Disease, Climate, and Abundance Impacts on Bighorn Sheep Lamb - Ewe Ratios in New Mexico

Abstract

Understanding the impacts of Mycoplasma ovipneumoniae on bighorn sheep populations is paramount to ensuring population persistence. In New Mexico, most bighorn sheep (Ovis canadensis) populations were thought to be free of this pathogen prior to 2017; however, infections in multiple herds since that time have raised concerns surrounding impacts to population size and lamb survival. We evaluated the impacts of M. ovipneumoniae infection, abundance, climate, and predation on bighorn sheep lamb:ewe ratios across New Mexican populations. We characterized these relationships using lamb:ewe ratios from aerial and ground surveys from 2000 to 2022 and M. ovipneumoniae testing data from 2007 to 2022 (n = 466 individuals) for both Rocky Mountain (O.c. canadensis) and desert bighorn sheep (O.c. mexicana). From 2007 to 2022, we sampled n = 466 bighorn sheep across 19 populations in New Mexico for M. ovipneumoniae exposure. While timing of initial herd infections varied across populations, one population sustained active infections for over 15 years. We found reduced juvenile:female ratios post M. ovipneumoniae exposure for both desert and Rocky Mountain bighorn sheep populations. Evaluation of the effects of population size and environmental conditions on lamb:ewe ratios indicated varying impacts for each subspecies. Notably, population size was negatively related to lamb:ewe ratios for Rocky Mountain bighorn only after populations were exposed to M. ovipneumoniae. Additionally, climatic conditions in the previous lambing season and pre-parturition timeframe were associated with lamb:ewe ratios for Rocky Mountain populations while lamb:ewe ratios of desert bighorn appeared to only be affected by preparturition climatic conditions.