Batesian mimicry, in which harmless organisms resemble unpalatable or harmful species, is a well-studied adaptation for predation avoidance. The females of some Batesian mimic species comprise mimetic and non-mimetic individuals. Mimetic females of such polymorphic species clearly have a selective advantage due to decreased predation pressure, but the selective forces that maintain non-mimetic females in a population remain unclear. In the swallowtail butterfly, Papilio polytes, female polymorphism is controlled by the H (non-mimetic) and h (mimetic) alleles at a single autosomal locus. Here, we examined if the dominant H allele has a deleterious effect on the pre-adult survival rate (egg-to-adult emergence rate). We repeated an assortative mating-like treatment—i.e. breeding of males and females whose mothers had the same phenotype (mimetic or non-mimetic)—for three consecutive generations, while avoiding inbreeding. Results showed that pre-adult survival rate decreased over generations only in lines derived from mothers with the mimetic phenotype (hereafter, mimetic-assorted lines). This lowered survival was due to an increased mortality at the final instar larval stage and the pupal stages. Interestingly, the pre-adult mortality in the mimetic-assorted lines seemed to be associated with a male-biased sex ratio at adult emergence. These results suggest that the dominant H allele displays a mildly deleterious effect that is expressed more strongly in females and homozygous individuals than in heterozygous individuals. We propose that this cost of mimicry in larval and pupal stages contributes to the maintenance of female-limited polymorphism in P. polytes.

Papilio polytes survival data in the laboratory

Survival data for butterfly species used in this study.