Can Sex Ratio be Defined or Determined? The Case of a Population of Checkerspot Butterflies

The sex ratio of adults in a single generation in a demographic unit of Euphydryas editha was strongly skewed toward males. This appears to be caused by higher pre-adult mortality in females, a higher rate of female emigration, and possibly higher female adult mortality. The impact of this and related factors on determining Ne is discussed. Although Ne in E. editha populations generally appears to be far below twice the number of males, it also seems to be well above the range where drift would be expected to be a major evolutionary factor. It is suggested that the factors that skew sex ratio and reduce Ne in E. editha may well operate in other univoltine insects.