Contact call variation along A) longitudinal and B) latitudinal gradients. Black is the highland Z. poliogaster, white is the lowland Z. abyssinicus. C) Comparison of call distance (calculated as Euclidean distances from the first two principal coordinates axes for all pairs of sites) in response to geographic distance (degrees) for both species, D) respective interspecies comparisons. Regressions in A: Z. poliogaster: r2 = 0.59, p = 0.02; Z. abyssinicus: r2 = 0.38, p = 0.03. Regressions in B: Z. poliogaster: r2 = 0.44, p = 0.05; Z. abyssinicus: r2 = 0.56, p = 0.006.