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Non-lethal Methods of Controlling Deer Population Growth
Plant Science Day 2002 Short Talk
The large numbers of white-tailed deer in Connecticut have led to numerous problems including damage to agricultural crops and landscapes, increased deer-vehicle collisions, and the spread of disease. My talk today will focus on "Non-lethal Methods of Controlling Deer Population Growth". Deer population management using reproductive control has received a lot of attention from the popular press, and some of it can be quite confusing. By the end of this talk I hope that all of you will have a better idea of where we stand in terms of non-lethal deer management. I will also describe some of the techniques we are experimenting with at The Connecticut Agricultural Experiment Station. Iíve also put up some posters that describe the research Iím conducting.
I will begin with this slide to give you an idea of deer population size both historically and currently. White-tailed deer was hunted close to extinction through much of its range by the end of the 1800s. Their numbers began to rebound during the later part of the 20th century because increasingly restrictive laws were enforced by game wardens to protect deer. Another factor that contributed to the increase in deer numbers is that by 1900, thousands of marginal farms had been abandoned leaving areas ideal for deer.
This slide shows the distribution of deer-related accidents in 2000. Deer-vehicle collisions in Connecticut have been increasing steadily over the last decade. In the year 2000, over 3,300 deer-vehicle collisions were reported in Connecticut. Most of the accidents occurred between October and December, corresponding with the deer breeding-season. The towns that recorded the highest deer-vehicle collisions in the State included Madison, North Branford and Groton.
Some methods being used to reduce deer damage include:
Regulated hunting is the most widely used method of white-tailed deer control. While it is effective in some places, it can have the opposite effect in other places. Some of the limitations of hunting are:
Live capture and relocation Ė this always seems like a comfortable option, you can get rid of problem deer without killing them, and no one is upset. The problems with this method of deer control are
Reproductive control: By suppressing reproduction in a population to a level below that of natural mortality, it is possible to achieve a decrease in population size. Most of the research on non-lethal control in deer has focused on females. The two basic methods of controlling female reproduction in deer are by using immunocontraceptives and by using contragestation agents.
Before I explain the different methods of reproductive control, I would like to explain the 3 distinct seasons in deer biology in Connecticut. This is relevant because techniques used to control reproductive output are dependent on these seasons. Sept.-Dec. is the breeding season; Jan.-April is the herding season, when most females are pregnant; and May-Aug., is the fawning season.
As I mentioned earlier, there are 2 tested methods of reproductive control in female deer, the first of which is using Immunocontraceptive vaccines. These vaccines work similar to human contraceptives. The disadvantages of immunocrotraceptives are that females need to be treated twice the first year, and treated females need to be given a booster shot each subsequent year of the program. Females also need to be treated just before the breeding season.
The other method of female reproductive control in deer is the use of contragestation agents. A contrgestation agent is one that causes spontaneous abortion. The disadvantages of contragestation agents are that like immunocontraceptives, females need to be treated every year, during the breeding season. If treated early in the pregnancy, females will simply get pregnant again. If treated late in the pregnancy, it is often dangerous to the health of the female (females have an unusually long labor. Normal labor is about 20-25 min., with this treatment Ė 24-28 hrs!).
Now moving on to my research - most of my research is conducted on south central water authority property in North Branford. One of the reasons for selecting this property as a site for my research is because it has a high density of deer. Hunting is prohibited here, which makes it easier for me to document changes in population size. This site forms one of the more typical areas where non-lethal techniques are applicable. If you look at this topo map of the study site, you can see that the southern boundary of the property borders residential communities. As I mentioned earlier, suburban communities in forest fringes are ideal habitat for white-tailed deer.
I have been collecting demographic information such as number of males, number of females, number of yearlings and number of fawns, using camera traps. Iíve set up feeders throughout the site. These feeders have been programmed to dispense corn once a day. Out here you can see a camera. This is a motion sensitive camera that is triggered when an animal comes in to feed. Data using the cameras indicated that for every adult male, there are about 3 adult females. This data is consistent with other parts of the state.
Before I could initiate a non-lethal population control research program, I also needed information on deer movement patterns and behavior within the site. To this end, we have captured over 70 deer and fitted them with cattle tags for individual identification. To record movement patterns, we have also fitted some of the deer with GPS collars and radio collars. This collar is a GPS collar. These collars record the exact location of the animal every 2 hours by tracking satellites.
This poster shows the home range of some of the females that had been fitted with GPS collars. Each color represents the home range of a different female. What I found was that the home range sizes vary widely among females, ranging from 30 acres to 3395 acres. Typically, suburban deer found in areas of very high deer densit, have relatively small home ranges. The data revealed that most suburban female deer had very well-defined movement patterns - when moving from forested areas to feed in peopleís back-yards, they typically chose specific gardens and went to the same locations on repeated nights.
Now moving on to the sterilization technique that Iím testing. The method Iím investigating involves sterilizing large males. Most of the research on reproductive control in deer has focused on females. The main reason for focusing efforts on females instead of males is that hunting records have shown that removing males from the population does not adversely affect the reproductive rate of females. However, hunting involves removal of males from an area, and this could lead to males that would otherwise not have access to females to mate. Sterilization would allow for the treated males to remain in the area. In species such as white-tailed deer, where a hierarchy exists in which dominant males monopolize most of the mating, greater efficiency could come from sterilizing large males. The fact that suburban deer have small home ranges makes it easier for males to guard females. The common method of sterilizing male domestic animals is by castration. However, castration will obstruct the production of hormones, thus such males will no longer produce antlers or engage in mating or mate guarding behavior. The method that we are using to sterilize males does not affect hormone production, thus the behavior of the treated male is not altered as a result of the treatment.
By retaining treated males in the population, these individuals will continue to use resources and participate in mating behavior, thus reducing the reproductive output of females.
Males are captured using a dart gun, where the dart contains tranquilizing drugs. Once the animal has been darted, it can take anywhere from 5 minutes to about 30 min. for the animal to go down. We track the darted animal via a transmitter that is attached to the dart, using an antenna and receiver. Since most of the capture occurs after dark, this is especially helpful. Once we find the animal, we fit a mask over the eyes to keep them calm, in the event that they are not completely out. We then draw some blood to maintain a DNA profile on the animals, and fit cattle tags for identification. We then sterilize the animal.
Hereís a quick lesson in male deer reproductive physiology Ė sperm are stored in the testes, and transported through the epididymus and the vas deferens. As I mentioned earlier, deer are seasonal breeders. So their reproductive organs only function from around July to January, after which the organs remain dormant. The procedure Iím testing to sterilize males is very simple, Iím blocking the epididimus (which is located here). The procedure involves injecting a scarring agent through the skin, which then forms a plug, thus prevents the flow of sperm.
Potential advantages of this sterilization technique:
There is also promising research being tested elsewhere. The National Wildlife Research Center, which is a branch of the USDA, is now working on a single dose immunocontraceptive. There is also research being conducted on the surgical sterilization of females.
These studies that I have undertaken are still at its infancy. It will take a few years to measure the success of these techniques. Reproductive control techniques are not quick fixes, and should be viewed as long-term solution. The function of such research is not to find a technique cheaper than hunting, but to provide alternatives to communities that are looking for other options, especially in areas where residential densities preclude hunting as an option.