Caribou are an important source of sustenance for the natives of Alaska's North Slope, where four main herds make up a total of around 690,000 caribou. If numbers were to dwindle, the food chain, local economy, and cultural practices would be greatly affected, so biologists keep close track of the herds by using radio or satellite collars and aerial photography.

In the north, satellite collars are typically used because their signal can be picked up even in the dark and during snowstorms. Within each lightweight collar is a transmitter that emits a signal to a passing satellite once a week for eight hours at a time—with enough battery power to last about 18 months. The satellite calculates the location of the caribou and sends the information on to scientists at one of three ground stations found in Alaska, Virginia, and France.

Attaching the collars is no easy task. In a river capture, biologists take a small boat out to a point where caribou are swimming across. They pick out one, sneak up behind it, and lasso its antlers. Once captured, the caribou is led closer to shore, so it will feel safe, and there the collar is put around its neck. This process, which takes only a couple of minutes, causes less stress than another common method in which a net is shot over the caribou from a helicopter, and the animal is tied up while it is being collared.

Once biologists locate the group of caribou with the largest percentage of collared animals, they venture out in small airplanes and photograph them using an aerial camera. Then other collared caribou not in the main group are found and photographed. In a time-consuming process, the photos are carefully laid out and each caribou is counted one by one. Scientists then analyze all the data to determine whether populations are decreasing or increasing.

Many people fear oil exploration and development on the North Slope will disturb local herds, particularly the Porcupine herd—named after the Porcupine River—which calves on lands in the 1002 Area of the Arctic National Wildlife Refuge. Scientists aren't sure if activities related to the oil industry have affected the herds in other areas of the slope, but they have observed that there is a tendency for calving females to avoid oil drilling areas. Other effects are still to be discovered, in part by these monitoring efforts.

—Emily MacDowell