Spatial and Temporal Interaction of Male and Female Grizzly Bears in Northwestern Montana

Abstract

Spatial requirements of grizzly bears (Ursus arctos horribilis) in Montana are poorly understood, yet habitat management is based on attributes of female home ranges. We evaluated home range size, overlap, and spatial/temporal use of overlap zones (OZ) of grizzly bears inhabiting the Swan Mountains of Montana. Annual home ranges of adult males were larger (x = 768 km 2 ), and adult female ranges smaller (x = 125 km 2 ), than those of subadults. Overlap in annual home ranges of adjacent female grizzly bears averaged 24% (37 km 2 ), varied from 0 to 94%, and was less when one or both females had young. Female home range overlap was greatest when one of both members of a pair were subadults. Male home range overlap with females averaged 19% for adult males and 30% for subadult males. Most simultaneous use of the OZ occurred during summer. We investigated both spatial and temporal interaction of grizzly bears having overlapping home ranges. Thirty-seven of 49 (76%) adjacent female pairs showed symmetrical and random spatial use of the OZ indicating lack of territoriality. In one of 49 (2%) cases, simultaneous use of the OZ exceeded solitary use. Temporal use of the OZ was random in 44 of 49 (90%) female interactions. Avoidance behavior within the OZ of home ranges was indicated for 1 of 2 pairs of sisters following dispersal from their mother. Most male/female pairs exhibited symmetrical and random use of the OZ. In 12 of 21 (57%) cases where the female home range was enclosed within a male range, the male exhibited spatial attraction to the female range. There was no evidence of spatial avoidance of the OZ for male pairs. Habitat availability in different portions of overlapping home ranges helped explain the observed patterns of spatial and temporal interaction among grizzly bears. The overlap zone of home ranges had higher proportional availability of avalanche chutes, rock/forb lands, and slabrock than home range areas outside the OZ. These home range and behavioral characteristics occurred at a female-dominated population density of 2-3 solitary grizzly bears/100 km 2 .