Females roam while males patrol: Divergence in large-scale movements of polar bears during the spring pack-ice breeding season

Background/Question/Methods

Foraging and reproductive success of wide-ranging and solitary predators depends on both encounters with prey and encounters between mates, often during a narrow breeding season. The primary tool for studying movements in polar bears (Ursus maritimus) has been satellite telemetry; however, few data have been collected from adult males due to limitations in transmitter attachment. We report movement data collected from n=23 adult male polar bears in two separate subpopulations inhabiting dynamic pack ice (East Greenland and Baffin Bay) tracked over 5 spring breeding seasons, 2007-2011. Data were collected with a satellite-linked transmitters attached to the ear. We compared these movements with those of n=27 adult females tagged concurrently with satellite collars during the spring breeding season (April-May), and analyzed velocities, tortuosity of movements, kernel home range sizes, and habitat selection with respect to sex, age, reproductive status and body mass. Our objectives were to examine differences in movement behavior between the sexes during the breeding season and examine how this may impact potential mate encounters at low densities on the sea ice.

Results/Conclusions

There were no significant differences in the mean 4-day displacement between sex or population. Nor were there differences between sexes in habitat models estimating selection for satellite-derived sea ice concentration (%), distance to sea ice edge, and distance to shore. However, there were consistent and significant differences in linearity of movement, measured with a persistence index (0<PI<1), which was significantly higher for adult females (mean=0.43, sd=0.23) than adult males (mean=0.01, sd=0.19, p<0.001) in all years and in both populations. The 95% kernel range for females was also significantly larger than for males in both populations (p=0.001).  We suggest that the more tortuous breeding season movements of adult males allow for a high probability of female mate encounter while avoiding competitive interactions with other males. Linear movements of females (independent of reproductive availability) are consistent with a prioritized search for sparsely distributed, low-probability prey (e.g. seals) over mate encounters. We interpret these results as the first evidence for some level of territoriality in pack-ice breeding male polar bears, which may have significant nearing on the rate of pair formation and the impacts of Allee effects.