Desert bighorn sheep bask on a platform between steep rocky slopes at Colorado National Monument. Click here to see larger image. Photo by Jeff Mitton.
By Jeff Mitton
After photographing landscapes at dawn, I was returning via the Rim Drive to my campsite at Colorado National Monument (CNM).
A flagger controlling traffic at a road repair site told me that I would be allowed to pass in a few minutes, so I got out of my vehicle and started a conversation with her. I am indebted to Ana Jarvis, for when I told her that I was doing photography, she walked me the 10 feet to the edge of the cliff and pointed out a herd of bighorn that was not visible from the road.
Twenty-four desert bighorn were resting in the morning sun on a ledge between steep slopes of sandstone. Most of them were reclining, though several were standing and looking downslope, perhaps considering moving into Monument Valley. A single ewe lagged behind the herd, 50 feet above them, and another ewe was below, walking slowly down the steep slope.
With the exception of pictographs and petroglyphs of bighorn, no evidence places bighorn sheep in the last several centuries in either CNM or the Black Ridge Canyons Wilderness Area immediately to the west."Bighorn sheep, Ovis canadensis, are comprised of four subspecies. Sierra Nevada bighorn, O. canadensis sierrae, and Rocky Mountain bighorn, O. canadensis canadensis, are named for the mountain ranges that they occupy. Mexican bighorn, O. canadensis mexicana, occupy southern California, Arizona, New Mexico and parts of Mexico. Desert bighorn, O. canadensis nelsoni, occupy the Colorado Plateau and Great Basin. Rocky Mountain bighorn are in Rocky Mountain National Park and desert bighorn are in Colorado National Monument .
With the exception of pictographs and petroglyphs of bighorn, no evidence places bighorn sheep in the last several centuries in either CNM or the Black Ridge Canyons Wilderness Area (BRCWA) immediately to the west. Desert bighorn have historically occupied the area near the confluence of the Green and Yampa rivers, to the north.
Desert bighorn were established in CNM and BRCWA by three introductions, all taken from the area around Lake Mead. The first introduction, in November 1979, brought 12 sheep from the Nevada side of Lake Mead to Devil's Canyon in BRCWA. The second introduction, in June 1980, brought 16 sheep from the Arizona side of Lake Mead to Monument Valley in CNM. The third introduction, in November 1981, brought nine sheep from the Black Mountains, east of Lake Mead, to Devil's Canyon in BRCWA.
These introductions provided the initial gene pool of the Black Ridge Desert Bighorn Herd, a management unit that includes both the CNM and the BRCWA. Within that management designation, somewhat independent herds occupy Fruita Canyon, Wedding Canyon and Rim Drive in CNM and several areas in the BRCWA. A census taken in 2009 and 2010 recorded approximately 50 sheep in CNM and a total of 200 sheep in the entire Black Ridge desert bighorn herd.
At this time of year, bighorn are usually segregated into two types of herds. Adult males form small herds, and adult females, juveniles and lambs assemble into larger herds. The herd in the photo has adult females, juveniles of both genders and four lambs.
One of the females in the photo has a purple ear tag (No. 150) and a collar. Kim Hartwig from the National Park Service at CNM provided the information on sheep introductions, but she directed me to Stephanie Durno at Colorado Parks and Wildlife for information on Lady 150 and studies of the Black Ridge Herd.
Lady 150 was one of three ewes tagged and collared in Devil's Canyon in January of this year to provide sentinels for disease outbreaks, major habitat shifts and to aid location of herds for annual surveys. Durno was surprised that Lady 150 had migrated to join a different herd since January. Annual surveys record gender ratios, diseases and other measures of health, and these data are used to compare the Black Ridge desert bighorn herd with the Dominguez-Escalante herd to the south.
Bighorn sheep are shy — they usually move away from humans. It is apparent in the photo that several of the sheep were watching me, but they were not alarmed. Perhaps they could assess my speed and agility and were also able to distinguish a Canon from a rifle.
Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.
June 15, 2016