Bighorn sheep may have adapted to mountain lions

Although still a favorite snack for hungry mountain lions, bighorn sheep may have adapted to the presence of the predator, which may account for steadier herd population numbers on the Kofa National Wildlife Refuge.

According to estimates by the Arizona Game and Fish Department, there are currently about 400 bighorn on the refuge, down from a high of about 800 in 2000.

“It does appear that bighorn sheep numbers have stabilized on Kofa, maybe with a slight decreasing trend,” said John Hervert, Yuma area wildlife program manager for the Arizona Game and Fish Department.

In 2000, there were an estimated 812 sheep on the refuge. By 2003, the bighorn sheep population was estimated to be around 600, eventually falling to below 400 in 2009.

“The rate of loss appeared to be much greater three or four years ago,” Hervert said.

“The bighorn sheep are probably learning to be more vigilant then they were a few years ago, and they may have abandoned areas that they formerly used where they are more susceptible to lion predation. In other words, they know they are vulnerable if they hang out in the shrubby hillside.”

Instead, the sheep may now only congregate on cliffs where they can see the predator coming, he added.

Mountain lions are believed to have been in the refuge for about the last 10 years.

“Back in the ‘90s we actually did two rather intensive surveys looking for mountain lions in the Kofa and found zero signs,” Hervert said.

“So we were relatively confident they were not in the Kofa, at least so infrequently we couldn't find their sign. Now I can take you up any number of canyons and show you lion tracks any day of the week.”

The suspected arrival of the lions coincides with the rapid decrease in the sheep population.

“We do suspect a number of factors that play a role in bighorn survival,” Hervert said. “One that changed since the ‘80s and ‘90s was the arrival of numerous mountain lions into the area.”

The refuge, even though it is quite large, may not be enough to support a mountain lion year round, so they come and go, Hervert said.

“There are nearby resident populations, as we call them, to the north and to the east. They would have had to have crossed an interstate highway or two, but for a predator that is not at all unheard of.”

No matter when or how they came to arrive in the Kofa, the mountain lions have been very detrimental on the herd population, affecting other areas as well, Hervert said.

When the Kofa herd was larger, the sheep had been an important source for replenishing herds in other mountain ranges all over the southwest.

“We've been trying very diligently to reestablish bighorn sheep through historic ranges throughout Arizona and the southwest,” Hervert said. “Some of our sheep have gone as far east as Texas. Bighorn sheep are very much in demand.”

Due to the thinning herd, transplants of sheep from the Kofa Refuge were halted in 2005.

“We have a lot of work to do and that is why it is important to have sources of bighorn sheep for transplant,” Hervert said.

In an effort to allow more bighorn to thrive on the refuge, the U.S. Fish and Wildlife Service continues to enforce a mountain lion management plan which requires the removal of mountain lions in the area if they kill too many sheep.

If a mountain lion kills two or more bighorn sheep within a six-month period, it is lethally removed from the refuge. That plan, which has proved controversial with many conservationists, will stay in affect until the herd reaches a population of 800.

Hervert estimates there are currently about five mountain lions present on the Kofa Refuge at any given moment, while three lions have been removed from the Kofa Refuge since January, Hervert said.

“They are basically shot — lethally removed. There is no place to take them to release them. That has been considered in various circles as an option, but the problem is Arizona is full of mountain lions and they are territorial. What that means is that they will kill each other to defend a territory.”

The bighorn sheep in the Kofa Refuge currently have a higher priority than the mountain lions, Hervert said.

“It is an ongoing management program. Really, the big picture, why are we trying to recover bighorn sheep on Kofa and remove mountain lions? Mountain lions are very abundant and there is really no real threat of them going extinct in Arizona. They are a very adaptable creature. Bighorn sheep, on the other hand, have been reduced from historic levels considerably.”