect of fertility control agents.8 They conclude that more than 50% of the females will need to be maintained infertile to achieve meaningful population reductions even when fertility rates are low. Their models demonstrate that fertility control using long-lived or permanent sterilization can be more effective than culling in regulating deer numbers.
Since 1994 the Deer Task Force of the city of Highland Park has explored efficient and publicly acceptable means of controlling their deer population. Residents are deeply divided about the use of lethal cull for population control, and this has prompted the city council to fund the current project. Our working hypothesis is that tubal ligation of 80–90% of Highland Park breeding does will lead to a declining population and reduce the current deer density from 9/mi2 to 5/mi2 in a 4-yr time period without significantly affecting behavior at the level of the social group.
General Study Area
Highland Park, IL (HP) is a 12.5-mile2 residential community bordering Lake Michigan. U.S. 41 is a major north-south road that provides a significant barrier to deer movements between the control area (4 miles2, west) and the treatment area (8.5 miles2, east).
Current Population Estimate
Based on aerial surveys, and correcting for a 60% aerial detection rate, we estimate an average population density ranging between seven and 10 deer per mi2. Based on Hobbs et al. models,8 we estimate that we will need to sterilize 45–70 does during the 4-yr course of the study.
Capture, Handling, and Anesthesia Techniques
Deer are captured using Clover traps baited with corn and apples,15 October through April of each year (from 22/Jan/02). Traps are managed and checked three times per day by the HP Police Department. Deer are removed from the traps by collapsing the trap and administering anesthetic agents by hand syringe, using 1.5–2.5 mg/kg xylazine HCl (X-ject E®, Burns Veterinary Supply, Westbury, NY) i.m. combined with 2–3 mg/kg tiletamine/zolazepam (Telazol®, Fort Dodge, IA) i.m. for treatment animals or 2–8 mg/kg ketamine HCl (Ketaset®, Fort Dodge, IA) i.m. for control animals.2 Deer are then blindfolded, removed from the trap, and examined. All deer are ear-tagged, and all does are collared with a 480 g radio transmitter with a mortality sensor (ATS, Model 2520). All deer have blood and either flank or abdominal muscle biopsies harvested for later genetic analysis. Deer are also weighed, aged,20 measured (chest girth and hock-toe length), and examined for ectoparasites. Deer are returned to the capture site, and the xylazine is reversed with 0.4 mg/kg yohimbine i.v. (Yobine®, Ben Venue Laboratories, Bedford, OH). Deer are observed until standing.
Treatment does are transported to an ambulance modified for a surgical suite. Does receive 44,000 IU/kg penicillin G benzathine/procaine (Hanford’s MFG Company/USVet, Syracuse, NY) 20% i.m., 80% s.c. and 0.1 to 0.2 mg/kg butorphanol i.m. (Torbugesic®, Fort Dodge, IA). Animals are intubated and maintained on 1% isoflurane in oxygen to effect. Anesthesia monitoring includes temperature, respiration, heart rate, O2 saturation, and end-tidal CO2. Tubal ligations are performed using either laparoscopic or standard laparotomy techniques. For laparoscopy, does are placed in left lateral for a right flank approach. Tubal ligation is accomplished using a three-port technique. Two hemostatic clips are applied to each isthmus, and the oviducts are transected. Closure is standard. For laparotomy, a 4–9 cm midline approach is used. Each oviduct is exteriorized, the isthmus is double ligated, using silk or nylon, and transected, removing a >1 cm section of oviduct. Closure is standard, preplacing near-far-far-near sutures in the linea alba for gravid does. No concurrent pregnancies are terminated.
Does are located every day by radio telemetry for the first 14 days, then a minimum of 1.5 times per week to obtain 30–40 relocations/deer/reproductive season. Annual trapping provides a means of assessing deer abundance using capture-recapture methodology. Additionally, deer-vehicle collision rates often provide an effective index to population density.
As of 4/20/02, 69 deer were captured in 510 trap days (13.5% trapping success). Additionally, 15 deer escaped prior to immobilization. Seventeen were recaptures (24.6%) with one doe recaptured three times and two others recaptured twice, and two additional captures were not handled. Nineteen treatments, 12 controls, and 19 bucks have been processed. Six of the 69 trapped deer are known to be deceased: one 2.5-yr-old doe died 3 days post laparoscopy with no gross abnormalities; one 7-yr-old doe was found asphyxiated in a trap due to a trap malfunction; one recaptured 10-mo-old buck (trapped together with another buck) asphyxiated in the trap at induction; one 10-mo-old doe died 16 days post laparotomy due to entrapment in a swamp by a cable snared around the right metatarsus; one 2.5-yr-old control doe was culled by the city of Lake Forest; and one 10-mo-old treatment doe was killed by a car after apparently dispersing 12 miles north with a treated sibling. Estimated home ranges during February to April have been <3 miles2 with ∼75% of deer at <0.5 miles2.
Both laparoscopic and standard laparotomy techniques have been utilized for tubal ligations in this field study. The laparotomy provides several advantages over the laparoscopy, including a significant simplification of the procedure with inexpensive equipment, no need for a sterile assistant, no need for additional training, improved efficiency and safety (no trocarization), especially in advanced pregnant does, as well as a potentially longer surgical season. Most importantly, the laparotomy is a technique that is easily transferable to any veterinarian. The potential disadvantages to the laparotomy include a longer surgical time, a longer post-operative recovery period, and an increased likelihood of a catastrophic dehiscence. Laparoscopy was the preferred method for the tubal ligations in the MCZ pilot study and could be expected to provide a quicker surgical time, a reduced recovery time, and, presumably, less morbidity and mortality. The disadvantages of laparoscopy include expensive and delicate equipment with additional sterilization requirements, and the risks of extraperitoneal or intestinal insufflation as well as unobserved intraabdominal hemorrhage or intestinal perforation. In our hands, the laparoscopic surgical technique, even after significant training, took longer and was more difficult than the standard laparotomy. The first three surgeries were performed laparoscopically, and all three required an assistant to hold the camera (a camera holding device was later concocted, but never used). The second surgical doe was about 60 days pregnant with triplets and laparoscopic uterine manipulation proved too difficult, so the surgery was converted to a ventral right flank laparotomy, and sterilization was successful. For tubal ligation to become a realistic population management tool, it is our opinion that the procedures must be as uncomplicated as possible.
There is little doubt that three of the six known deceased deer died as a result of our intervention. In each case we have implemented significant modifications to either the traps or our procedures to optimize deer safety and prevent additional mortality.
Hobbs et al.8 indicate that approximately 10% of the population would need to be sterilized per year in order to achieve a 50% population-wide sterilization rate and a net reduction of the population. They further show that when recruitment is 0.6 does/doe, and survival of adults is high (90%), then nearly 90% of the does must be rendered infertile to approach a target population density of less than 25 per sq. mile. In this study our goal is to sterilize 80–90% of the does within 3 yr. For this technique to be successful in long-term reduction of the deer population, a maintenance effort of annual sterilizations of 5–10% of the does is required, depending on recruitment levels. It is interesting to note that Hobbs et al.8 and Boone and Wiegart3 caution that treating small, closed populations with irreversible agents increases the likelihood of population extinction relative to treatment by culling. We feel it unlikely that we would be able to trap and sterilize all resident does, nor is it our intent, but even if this did occur, there are sufficient local reservoirs of deer for this to be a temporary condition. This project is designed to investigate the efficacy and practicality of utilizing surgical sterilization in local deer management. If proven successful, municipalities may have an additional means of non-lethal control for consideration.
This work is supported by a grant from the city of Highland Park and the efforts of the HP Police Department.
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