Seasonal differences in onset of surface activity of Ord's kangaroo rat (Dipodomys ordii)

Activity patterns of many nocturnal mammals are synchronized to daily cycles of light and dark. Light intensity is an important cue for nocturnal mammals because of the interplay between illumination and risk from visual predators. Studies suggest that nocturnal rodents are at greater risk from visually oriented predators before full darkness than after full darkness. We examined onset of surface activity of Ord's kangaroo rat (Dipodomys ordii) over 3 seasons in central Nebraska. To determine surface activity, we used a nonobtrusive procedure-buried timers near burrows. Although initiation of aboveground activity was significantly correlated with sunset from season to season, mean onset of surface activity differed among seasons: 1 min before the start of full darkness in summer, 4 min before full darkness in autumn, and 15 min after full darkness in winter. Despite apparent costs of emerging before darkness, 61% of kangaroo rats in summer and 63% in autumn emerged before full darkness. In winter, however, only 19% of kangaroo rats began surface activity before full darkness. We suggest that emergence behaviors of nocturnal rodents from daytime shelters are plastic and probably linked to seasonal trade-offs between costs of predation and benefits of reproduction and food abundance.