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

The Japanese subterranean termite Reticulitermes speratus exhibits a female-biased alate sex ratio. Colony foundation by female–female alate pair, and biased reproductive values between king and queen resulted from king displacement have been proposed as the causes of the biased sex ratio. These causal hypotheses can be tested by examining alate sex ratios and their geographic variation. Considering the wide north–south distribution and 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 the sex ratio variations of R. speratus alates in 157 colonies from 16 populations in the Japanese archipelago. Variations in head width and dry weight were examined in 64 colonies from 10 populations. The alate sex ratio was biased toward females. No colonies without males were found, contradicting the hypothesis that parthenogenetic colonies founded by female alates bias the sex ratios. While the female bias of numerical sex ratio was slightly more pronounced at lower temperatures (higher latitudes), the sex-investment ratio did not follow this trend. There was no clear support for the hypothesis that king displacement causes sex ratio bias. Interestingly, a distinct split sex ratio was observed at low-temperature populations, indicating the existence of another factor causing disruptive selection. The head widths and dry weights of alates were larger at lower temperatures (higher latitudes). This is the first report of an intraspecific latitudinal body size cline in termites, which is common with a temperate ant species and many ectotherms.