Temporal geographic variation in the lambing season of bighorn sheep

Abstract

Temporal geographic variation in lambing seasons was statistically assessed for 22 populations, including 5 ecological races, of North American bighorn sheep (Ovis canadensis ssp.) from the Canadian National Parks (52° N) to western Texas (30° N). Throughout their distribution, bighorn lambing occurs coincident with the period of vegetative development when the environmental regime ameliorates neonate survival. Analyses generally demonstrate later and shorter lambing seasons in higher latitudinal populations (P < 0.001). Inception of lambing occurs later with colder temperatures, increased snowfall, at higher latitudes and elevations, and with later and shorter growing seasons [Formula: see text]. Additionally, a significant (P < 0.001) divergence in the reproductive "strategy" (median onset and duration of lambing) exists between bighorn herds of the Sierra Nevada Mountains, California (37° N) and the Desert National Wildlife Range, Nevada (36° N) as a result of two distinct, but adjacent environmental regimes. Lambing in northern populations is cued to a brief, relatively predictable period of plant growth. Southern bighorn protract lambing such that some recruitment coincides with relatively unpredictable plant growth, triggered by erratic rains.