Low Mating Frequency in Predominantly Female Populations of the Butterfly, Acraea encedon (L.) (Lep.)

Abstract

During mating a male butterfly leaves a spermatophore in the bursa copulatrix of the female; one spermatophore results from each mating, and by dissection it is possible to determine how many times a female has mated. In butterflies with a normal I:I sex ratio most females mate at least once, and samples collected in the field contain very few unmated females. Most females from four Uganda populations of Acraea encedon containing only 2-6 males to every I00 females were found to be unmated. In contrast, most of the females from a population in Sierra Leone with a sex ratio of about 0.I6 had been mated, and all those in a small sample from a population with a normal sex ratio had mated. Statistical analysis of spermatophore counts suggests that matings subsequent to the first occur at random, but at the same time it would seem that the predominantly female populations of Acraea encedon, which in some cases are known to have survived for at least 50 breeding generations, owe their persistence to some form of frequency-dependent selective mating such that females producing males as well as females are at an advantage. It is suggested that spermatophore counts could be used in any possible future investigation of selective mating in Acraea encedon. The behaviour of individuals in an isolated population in Sierra Leone with a normal sex ratio suggests that reproductive isolation has been evolved, and that the sex ratio trait has been unable to spread into this population.