Two choruses of frogs?

Extract

In September 1991 I came across two parties of frogs in the bulrushes on either side of a still little pool at the Ain Qilt, some ten miles east of Jerusalem. The two parties were calling to each other in turn, as though singing antiphonally. The remarkable fact which struck me was that each group had a different chant, the one distinctly chanting only βρεκεκεκέξ, while the other replied equally distinctly with a consistent κοάξ, κοάξ. I observed this phenomenon for some ten minutes, but was not able to ascertain other differences between the two groups, such as sex, age, or temperament; but I thought that this fact, whatever its explanation, might be a significant contribution to field-research on Aristophanes.