Adult Sex Ratio in the Parnassius Mnemosyne Butterfly: Effects of Survival, Migration, And weather

Abstract

Sex ratio biases in animal populations influence the genetically effective population size, and thus are of interest in conservation. A butterfly group in which many authors report biases towards males is the genusParnassiusLatreille, 1804 (Papilionidae). Using a vulnerable woodland species,P. mnemosyne, we carried out a detailed marking campaign designed to eliminate biases towards individual sexes on marking. We then estimated the numbers of males and females using constrained linear models (CLMs) (Cormack-Jolly-Seber and Jolly-Seber in MARK); compared details of mobility between males and females using the Virtual Migration (VM) model; and built CLMs containing weather variables in order to directly assess weather effects on survival. The estimated population size was 4000 adults, with a male: female sex ratio of 1.5-1.6. Both daily and average catchability were higher for males, while the residence values (i.e., survival) were higher for females. Migration parameters were similar for the sexes, with slightly lower male survival within patches and slightly higher male emigration. CLMs with weather substituted for or added to marking days performed worse than models with mere marking days, and although weather affected the sexes differently, males still retained lower survival. The surplus of adult males in the studied population ofP. mnemosynewas real, not caused by increased male survival or a difference in mobility. Therefore, the bias toward males must appear prior to adult emergence, probably during the larval period.