The northern red-legged frog, or Rana aurora, is found along the western coast from British Columbia to Northern California. Their typical breeding season lasts from January to March. The foothill yellow-legged frog, or Rana boylii, is found along the western coast from northern Oregon to central California. Their typical breeding season lasts from April to July. What mechanism might keep Rana aurora and Rana boylii from mating?