Caribou on the Move Ratios & Proportions Article for Students

There’s a chill in the air, leaves are turning colors, and food is getting scarce along the North Slope of Alaska. That’s how the 165,000 caribou in the Western Arctic Herd know it’s time to get moving. Over the course of a year, Alaska’s largest caribou herd makes some of the longest migrations of any land animal—traveling up to 2,400 miles.

The caribou’s migration route covers an area of 157,000 square miles. In the spring, the caribou trek to their calving grounds, where they give birth. In the summer, they move north to evade mosquitoes and other pests. By the time winter hits, they trot across the tundra to find food that isn’t covered in snow and ice.

A chill is in the air. Leaves are turning colors. Food is hard to find in northern Alaska. These changes let caribou know it’s time to get moving. There are 165,000 caribou in the Western Arctic Herd. It’s the largest in the state. Over the course of a year, this herd makes some of the longest migrations of any land animal. They can travel up to 2,400 miles each year!

The caribou’s migration route covers an area of 157,000 square miles. In the spring, the caribou head to their calving grounds. This is where they give birth to baby calves. In the summer, they move north to get away from mosquitoes and other pests. By the time winter hits, they cross the frozen tundra to find food that isn’t covered in snow and ice.

The Western Arctic Herd is one of dozens of herds that roam across northern Alaska and Canada. But all of the herds have been facing problems over the years.

The population of large migratory caribou herds naturally grow and shrink dramatically. The first modern count in 1970 found 242,000 caribou, though biologists thought the herd was declining. They were right. By 1976, there were just 75,000 animals—the lowest ever seen. To help boost their numbers, wildlife managers reduced the number of caribou a hunter could take to just one bull a year, which was very controversial at the time. It helped the herd, though, and the population climbed to nearly 500,000 by 2003. Hunting was allowed to increase as the herd grew.

Since that rebound, many of the herds—including the Western Arctic Herd—have steadily declined because of factors like climate change, human influence, and a natural boom-bust population cycle. “You look at that big population swing across the last 50 years and it’s really stark,” says Matthew Cameron, a wildlife biologist at the National Park Service who studies the herd. “The things that could be affecting it—predators, nutrition—we’re looking to understand all those.”

There are dozens of herds of caribou living in northern Alaska and Canada. All of them have been facing problems. Their populations naturally grow and shrink dramatically over time. The first modern count was in 1970. Biologists counted 242,000 caribou, but they thought the herd was declining. They were right. By 1976, there were just 75,000 animals. That’s the lowest ever seen!

Wildlife managers wanted to help boost caribou numbers. So, they reduced the number of caribou a hunter could take to just one bull a year. This plan was very controversial at the time. But it helped the herd. The population climbed to nearly 500,000 by 2003. Hunting was allowed to increase as the herd grew.

Since that rebound, many of the herds have steadily declined. This is due to factors like climate change, human influence, and a natural boom-bust population cycle. “You look at that big population swing across the last 50 years and it’s really stark,” says Matthew Cameron. He’s a wildlife biologist at the National Park Service who studies the Western Arctic Herd. He and other biologists are studying the things that could be affecting the herd, like predators or what they’re eating. "We’re looking to understand all those,” he says.

Researchers count the animals each year and equip some with GPS collars. The collars transmit a caribou’s position via satellite. “Our goals are to track annual patterns and range of the herd,” says Cameron.

In past years, the biologists would wait until the fall, when the herd crossed the Kobuk River at a place called Onion Portage. As the herd swam, biologists in boats would motor alongside the animals and attach the collars.

More recently, the herd began crossing later and later in the year—or not crossing at all. By the time they arrived, the river was full of ice. This made the tagging procedure dangerous for
the biologists.

Now, each spring, biologists fly over the herd in helicopters. They shoot nets at individual caribou from above to catch them. Then the biologists land, tag the animals, and release them back to the herd. Data collected from the collars showed that the herd traveled 1,865 miles last year.

Researchers count the animals each year. They also put GPS collars on some of the caribou to track their movements. “Our goals are to track annual patterns and range of the herd,” says Cameron.

