Aposematic polymorphism in the tropical butterfly Danaus chrysippus: A review

In theory, the selective attacks by experienced predators are expected to drive aposematic prey species towards colour monomorphism because mutant individuals with divergent colour forms will not be recognized as distasteful and thus will be targeted by predators. The tropical butterfly Danaus chrysippus represents a notable exception to this rule since it is aposematic but shows extensive colour pattern polymorphism in the region of East and Central Africa. In this paper, we present a critical review of the various hypotheses that were suggested to explain the origin and maintenance of colour polymorphism in D. chrysippus. Three different approaches to this problem were revised and criticized: first, that polymorphism is a non-adaptive consequence of past geographic isolation that resulted in the formation of a hybrid zone, second, that polymorphism represents an evolutionary escape route when an aposematic colour form is heavily loaded by batesian mimics and third, that polymorphism is maintained because D. chrysippus is subjected to spatially and temporally variable Müllerian selection as a result of recurrent population extinctions in the mimetic Acraea species, driven by the spread of the male-killing Wolbachia bacterium. It has been argued that the three hypotheses are not mutually exclusive; alternatively, each hypothesis targets a distinct problem and exploits a distinct set of key observations. The study concluded that the final solution to the long standing problem of aposematic polymorphism in D. chrysippus might be achieved through the adoption of a pluralistic approach that integrates all the three hypotheses.