Combination of local selection pressures drives diversity in aposematic signals

Abstract

The diversity of aposematic signals is one of the most difficult phenomena for understanding the evolution of such signals because aposematic animals are most effectively protected when they are common. Theoretical and experimental studies predict that a combination of local selection pressures could maintain variation in aposematic signals. However, the application of this hypothesis to large-scale geographic variation in aposematic signals, other than mimicry systems, is yet to be tested empirically. I investigated geographic variation in morphological and behavioural aposematic signals of the newts, Cynops pyrrhogaster, and in predation pressures on them in populations ranging over 800 km of latitude. Field experiments demonstrated that local differences in predation pressures explain well the island-mainland variation in the aposematic colouration and behaviour of newts. Furthermore, I found a latitudinal gradient in aposematic colouration but not in behaviour, independent of predation pressures. The results suggested that island-mainland variation in aposematic signals resulting from local differences in predation pressures might also be shaped by several factors, such as temperature, body size variation, and genetic differences, and such factors might act on each aposematic trait differently.