1) Sometimes wolves wander outside the boundaries of the park or refuge and need to be brought back. In this case, the wolves are usually put in an acclimation pen for a few weeks before they are set free again.
2) Radio collars need to be replaced. The batteries in these collars wear out after a couple of years.
3) Injuries. Wolves sometimes get injured or become ill. Biologists might try to catch the injured or sick wolf so a veterinarian can treat it.
4) All of the pups born in the wild need to be captured and fitted with radio collars. Biologists usually wait until the pups are several months old so they don't outgrow their collars.
5) Wild wolves are sometimes captured and "translocated" to new areas. This was the case for the original Yellowstone wolves. The wolves were caught in Canada, flown to the Yellowstone National Park, and released.
There are basically two methods used to capture wolves
aerial darting and leghold traps.
Aerial darting is usually done from a helicopter. The wolves are caught out in an open area. A biologist or veterinarian is strapped into the helicopter's open doorway as it flies next to the fleeing wolves at about the same speed they are running. The shooter carefully fires a tranquilizer dart into the muscle of the running wolf usually the wolf's rump.
After the wolf is darted the helicopter follows it until the tranquilizer takes effect. The helicopter either lands and retrieves the wolf, or they notify a ground crew and tell them where the tranquilized wolf is located.
Aerial darting is not always possible because wolves
tend to live in areas of dense cover and catching them
in the open is not easy. The alternative is to use
steel leghold traps.
Leghold traps have been used for hundreds of years to capture wolves. They are still the most effective way to capture wolves in the wild. It's odd that one of the methods used to exterminate the wolf is now being used to help save it from extinction.
A leghold trap is made up of two halves hinged in the middle with a powerful spring. To set the trap, the biologist spreads the two halves flat and locks them open. In the center of the trap is a "tripping mechanism" called a pan. When the wolf touches the pan with its foot, the two halves of the trap snap together and hold the paw firmly in place. Attached to the trap is a chain to stop the wolf from running away with the trap on its foot.
Leghold traps are buried just beneath the surface of
the ground. The trap lines (series of traps) are set
in areas and along trails that are frequented by the
wolves. The traps are baited with a special scent that
only a wolf could love. This scent is sometimes called
a "wolf attractant." The biologists check
the trap lines every day. This assures that the captured
wolves spend as little time as possible in the trap.