Murder Mystery on Whiskey Mountain

by Molly Absolon

Seventy-seven, I repeated. That was my number to remember, 77 ewes. John Mionczynski was tracking the lambs, but so far we'd only seen two. The bighorns looked healthy - their dusty brown coats thick and their bodies sleek - but their looks belie the numbers, and the numbers tell the story. These sheep are dying off. Lambs are not replacing the aging ewes and rams, and if the trend continues, the herd could disappear.

We were counting Rocky Mountain bighorn sheep on Sheep Ridge, southeast of Dubois, on a blustery February day. The Sheep Ridge band is part of the Whiskey Mountain herd, which used to be the largest wintering group of bighorns in North America. The group was so successful that from 1949 to 1995, 1,900 sheep were trapped and transplanted to parts of the animals' former range in an attempt to reestablish populations. In those days, hunters vied for as many as 84 ram tags for the area. Today that number has shrunk to 24, and sheep haven't been trapped on Whiskey Mountain since 1995.

The collapse of the herd began in 1991. Following a two-week cold snap, sheep started dying. Ultimately, a quarter to a third of the 1,600 animals were dead. Biologists watching the herd believed the animals died from pneumonia caused by the bacteria Pasteurella.

"Typically after a Pasteurella die-off, you have poor lamb recruitment - or survival - for three to five years," Wyoming Game and Fish wildlife biologist Pat Hnilicka says. "So we watched the herd. There were poor numbers for three years, then five, then six. Some years only four or five lambs per 100 ewes would make it. Normally you expect to have an average of 35 lambs per 100 ewes."

The poor recruitment rate prompted the Whiskey Mountain Technical Committee - of which Hnilicka is the current chair - to look into what was going on. The technical committee is comprised of representatives from the Wyoming Game and Fish Department, the Bureau of Land Management and the U.S. Forest Service. In 1997, the committee decided to see if the herd's problem was caused by reproductive failure. Biologists found that 95 percent of the ewes were pregnant, so there seemed to be no trouble with fertility. The next logical question was why the lambs were not surviving. In 1998, John Mionczynski was hired to find that answer.

"My job was to watch and follow the sheep," Mionczynski says. "As soon as we got symptoms, I could collect the lamb, go to the lab and we would find the pathogen that had caused death. Very simple; it sounded like it was going to be a two-month job for me."

Four years later, Mionczynski is still on the job.

"It's a murder mystery," he says as we drive around the sheep's winter range. "We went in thinking the culprit was Pasteurella, now we're looking at mineral deficiencies, weather patterns, forage, predators….We can't prove anything yet, but we have an intriguing theory."

The pieces started falling into place in July 1998. Up in the herd's summer range, lambs started getting sick. They became stiff, their muscles atrophied and their coats stretched thin over gaunt bodies. At times, the animals coughed and appeared to have difficulty breathing. Mionczynski wasn't sure what he was dealing with, but he was convinced that all the lambs were going to die.

While Mionczynski was up in the high country with the sheep, Hnilicka was conducting research. In mid-July, he received a report that a collared ewe-Y28- had been seen at a natural mineral lick several thousand feet below and eight miles from the sheep's summer range. At first, Mionczynski didn't believe the report. He'd seen Y28 only the day before. But then another collared ewe was reported at the mineral lick, and another.

"It seemed as if the ewes knew that when their lambs got sick, they needed to get down to the mineral lick. We saw ewes leave with sick lambs, and when they returned they either no longer had lambs, or their lambs were healthy," Mionczynski says. "So I started following them. On the way I found two mountain lion ambush sites with fresh scat and sheep bones. It appeared as if the lions were bypassing their regular diet of mule deer in favor of sick lambs."

The migration of the sheep was unexpected and pointed the researchers toward mineral deficiencies as the culprit for the lambs' sickness. Eventually the mineral in question was narrowed down to selenium. Mionczynski and Hnilicka combed through research papers to see if there was any information about the effects of selenium deficiencies on bighorn sheep. They found nothing. There was, however, literature on the effects on domesticated animals and humans. Selenium deficiencies are believed to cause everything from hardening of the arteries and periodontal problems to white muscle disease and viral infections. The symptoms of white muscle disease matched the symptoms Mionczynski observed in the lambs during the summer of 1998.

But Mionczynski and Hnilicka had no proof. By the time the pieces began to fall into place, the lambs had either recovered or perished. Nor could Mionczynski explain why for the first time in its known history, the herd was suffering from a selenium deficiency. All he knew was that a lot of lambs had died.

"We documented 88 ewes with lambs in 1998," Mionczynski says. "Of those, 9 lambs survived. A normal herd would have a minimum of 30 surviving lambs, and could go as high as 50 or 60."

The Whiskey Mountain Technical Committee decided to put selenium blocks out in 1999 to see if that would improve the lambs' survival rate. As a control, some parts of the herd would not have access to the blocks.

Lamb numbers were better in 1999. In fact, Mionczynski observed no sign of white muscle disease and the survival rate was up to 38 lambs per 100 ewes for sheep with access to mineral blocks, while those without had 26 lambs per 100. The sheep on the blocks looked healthier, but the numbers indicated that for some reason, all of them did better in 1999. Maybe it wasn't selenium. Or maybe for some reason there was more natural selenium in the forage in 1999.

The later seemed to be the answer. Samples of forage in 1999 showed as much as 70 parts per billion selenium compared to numbers around five parts per billion the year before. What made 1999 different?

Drought. 1998 was the last year with above-average rainfall. Since then rainfall has been scant and what does fall is often acidic, most likely from nitrates produced by burning fossil fuels. Laboratory tests have found that acid rain converts selenium from selenate to selenite - a form that plants cannot readily absorb. Mionczynski and Hnilicka don't know if this process happens in nature, but they do know that at times the rainwater they collect has had a pH as low as 4.0, an acidity level comparable to lemon juice.

"We can't prove anything," Mionczynski says, "but the pieces fit together. We have enough to convince us to continue. There are all sorts of questions to be answered, but for now, the sheep with access to selenium blocks are doing better, so we believe we're on the right track."

Doing better, but still just hanging on. The numbers of surviving lambs were down again in 2000. The sheep on the selenium blocks had 22 lambs per 100 ewes. The bands without had as few as four lambs per 100.

Kevin Hurley, a Wyoming Game and Fish Department wildlife biologist, is hesitant to endorse the selenium theory. "A lot of people want to jump on an easy answer. Those guys are doing interesting research, but I'm waiting for proof."

To help prove the theory, scientists at the department's research lab are planning to feed captive bighorns forage with selenium levels equivalent to what would have been available in 1998. If Mionczynski and Hnilicka are right, the lambs should develop white muscle disease. If not, the murder mystery remains unsolved.

Molly Absolon freelances from Lander. She recently joined WOC's board of directors. (See article) A version of this article recently appeared in High Country News.