A wolf pup growing up in Yellowstone National Park has a better chance of surviving if it has a big mama that lives in a big pack.
Those were the findings of a recent study published in the Journal of Animal Ecology that utilized 14 years of data collected on Yellowstone wolves.
“This is one of the few studies of large carnivores in the world to tease out what drives reproduction,” said Dan Stahler, lead author of the study and a biologist for Yellowstone’s Wolf Project team. “What’s fascinating is that, despite decades of research, we really didn’t know much about reproductive success and what are the triggers for failure.”
At last count, Yellowstone had 105 wolves in about 10 packs. Forty-five of the wolves were in the park’s Northern Range, while 60 were in the interior. This is the first time that wolves in the park interior have outnumbered their northern brethren. It’s the fourth year that wolf numbers have been around 100.
So what makes a big wolf mama? Stahler said genetics is obviously a key, as it is with any mammal. Beyond that, pack size is essential to providing a pup with good nutrition -- since larger packs are more adept at bringing down game -- as well as providing protection from other wolves and predators.
“We know that larger packs can out-compete smaller packs, it gives them more resources,” Stahler said.
A pack size smaller than four wolves seems to leave pups at a disadvantage. Beyond four, the benefits increase with the pack size, but more slowly.
“It’s very similar to humans, extended family can be key,” Stahler said. “Reproduction is hard for any mother.”
The size of Yellowstone’s wolf packs has dwindled from the early years of reintroduction when food resources, namely elk, were more plentiful. Wolves were returned to Yellowstone starting in 1995. Early packs grew as large as 16 and 20 members. Now a big pack is 10 to 13, Stahler said.
Disease -- especially canine distemper, which often kills pups -- is another factor controlling wolf populations in Yellowstone. Big packs also will lose members that break off to breed, since only the alpha male and female typically breed.
The researchers also discovered during their work that the color of a wolf's coat seems to be associated with its immune system. About 50 percent of Yellowstone’s wolves are gray, and 50 percent black. Black alpha females that give birth have lower success rates but their pups have higher survival rates, Stahler said.
“So there’s a tradeoff between reproduction and survival,” he said. “We’re still trying to unravel this. It’s still a mystery.”
Coat color is determined by a certain gene that is a mutation that occurred in domestic dogs thousands of years ago and was brought to North America by wolves, Stahler said.
Stahler said the research in Yellowstone should help wolf managers in surrounding states where the animals are hunted. This is the first year that wolves are being hunted in Wyoming.
If states are managing a population of at least 100 wolves, including 10 breeding pairs, then knowing that a pack of at least four has the best chance for breeding success can help assure managers that wolf reproduction should be good. Small pack numbers could lead to poor breeding success and prompt more conservative hunting regulations. Likewise, if females killed by hunters are small or there are indications of disease, more conservative regulations may be advised.
“Understanding the triggers for failure is valuable information, as it is for other species we manage,” Stahler said.
"Our results should reassure wolf conservationists," said Dan MacNulty, a Utah State University researcher and study co-author. "If female body weights are high and pack sizes are sufficiently large, wolves can successfully reproduce despite the impact of environmental factors, such as disease and competition."
Other authors contributing to the study were Yellowstone wolf biologist Doug Smith and Robert Wayne and Bridgett vonHoldt of the University of California, Los Angeles. The study was funded by the National Science Foundation, National Park Service and the Yellowstone Park Foundation, along with private donors.