When and why do bighorn sheep lambs die in Colorado herds

Abstract

We evaluated the use of vaginal implant transmitters (VITs) as a means of detecting, capturing, and radio collaring Rocky Mountain bighorn sheep ( Ovis canadensis canadensis) lambs to estimate survival and to facilitate carcass recovery to assess causes of mortality. We focused on one of several bighorn herds in Colorado, US, suffering from depressed recruitment that was not preceded by a classic all-age die-off. We captured, radio-collared, diagnosed pregnancy by ultrasound examination, and inserted VITs into 15 pregnant ewes from a herd residing near Granite, Colorado. We were subsequently able to collar a lamb from each of 13 VITs, and two additional lambs opportunistically from ewes without transmitters. As lambs died, we recovered and submitted carcasses for necropsy and laboratory assessment. All lambs captured and one additional lamb (carcass found opportunistically) were dead by about 130 d of age: 11 died of apparent pneumonia (all within 8-10 wk of age), one died from trauma after being kicked or trampled, one was killed by a mountain lion ( Puma concolor ), and three died of starvation likely caused by abandonment after capture. Pneumonic lambs had involvement of Mycoplasma ovipneumoniae and leukotoxigenic Bibersteinia trehalosi . The use of VITs and lamb collars enabled us to efficiently identify pneumonia as the predominant cause of depressed lamb recruitment in this herd; however, we urge care in neonatal lamb handling to minimize abandonment.