Studies in which the suprachiasmatic nucleus (scn) was surgically removed from laboratory mammals demonstrated that the scn is required for circadian rhythms. but these studies did not reveal whether circadian rhythms originate in the scn. to answer this question, researchers performed an scn transplant experiment on wild-type and mutant hamsters (mesocricetus auratus). for wild-type hamsters, the period between cyclic peaks in activity is 24 hours in the absence of external cues. hamsters homozygous for the τ (tau) mutation have a period lasting only about 20 hours. in this exercise, you will evaluate the design of this experiment and propose additional experiments to gain further insight. the researchers surgically removed the scn from wild-type and τ hamsters. several weeks later, each of these hamsters received a transplant of an scn from a hamster of the opposite genotype. these transplants restored rhythmic activity in 80% of the hamsters. for hamsters in which an scn transplant restored a circadian rhythm, the researchers measured the circadian period. the results are shown in the graph. each red line represents the change in the measured period for an individual hamster. part a - understanding the experimental design in a controlled experiment, researchers manipulate one variable at a time. what was the experimental variable in this study?