Pronghorn Die-off in Trans-Pecos Texas

Abstract

Pronghorn antelope (Antilocapra americana) numbers on the Marfa Flat, comprising 75,000 acres in Presidio County, Texas, declined from 484 in June, 1964, to 148 in June, 1965. Nearly 60 percent (274) of the original number died from causes other than hunting. Pronghorns were confined on this area by net-wire fences. Three years of below-average rainfall combined with heavy stocking of cattle, horses, and pronghorns caused severe range depletion and forced pronghorns to rely almost entirely on browse species for sustenance. Only tarbush (Flourensia cernua), creosotebush (Larrea divaricata), and snakeweed (Amphiachyris dracunculoides) were readily available. Twelve pronghorns were observed to be utilizing a diet composed almost entirely of tarbush during the winter and spring period. Lesions characteristic of tarbush toxicity were found in 83 percent of the animals examined. Resorbing embryos were found in three of four females that had conceived. Malnutrition coupled with tarbush toxicity was considered to be the cause of the losses. Reproductive rates were reduced from 52 fawns per 100 does in June, 1964, to 17 fawns per 100 does in June, 1965. Pronghorns on ranches with a variety of more desirable browse species suffered only minor losses. Adjustment of stocking rates to forage available, construction of fences allowing pronghorn movement during periods of food shortage, and the possibility of limited, temporary supplemental feeding are management practices that might prevent the recurrence of such losses.