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Abstract
Although the sex ratio of Odonata at emergence has received much attention, we are still far from understanding the exact causes of its variability and imbalance. In this paper we studied the sex ratios at emergence in natural populations of two Gomphus species based on samples of exuviae taken from two European lowland rivers. We hypothesized a possible relationship between the water temperature during larval development and the sex ratio at emergence. Sex ratio records exhibited no consistent bias towards one sex but varied between habitats and years in both species. We found correlations between sex ratio and water temperature in the year preceding emergence. Furthermore, the correlation between sex ratios and water temperature was in opposite directions in the two congeneric species, which may be attributed to differences in their voltinisms. We conclude that the effect of water temperature can be mediated through cohort-splitting; temperature-dependent development of minor cohorts, including unequal proportions of males and females due to the faster development of male larvae, affects the sex ratio at emergence. The supposed effect does not cause a long-term consistent bias, but may explain the year-to-year variations.
Acknowledgements
We wish to thank the Upper-Tisza Environmental and Water Directorate, personally Gáspár Bodnár, Károly Uray and the Tóth family, for providing the facilities for our studies; the Environmental and Water Management Directorate of the Tiszántúl Region for access to water temperature data. The publication was supported by the TÁMOP-4.2.2/B-10/1-2010-0024 project. The project was co-financed by the European Union and the European Social Fund.