State and federal wildlife agencies announced in January that the numbers of Mexican gray wolves in the wilds of Arizona and New Mexico increased from the previous year.

The reason experts knew of the increase was that most of the wolves are kept track of, or "marked" in wildlife science parlance, with radio transmitters. Transmitters are just one marking technique allowing biologists to gather information.

“On a larger scale, when we’re interested in looking at interactions over the home range or territory of an individual and it’s not as important to actually see the interaction itself, we’ll use radio collars,” says John Koprowski, a professor of wildlife and fishery science in the School of Natural Resources and the Environment at the University of Arizona.

“So individuals have their own radio frequency, they’re their own radio station and they broadcast a unique signal that we’re able to identify,” Koprowski says. He is not affiliated with the Mexican wolf program, but has worked on other marked animals projects.

Other animal marking techniques include painting fur with dye, ear tags, leg bands, satellite transmitters and even fluorescent dye.

“No matter what the particular goal of a project is, there’s a couple things that we tend to aim for,” says Robert Steidl, professor of wildlife ecology in the UA School of Natural Resources. “One is that we try to use marking approaches that are non-invasive to the animal.”

“Another technique is to come up with ways that animals can stay marked for long periods of time," says Steidl, also not affiliated with the wolf project. "If the goal is really about long-term demographic characteristics or population level processes, what we’re trying to do is come up with strategies that are both non-invasive and really long lasting."