The Sheep Ridge band of bighorns 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 re-establish populations. But sheep haven't been trapped on Whiskey Mountain since 1995, when it became obvious that something was wrong.
Following a two-week cold snap in 1991, sheep started dying. Soon, 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."
Biologists found that 95 percent of the ewes were pregnant, so there seemed to be no trouble with fertility. The next 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."
Yet, four years later, Mionczynski is still on the job. "It's a murder mystery," says Mionczynski, a wildlife consultant who has conducted field research since the 1970s. "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."
When the lambs began to sicken, they became stiff, their muscles atrophied, and their bones protruded through 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 all the lambs were going to die.
In mid-July, Mionczynski 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, he didn't believe the report. He'd seen Y28 only the day before. But then another collared ewe was reported near the mineral lick, and then another.
"It seemed as if the ewes knew that when their lambs got sick, they needed to get down to the mineral licks," Mionczynski says. "We saw ewes leave with sick lambs, and when they returned they either no longer had a lamb, or their lamb was healthy."
The migration of the sheep pointed toward mineral deficiencies as the culprit for the lambs' sickness. The mineral in question looked to be selenium. In domesticated animals and in humans, selenium deficiencies are believed to cause anything from hardening of the arteries and periodontal problems to white muscle disease and viral infections. The symptoms of white muscle disease - a kind of muscular dystrophy that attacks young domestic animals - matched those that Mionczynski observed in the lambs during the summer of 1998.
But why, for the first time in its known history, would the herd be suffering from a selenium deficiency?
In 1999, biologists decided to put selenium blocks out to see if that would improve the lambs' survival rate. As a control for the test, other parts of the herd did not have access to the blocks.
And lamb numbers were better that year. In fact, Mionczynski observed no sign of white muscle disease. The sheep with access to the blocks looked healthier, but for some reason, all of the sheep did better in 1999. Maybe the culprit wasn't selenium.
Or maybe there was more selenium in the forage in 1999.
Samples of forage in 1999 showed as much as 70 parts per billion of selenium, compared to around 5 parts per billion the year before. What made 1999 different?
Drought. In the Whiskey Mountains, the rain 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. Rainfall slacked off in 1999, which may have allowed the plants to soak up more selenium.
"We have enough (evidence) to convince us to continue," says Mionczynski. "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 are on the right track."
Not everyone is convinced. Kevin Hurley, who like Hnilicka is a wildlife biologist for Wyoming Game and Fish, is hesitant: "A lot of people want to jump on an easy answer. Those guys are doing interesting research, but I'm waiting for proof.
"Right now, bighorn sheep are doing well in the West," Hurley adds. "In Wyoming, things are a little different. We have 15 herds. Eight of these are native, seven have been re-established. The Whiskey Mountain herd is not unique in having difficulty, although it is the one native herd in trouble. All seven of the transplanted herds are also struggling."
Even if the selenium deficiency theory proves correct for the Whiskey Mountain herd, Hnilicka does not believe it is necessarily the cause of problems in other bands of sheep.
"I could foresee this happening anywhere you have similar geology, but the concentrations of selenium depend in part on the stratigraphy of the area," Hnilicka says. "So it's hard to say if it could happen where the geology is different."
Like all wildlife biologists, Mionczynski and Hnilicka continue to watch, wait and patiently test their theories. The numbers of surviving lambs in the Whiskey Mountain herd 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.
Scientists at Wyoming Game and Fish department's research lab are now 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 mystery remains unsolved.