Pregnancy incentives, predation constraints and habitat shifts: experimental and field evidence for wild bighorn sheep

Abstract

Behavioural and ecological consequences of pregnancy in a medium-sized herbivore were examined by testing the hypothesis that pregnant females should move to areas that maximize nutrient intake once the demands of gestation are high, even at the risk of increased predation. Observational and experimental data were contrasted for six populations of bighorn sheep, Ovis canadensis, living in heterogeneous environments with natural predators at Great Basin ( N=4), Sonoran Desert ( N=1) and northern prairie ( N=1) study sites. Manipulations included the alteration of dietary protein (via alfalfa supplements) in habitats that varied in predation risk when females were at different stages of pregnancy and, hence, protein requirements. Both sexes left the mountainous terrain during late winter to feed on bajadas where emergent new grasses were highest in crude protein. In these areas females were more than twice as likely as males to flee from natural predators. Females also (1) remained closer to mountainous slopes while feeding and (2) incurred greater losses in feeding time when away from slopes than males. These data suggest that predation pressure affects the sexes differently. Results of the experimental manipulations were consistent with the posed hypothesis since protein supplements on bajadas were consumed by both sexes until emergent new grasses became more abundant on mountainous slopes. However, natural variation in female reproductive status allowed a further refinement of the extent to which pregnancy incentives and predation constraints affected habitat use. Ewes not accompanied by lambs or yearlings used areas where they were at risk from predators, such as bajadas, more frequently than those without young, observations that favour the tenet that potential predation pressure overrides increased nutrient demands during late stages of pregnancy. These data suggest that species of mammals with access to heterogeneous environments prior to parturition are ideal subjects in which to examine the extent to which habitat shifts result from balancing increased dietary requirements during late gestation and safety from potential predators.