The eastern grey tree frog (sound) looks exactly like the closely related Cope's grey tree frog (sound). The big difference between the two species is beneath the surface—the eastern has twice the number of chromosomes as does the Cope's. Having more sets of chromosomes makes the cells of the eastern frog larger than the cells found in the Cope's. And those bigger cells makes the eastern's song just a little deeper. Now University of Missouri researcher Carl Gerhardt and his student Mitch Tucker have determined that the slight difference in the calls—here's the eastern again and here's the Cope's (sound)—is how the females know which species' males to buddy up with: the ones with the same chromosome number that they have. The work is in the poceedings of the Royal Society B-Biological Sciences. Speciation is often caused by a geographic barrier that keeps populations from mating. But the tree frog situation may be a rare case in which chromosome duplication, and its subsequent effects, presented a productive barrier. As in humans, it comes down to whether he calls.