Effects of maternal condition, disease status, and behavior on survival of juvenile bighorn sheep

Abstract

Nutrition integrates and interacts with a variety of biotic and abiotic factors that modulate performance of large-herbivore populations. Accordingly, studying patterns of herbivore performance through the lens of nutrition can shed light on the complex mechanisms that drive population dynamics. We studied bighorn sheep (Ovis canadensis) and their habitat and quantified relationships among 1) the foodscape (defined herein as spatiotemporal variation in the quality and availability of forage plants); 2) female behavior (i.e., use of the foodscape and rugged escape terrain), condition (% ingesta-free body fat), and disease status (infection status and antibody levels); and 3) juvenile survival. We conducted intensive vegetation sampling and used generalized additive modeling to map the foodscapes available to bighorn sheep during summer (May–September) over 2 years in 2 population ranges in Idaho, USA: the East Fork of the Salmon River and the Lost River Range. In each study area, we used global positioning system (GPS) collars and field observations to monitor adult female behavior and lamb survival, quantified nutritional condition and disease status of maternal females in late winter, and used known-fate survival modeling to test for effects of female traits and behavior on lamb survival. Adjusted R² values for foodscape models ranged from 0.34 to 0.61. Collared females consistently selected rugged escape terrain in both study areas, but we found little evidence of selection for the foodscape. We did not detect any consistent effects of maternal space-use behavior on lamb survival. In contrast, nutritional condition of females in late winter had a strong, positive effect on summer lamb survival: lambs born to females in relatively good condition (15% ingesta-free body fat, ~94% probability of lamb survival) were roughly 4 times more likely to survive the summer months than lambs born to females in poor condition (5% ingesta-free body fat, ~25% probability of lamb survival). In addition, whereas maternal infection with the pathogen Mycoplasma ovipneumoniae had no discernable effect on juvenile survival, lambs born to females that showed no sign of exposure (antibody) to M. ovipneumoniae (~99% probability of survival) were roughly 3 times more likely to survive the summer than lambs born to females that had mounted a strong immune response (35% probability of survival). Our work adds to a growing body of literature linking nutritional condition to juvenile survival and other key vital rates, and highlights the importance of efforts to better understand the role of nutrition in the context of disease and other factors limiting performance of herbivore populations.