In the past, biologists would wait until the fall to put tracking collars on the caribou. That’s when the herd crossed the Kobuk River at a place called Onion Portage. Biologists in boats would drive up to the animals as they swam. Then they would attach the collars in the middle of the river!

More recently, the herd began crossing the river later in the year—or not crossing at all. By the time the caribou arrived, the river was full of ice. This made the collar procedure dangerous for the biologists.

Now, biologists fly over the herd in helicopters in the spring. They shoot nets from above to catch a caribou. Then the biologists land, put a collar on the animal, and release it back to the herd. Data collected from the collars showed that the herd traveled 1,865 miles last year.

Ivan Savini/Alamy Stock Photo

In the winter, caribou eat up to 7 pounds of lichen per day. That would fill 2 garbage bags!

Caribou have long provided food for 40 different subsistence hunting groups in the region. Subsistence hunters take only what they need to survive. Many of those people are Alaskan Natives. “Caribou are used for food, clothing, art, and much more. They are a critical piece of the culture of the region,” says Kyle Joly, a wildlife biologist with the National Park Service.

To monitor the herd’s health, representatives from Alaskan Native groups have joined forces with biologists and state and federal wildlife managers. Every year, they meet to discuss threats to the herds and new research. The group talks about how many calves were born in the spring and how many caribou they saw when the herd gathers in the summer to avoid insects.

One thing that could be affecting caribou numbers—and where they travel—is changes to their food source. In the winter, caribou mainly eat lichen, an organism that is part algae and part fungi. But it can take up to 60 years for lichen to regrow. As the climate warms, shrubs begin to grow, shading the flat open areas where lichen once grew. That might be forcing the herds to move elsewhere to find food.

Knowing more about where the caribou move to get what they need could help biologists protect them. That is especially important as the climate warms. Studying the herds, and the Western Arctic Herd in particular, is valuable for many reasons, says Joly. The most important, he says, is to “ensure the long-term conservation of the herd.”

Caribou have long provided food for 40 different subsistence hunting groups in the area. Subsistence hunters take only what they need to survive. Many of those people are Alaskan Natives. “Caribou are used for food, clothing, art, and much more. They are a critical piece of the culture of the region,” says Kyle Joly. He’s a wildlife biologist with the National Park Service.

Monitoring the herd’s health is important to many people in the region. Over the past years, people from Alaskan Native groups have joined forces with biologists to help the herd. Every year, they meet to discuss threats to the herds and new research. The group talks about how many calves were born in the spring. They also review how many caribou were in the herd when gathers in the summer to avoid insects.

One thing that could be affecting caribou numbers is changes to their food source. In the winter, caribou mainly eat lichen. This organism is part algae and part fungi. But it can take up to 60 years for lichen to regrow after it’s eaten. As the climate warms, shrubs begin to grow. The shrubs create more shade on the flat open areas where lichen once grew. This shade makes it harder for new lichen to grow. These changes might be forcing the herds to move elsewhere to find food.

Knowing more about where the caribou move to get what they need could help biologists protect them. That is especially important as the climate warms. Joly says studying the herds, and the Western Arctic Herd in particular, is valuable for many reasons. The most important, he says, is to “ensure the long-term conservation of the herd.”

Use this information to learn more about the Western Arctic Caribou Herd’s migration. Round answers to the nearest hundredth when necessary. Record your work and answers on our answer sheet.

A. For the year’s first 100 days—January to April—caribou average 0.10 miles per hour as they graze on lichen. What distance do they travel?

B. Some years, the herd must go farther north to find lichen. If the caribou travel 280 miles walking at the same speed, how many days would it take?

After winter, caribou speed up to 0.28 miles per hour to reach their calving grounds. If they travel for 8 days, how many miles do they cross?

New caribou moms slow down so their calves can keep up. If a new mom slows down to 0.17 miles per hour for the first 10 days of her calf’s life, how far does she travel?

If the caribou move an average of 0.22 miles per hour over 105.6 miles to get from their spring calving area to their summer grounds, how many days will it take them to get there?

A. Caribou travel the fastest in the summer, moving up to 0.62 miles per hour to get away from pesky insects. If they average 0.43 miles per hour across 17.2 miles, how many hours do they spend walking?