Wolf Management: Non-lethal Control
7. Contraception
People have experimented with contraception, or fertility control, for controlling wildlife populations since the 1950's. Contraception is an increasingly active field, stimulated in part by public concern over killing animals to control their populations.
The aim of wolf contraception is to reduce the birth rate of wolves who live close to livestock. The assumption is that a reduced birth rate should decrease any need to prey on livestock to feed a growing number of wolf mouths. Another, more controversial, assumption is that it might allow wild ungulates in North America, like caribou (Rangifer tarandus, called reindeer in Europe) to increase in number because there will be fewer wolves preying on them.
How It Works
The basic procedure of wolf fertility control at present is vasectomising the male breeder and ligaturing the fallopian tubes of the female breeder in a wolf pack; in other words a 'snip and tie' that results in sterilisation. This is exactly the same procedure undergone by many pet dogs and cats and voluntarily by some humans.
Usually only a single male and female pair breed per wolf pack. The pair monopolises the pack's breeding by actively inhibiting other pack members from reproducing. Thus if the breeding pair can be prevented from bearing young, the pack's birth rate will drop, at least until such a time as another pair take over breeding.
Does It Work?
Major questions are whether a neutered pair of breeding wolves would continue their normal behaviour but not give birth to cubs and whether the pack would hold together and maintain its territory. Fertility control would be useless if the neutered pair was replaced by a new breeding pair or if the pack broke up and was replaced by a new non-treated pack.
Experimental Field Work
To come up with answers to these questions, five adult male wolves in Minnesota were caught, transported to a veterinary laboratory, vasectomised, and returned to where they were found after fitting them with radio collars (Mech et al 1996). The wolves were monitored over a number of years in the late 1980's and early 1990's.
The researches discovered that breeding pairs stayed together, packs retained their territories and pack sizes decreased or did not grow. The results support vasectomy as being a viable technique for control of wolves.
Applied Field Work
A larger application of wolf contraception was tested beginning late 1990's. The Fortymile caribou herd of Alaska and Yukon used to number several hundred thousand animals. By the late 1990's the herd was down to 25,000 head. The goal of the Fortymile caribou management plan was to increase caribou numbers. Reasons for the decline of the caribou are not fully understood but a major factor limiting herd growth is hypothesised to be wolf predation on calves. Of over 8,000 calves born in 1994, 5,000 were killed by predators, mainly wolves. Therefore, it was reasoned, the number of caribou should increase by reducing the number of wolves.
The caribou management plan called for breeding pairs of up to fifteen wolf packs in the caribou calving grounds to undergo vasectomies and tubal ligations. Wolves were chased by helicopter and tranquillised by a shooter leaning over the side firing a dart gun. Darted wolves were surgically snipped or tied in a tent set up on site or flown to a building in a nearby village and then returned to where they were caught. As usual the wolves were radio collared for follow up monitoring. At the same time, to reduce the size of the population quickly, other members of their packs were caught and translocated at least 160 kilometres (100 miles) away so they would not return to their packs.
Like the Minnesota study, the results so far suggest that sterilisation does not influence the basic social and territorial behaviour of the wolves as the packs seem to be holding together. It is expected that the wolves will maintain their territories and take fewer calves, allowing the caribou herd to increase its population.
An Alternative To Snip & Tie?
Snip and tie, or vasectomy and tubal ligation, are time consuming surgical procedures. Immunocontraception is a different approach to fertility control based on drugs which block reproduction.
Immunocontraceptives make use of the body's immune response to prevent pregnancy. Immunocontraceptive are like vaccines against diseases. A vaccine is a substance composed of the dead or greatly weakened viruses or bacteria which produce a disease or is composed of their toxins (harmful excretions). On being injected with a vaccine, a body reacts by producing antibodies, molecules that attack the injected vaccine and, importantly, attack similar substances which may already be in the body or may one day invade the body.
Different kinds of immunocontraceptives are being developed. Among them are ones which prevent pregnancy by blocking fertilisation. A way an immunocontraceptive like this can work is by modifying the membrane surrounding an ovum. During fertilisation certain sites on the membrane permit sperm to attach to the egg. However, the body produces antibodies in response to an immunocontraceptive which themselves attach to these sites, distorting them and preventing the sperm from connecting. Thus fertilisation is blocked.
Advantages & Disadvantages
Although still a long way from being achieved, among the ideal features of a wolf contraceptive are:
- it does not alter basic wolf behaviour
- is not harmful to non-target animals or to wolves who eat several doses
- is reversible, that is its contraceptive effect wears off after a suitable period (not all contraception control need be permanent)
- is easy to handle - a pill or capsule would be perfect
- can be dispensed easily and without contact with the wolves, such as wrapped as an appetising delicacy and scattered from an aircraft for wolves to find and eat
- is cheap and plentiful.
Contraception can be cheaper than killing wolves. A wolf cull has to continue for several years because wolves quickly regain lost numbers through reproduction and immigration. Relatively few wolves need be sterilised and once done it does not necessarily have to be repeated until the wolf is replaced by an untreated wolf.
However, a draw back for snip and tie is that individual wolves must be captured and surgically operated on, a time consuming expensive method, traumatic for the wolves, with a risk of infection and death. Capture and surgery is viable for small populations but too costly for controlling a large wolf population over a wide region. Development of a non-surgical technique, a drug eaten by the wolves for instance, would be more practical. Immunocontraceptives might fill this niche.
Some people will object to sterilisation on the grounds that it shows disrespect for animals. Some people may similarly object to immunocontraception (not the same as sterilisation, as a drugs influence can wear off).
Contraception has potential in the future to offer a refined and humane way for controlling wolf populations in situations where control might be thought necessary and appropriate.
© Wolf Trust 2004. All rights reserved.