Wildlife overpopulation and overcrowding from horses, bison, deer elk, and other species can cause damage to native habitat and affect the diversity and abundance of other species that share the same space.
"Hunting and culling have traditionally been used to regulate animal numbers in the wild, but other approaches that meet public approval are needed in parks and urban areas where these methods are not feasible," says Dr Terry Nett, a researcher at Colorado State University's Animal Reproduction and Biotechnology Laboratory and the Department of Biomedical Sciences.
"Controlling the fertility of female mammals may eventually prove to be an effective alternative for population control, but more testing is needed before this goal can be realised."
One approach has been shown to be 100% effective as a contraception in mule deer and elk for one breeding season while the other method may provide contraception for two to three years and is between 75% and 90% effective.
Recent research by the university's College of Veterinary Medicine and Biomedical Sciences on female elk showed that a drug called leuprolide, which is used to treat endometriosis and fibroid tumors in the uterus, and a vaccine called GonaCon show promise as wildlife contraceptives.
Each influence a hormone called gonadotropin-releasing hormone, or GnRH, and suppress ovulation in female mammals by altering normal function of the pituitary gland. A slow-release formulation of leuprolide prevents the pituitary gland from functioning for six months to a year, preventing pregnancy for one breeding season.
The GnRH vaccine, GonaCon, causes an immune response that neutralizes the animal's own GnRH and prevents the pituitary gland from stimulating the ovaries for up to three years in some species.
Both can be administered by dart, eventually making the technology more useful to wildlife managers.
Research shows that neither method significantly impacts social behavior of the herd during the breeding season. However, GonaCon may increase individual male and female interactions during the breeding season.
Ongoing studies at Colorado State will determine how long the approaches impact fertility. Studies will also look for long-term impacts on reproductive system development in calves that are the offspring of treated females. The growth rates and survival rates in the offspring of previously treated elk do not appear to be affected in preliminary studies.
While the GnRH vaccine does not impact current pregnancy in animals, leuprolide must be administered when the animal is not pregnant.
Research at Colorado State University into wildlife fertility suppression over the last 10 years has included partnerships with the National Wildlife Research Center USDA/APHIS, Colorado Division of Wildlife, National Park Service and the University of Wyoming's School of Pharmacy.
Key collaborators include Jenny Powers, a Colorado State Biomedical Sciences and National Park Service researcher; Dan Baker, a former Division of Wildlife researcher and currently a faculty affiliate in the Department of Biomedical Sciences at the university; and Lowell Miller, a research scientist with the National Wildlife Research Center, USDA/APHIS.