Differences in endogenous activity within the genus Phodopus

Timing, distances, and durations of above-ground activity were monitored during three successive years in wild populations of dwarf hamsters (Phodopus). Both P. campbelli (Djungarian hamsters) and P. sungorus (Siberian hamsters) were nocturnal. However, P. campbelli arose earlier (before dark or soon after dark versus well after dark) and remained above ground 2.5 times longer than P. sungorus. Longer expeditions allowed female P. campbelli to travel 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 larger areas. Patterns of wheel running by females in the laboratory confirmed the earlier onset and longer duration of activity in P. campbelli, demonstrating that differences between species in the wild were not simply responses to different environmental stimuli. Activity before and after dark and prolonged activity throughout the dark are interpreted as evidence that resource acquisition in the colder, drier, more seasonal habitat of P. campbelli requires more energy and time than resource acquisition in habitat of P. sungorus. As such, constraints on energy and time during the breeding season may have been important in the evolution of biparental care in P. campbelli but not P. sungorus.