Biologists investigating a decline in Dall sheep in Interior Alaska suspected coyotes might be playing a role. Their work revealed some surprises about the adaptable, opportunistic Alaska coyote and its interactions with other predators and prey.
By the mid-1990s, Dall sheep numbers in the Central Alaska Range and the Yukon dropped to about half of what they had been a decade earlier. Hunters and wildlife managers were concerned. To understand the causes for the decline, biologists needed to learn more about sheep, especially lambs and their predators.
Laura Prugh, a graduate student at the University of British Columbia, teamed up with Alaska Department of Fish and Game biologist Steve Arthur of Fairbanks. Prugh's goal was to examine how the snowshoe hare population cycle affected coyotes, and how in turn that affected alternative prey such as Dall sheep.
Hares are the mainstay for coyotes, but hare populations fluctuate on 10-year cycles, crashing dramatically and forcing coyotes to find other options.
"We were concerned coyotes might switch to Dall sheep when hares crashed," Prugh said.
The major cause of mortality for Dall sheep lambs is predation by coyotes and golden eagles. Most of that predation occurs in late spring when the lambs are young and most vulnerable. But Prugh and Arthur did not find any increase in predation when hare numbers were low - in fact they found just the opposite.
Dall sheep lamb survival actually increased in years when hares crashed. "Coyote survival decreased, and they failed to reproduce," Prugh said. "Predation on sheep decreased three-fold."
Overall, there are fewer coyotes in those lean years. Prugh found when hares crashed, the coyote population declined by half. Without hares as a dietary mainstay, coyotes had fewer pups. Arthur and other researchers found the same is true of golden eagles. When prey is scarce, the big raptors simply don't lay eggs.
Prugh and Arthur found that coyotes in their study area, in the foothills of the Alaska Range south of Fairbanks, tended to den in the alpine, and have their pups at the same time sheep are lambing. Nabbing a few lambs in the spring is probably a boon for a coyote with a litter of pups, but it's not a major part of the coyotes' diet.
"When you look at the coyotes' diet over a whole year, Dall sheep are pretty minor, 3 to 5 percent," Prugh said. "Coyotes couldn't make a living off sheep."
Coyotes are quick to respond to fluctuations in prey. "They just have more babies - if there's a lot of prey and not much competition. They have really big litters," Prugh said. "The record is 19 pups in one litter. Even if you knock them back they'll bounce back, they're even more resilient than wolves."
Coyotes are newcomers to Alaska, arriving about 100 years ago. Populations were first reported in Southeast Alaska, then expanded north into the upper Tanana Valley, from which they spread in all directions.
"They extended their range throughout North America coincidental with the arrival of Europeans," Prugh said. "Their original range was the Central Plains, and they've expanded in all directions."
Coyotes likely benefited from forest clearing and agriculture, which increased rodent populations. Road corridors may have also helped expand their range. A major factor was the decline in wolf populations.
"Wolves kill coyotes - where they coexist, wolves they are the major mortality factor," Prugh said. "Coyotes tend to live in the 'No mans land' between wolf packs."
When wolves were reintroduced to Yellowstone National Park in the 1990s, coyote numbers dropped significantly. Before the reintroduction of wolves, coyotes in Yellowstone subsisted largely on hares and rabbits. That changed after wolves arrived.
Wolves kill coyotes, but wolves also provide them with food. The smaller canids are adept at scavenging wolf-killed carrion. In Yellowstone, that's elk. In Alaska, it's moose and caribou. Prugh examined more than 1,500 coyote scats to determine the coyotes' diet. She found moose and caribou to be the second most common item in diet, after hares. In years of low hare abundance, scavenged carrion became the mainstay for many coyotes.
When hares crashed, coyotes didn't take more sheep, but they did target other prey. Prugh found an astonishing variety of prey items in scat, ranging from the unsurprising - carrion, marmots, grouse, ptarmigan, shrew, red squirrel and ground squirrel - to the bizarre - lynx, otter, goshawk, vegetable matter, porcupine and even bear.
Porcupine was unexpected, Prugh said. Porcupines are less than 1 percent the coyote diet when hares are abundant, but when hares crashed, porcupine went up to about 15 percent of the diet. In terms of average biomass, one porcupine equals seven hares or 350 voles. It's not a bad meal if a coyote can catch one.
"I suspect they might gang up on them," she said. "I've heard they'll try to back it up against a tree and grab by the face, flip it over and get into the underbelly. Steve found porky skins turned inside out, and I think one technique is to peel skin off."
Even for coyotes that have learned some tricks, the prickly rodents are a risk.
"We had one pair that was feeding their young porcupines, and the diet was about 40 percent porky," Prugh said. "But that fall, we found the mom dead, and her stomach was full of quills and punctured."
Nineteen coyotes were collared and equipped with transmitters. Prugh was also able to collect DNA from coyote scats and identify 56 individual animals, enabling the researchers to track the diets of specific coyotes.
Although coyotes are opportunists, they developed different hunting skills and strategies. Some focused on small rodents, others targeted carrion. In the long run, some strategies had their risks. Coyotes scavenging carrion sometimes had to face wolves.
"We found two coyotes killed by wolves, and two killed by porkies," Prugh said.
Coyotes will hunt cooperatively, but it depends on what they're hunting. Before wolves were reintroduced to Yellowstone, coyote packs with as many as 10 animals would successfully hunt elk. Prugh said with hares, a common strategy is for a pair to trot through the woods, flushing and then chasing down the prey. With small mammals, coyotes are most successful when hunting alone.
The typical structure is a mated pair and offspring, Prugh said, and most offspring disperse in the first year. In Alaska, coyotes are found mostly as mated pairs with an established territory. They have a stable social structure and tend to mate for life, but will fairly quickly replace a lost mate.
Lone coyotes are not unusual, but are generally transients without established territories. Prugh did not see packs of coyotes in Alaska.
"They form a strong pair bond," she said. "Once they set up a territory they're pretty stable. They can survive hare lows if they have a good territory. In our area, a territory is about 40 square kilometers, which is really big for coyotes. In Texas, you may have several coyotes in a square kilometer. There's a lot more food and greater diversity of prey there."
Riley Woodford is a writer with the Alaska Department of Fish and Game, Division of Wildlife Conservation.