Differences in endogenous activity within the genus Phodopus

Timing, distances, and durations of above-ground activity were monitored du ring three successive years in wild populations of dwarf hamsters (Phodopus ). Both P. campbelli (Djungarian hamsters) and P. sungorus (Siberian hamste rs) were nocturnal. However, P. campbelli arose earlier (before dark or soo n after dark versus well after dark) and remained above ground 2.5 times lo nger than P. sungorus. Longer expeditions allowed female P. campbelli to tr avel farther than both sexes of P. sungorus at the same speed. In contrast, P. campbelli males were active for the same amount of time as females but travelled almost three times faster, covering much greater distances and la rger areas. Patterns of wheel running by females in the laboratory confirme d the earlier onset and longer duration of activity in P. campbelli, demons trating that differences between species in the wild were not simply respon ses to different environmental stimuli. Activity before and after dark and prolonged activity throughout the dark are interpreted as evidence that res ource acquisition in the colder, drier, more seasonal habitat of P. campbel li requires more energy and time than resource acquisition in habitat of P. sungorus. As such, constraints on energy and time during the breeding seas on may have been important in the evolution of biparental care in P. campbe lli but not P. sungorus.