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Study shows lions prefer elk calves


April 20, 2000
Seeley Swan Pathfinder
Seeley Lake, Montana


by Mike Thompson,
Wildlife Biologist
Montana Fish, Wildlife & Parks
"Game Range Ramblings" column Seeley Swan Pathfinder

I've especially enjoyed my conversations with Pathfinder readers over the past few weeks because we're all trying to pinpoint the cause of declining survival rates in elk calves. I can bring information to bear, but the solution is still a mystery.

So, we're all in the same boat. However, a recent discussion at the Double Arrow escalated out of control, until someone accused another of being able to lick his own eyebrows. I was horrified when the accused admitted the crime. Since then, I've decided to abandon ship temporarily and retreat to the sanctuary of the scientific literature. I'll come out into the real world again when my stomach settles.

One thing the literature can help us with is my assumption that a declining calf:cow ratio is one possible symptom of predation by mountain lions. Is this really a safe assumption?

Well, in order for predation to cause a decline in the calf:cow ratio we observe in January on the Game Range, it is necessary for the predator to kill calves at a higher rate than it kills cows. Lions are certainly capable of killing adult elk, and I once observed a lion with a freshly killed, otherwise healthy cow elk that turned out to be 6.5 years old. So, evidence is required to support the assumption that lions prefer calves.

The first place to look for information on mountain lion predation is the pioneering work by Maurice Hornocker along the Middle Fork of the Salmon River in Idaho, back in the mid-1960s. He examined 53 elk killed by lions from 1964 to 1968. Twenty-eight (53%) of these were calves (i.e., 6-12 months old), while calves comprised only 20% of the live population. Conversely, Hornocker's lions killed only 16 cows (30%), even though cows comprised 68% of the elk population. These results support the assumption that lions select calf elk as prey items and that lion predation could lower the calf:cow ratio.

Thirty years later, Dr. Hornocker's student, Kerry Murphy, finished some landmark work of his own in Yellowstone National Park. Murphy examined 97 elk and mule deer (combined total) killed by lions in winter from 1987-1996 and found that 68% were elk calves, 21% were cows and 7% were bulls (in an area where mule deer were uncommon). That kind of selection for calves will definitely skew your calf:cow ratio downward.

While these results are compelling, they do not necessarily explain the calf:cow ratios we observe on the Game Range in January. Hornocker's study and the results presented so far in Murphy's study pertain only to the winter period. Although lions do prey on calves from mid-January through spring on the Game Range each year, this predation has no effect on the calf:cow ratios we observe in the second week of January. No, we're looking for calf losses that occur from the time calves are born (or maybe before!) until the helicopter arrives in early winter.

Additional results from Murphy's study shed some light. He was able to find 71 elk and mule deer (combined total) killed by lions in summer-fall. Thirty-two percent were elk calves. The second-highest category was mule deer does at 15%. Next came cow elk at 14% followed by mule deer fawns at 13%, mule deer bucks at 11% and bull elk at 10%. So, calves seemed to be a preferred item in summer as well, although not to the degree observed in winter.

However, Murphy and other researchers have found that lions kill less frequently in summer than in winter. Summer also seems to offer more dietary diversity. In Murphy's study, food items such as porcupine, beaver, marmot, hare, squirrel, coyote, black bear and lion (yes, they will consume each other) made up about 36% of the food items in lion feces during spring, summer and fall. In winter, these items made up only 12% of the diet.

The mystery remains unsolved. Certainly, lions have an eye for elk calves. And, they can impact an elk population. In research by Kyran Kunkel in the North Fork of the Flathead, lions were the most common cause of death for elk and white-tailed deer from 1990 to 1996. However, the possibility that lion predation on elk calves may be lighter in summer than winter leaves room to doubt that the declining trend in calf:cow ratios we observe each January is caused by lions alone.

Oh, but there is one more bit of evidence pointing toward lions. Maybe you'll recall from Pathfinders past that fawn:doe ratios in white-tailed deer and mule deer have seemed normal on the Game Range while calf:cow ratios have declined. Hornocker's research indicated that fawns comprised 35% of mule deer killed by lions in winter, compared with 53% for elk calves. Murphy found that mule deer fawns comprised 21% of the overall lion kill in winter where elk calves comprised 45%. In summer-fall, lions were slightly more likely to kill adult female mule deer than fawns in Murphy's study. And, in Kunkel's study, white-tailed deer younger than 2 years old were more likely to survive in the face of predation and other factors than deer 3-7 years old, while the opposite was true for elk.

So, it would fit with lion predation to see depressed calf:cow ratios at the same time that fawn:doe ratios remain largely unchanged. It doesn't prove anything, however, because I suspect the same can be said for predation by black or grizzly bears. But, that's another article.

On a discouraging note, I found no research on eyebrow licking in humans. But, at least I'm confident of one thing.

Eyebrow licking has nothing to do with a declining trend in calf:cow ratios.

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