Daily and Seasonal Patterns of Activity and Energetics in a Heteromyid Rodent Community

Abstract

Dipodomys microps, D. merriami, and Perognathus longimembris spend most of their lives solitarily and in the ground, coming out onto the surface only at night and foraging for as little as an hour. However, they are active within their burrows throughout the day, seldom remaining motionless for more than half an hour at a time during the warmer parts of the year. All three species rest at positions in their burrows which are generally at the nearest of available temperatures to the lower end of thermal neutrality, but not warmer than 30 degrees C.To do this, they occupy different depths at different seasons and in some cases different times of day. Daily vertical movements are most common in P. longimembris, because it maintains tunnels as near as 1 cm from the surface, where daily fluctuations in temperature are profound. In late fall, winter, and early spring all three species occupy nests, which serve to reduce metabollic expenditure. Energetic strategies differ interspecifically in relation to differences in bodyweight among the species: D. microps (56 g), D. merriami (35 g), and P. longimembris (7 g). The amount of surface activity in the populations and the frequency of movement measured in individuals decrease during colder parts of the year. This decrease is most apparent in the smaller species, and is related to the decreasing success at foraging later in the season and the increasing costs of metabolism at colder temperatures. Surface activity in D. microps and D. merriami continues throughout the year, at temperatures measured as low as —19 degrees C. However, for P. longimembris the costs of foraging in winter usually far outweight the yield, and the mice become continuously fossorial for up to 5 months of the year, living on seeds that they have cached in their burrows, and spending a great deal of time in dormancy. Seasonal cycles of weight and reproduction are not synchronous in D. microps and D. merriami, because of differences in diet and foraging behavior. Dipodomys merriami, which is primarily a seed eater, requires the green vegetation of annual plants to reproduce, and therefore this species increases body weight and reproduces at sporadic intervals according to the unpredictable availability of annuals. Dipodomys microps, however, reproduces in a brief, annually consistent period because leaves of perennial shrubs are its primary year—round food and water resource. This larger kangaroo rat forages over a smaller area for leaves than D. merriami does for seeds; but D. merriami runs 50% faster, which should compensate for the cost of moving greater distances. Dipodomys microps is most active during the first part of the night, but D. merriami becomes more active during the course of the night as the activity of D. microps decreases. Height and breadth of burrow hillocks and height of shrubs on hillocks were significantly larger for D. microps than the other species, but height of burrow hillocks and shrubs on them did not differ between D. merriami and P. longimembris. Perognathus longimembris resembles D. merriami more than it does D. microps in diet and reproductive pattern, but in winters of great food scarcity competition between P. longimembris and D. merriami is reduced by the total absence of P. longimembris from the surface.