Behavioural modulation of predation risk: moonlight avoidance and crepuscular compensation in a nocturnal desert rodent, Dipodomys merriami

The temporal and spatial distribution of above-ground activith in Merriam's kangaroo rats, Dipodomys merriami, radio-tracked during 10 winters varied in relation to phases of the moon. At the full moon, animals were more likely to be found in their day burrows at scheduled hourly radio fixes, and when they emerged, they stayed closer to home. At partial moon phases, above-ground activity was preferentially allocated to hours when the moon was down. As predicted from an extension of Rosenzweig's (1974, J. Mammal., 55, 193–199) model of optimal above-ground activity, the suppression of nocturnal activity at the full moon was partially offset by relatively great activity at dusk and dawn, and losses to nocturnal versus diurnal predators were distributed differently over the lunar cycle. Gross predation rates did not differ significantly among moon phases, but the kangaroo rats' relatively crepuscular activity pattern at the full moon both reduced their vulnerability to nocturnal predators, as compared with the new moon phase, and raised their vulnerability to diurnal shrikes.