Intraspecific variation of sex ratio and body size along latitude in the termite Reticulitermes speratus (Isoptera: Rhinotermitidae)

Abstract The Japanese subterranean termite Reticulitermes speratus exhibits a female-biased alate sex ratio and a wide north-south distribution area. The possible causes of the biased sex ratio can be tested by examining alate sex ratios and their geographical variation. Considering the need for adaptation to the winter fasting period, a body size cline of alates may be present in this species. In this study, we examined variations in the sex ratio, head width, and dry weight of R. speratus alates in 157 colonies from 16 populations on the Japanese archipelago. The alate sex ratio was biased toward females; however, no colonies without males were found, thus contradicting the hypothesis that parthenogenetic colonies founded by two female alates bias the sex ratios. The sex ratio bias was more pronounced at lower temperatures (higher latitudes), which supports the hypothesis that asexual queen succession causes sex ratio bias. Interestingly, a distinct split sex ratio was observed at low-temperature sampling sites, indicating the existence of another factor causing disruptive selection. The head widths and dry weights of alates were larger at lower temperatures and higher latitudes. This is the first study of an intraspecific latitudinal body size cline in termites, which is common in a temperate ant species and many ectotherms.