Sex ratio expression in ants: Empirical studies with Aphaenogaster rudis

Date of Completion

January 2006


Biology, Entomology




This work introduces the use of a Bayesian logistic regression model for analyzing ant sex ratio data and presents empirical research investigating sex ratio expression in Aphaenogaster rudis. The analytical method can be used to test how explanatory variables influence sex ratio expression and to produce estimates of population level sex ratios that are more accurate than those previously obtained using other methods. ^ My empirical research is designed to explain how environmental factors influence sex ratio expression. A review of A. rudis' biology and natural history shows that certain characteristics of this species, such as its stable social structure, make this ant an ideal study system for this work. ^ My field and laboratory experiments showed that A. rudis colonies did not alter their sex ratio when provided with supplemental food. This lack of a treatment response contrasts with findings from a similar A. rudis study and contradicts predictions of numerous resource based sex ratio models. The observed sex ratios were consistent with worker control of sex allocation. ^ A two year observational study showed that the sex ratios did not differ between two A. rudis field populations. There was a large change in the sex ratios between years and the changes were similar in both populations. In 2002 the sex ratio (0.67, female investment) fell between the values predicted under queen control (0.50) and worker control (0.75) of sex allocation. In 2003 investment was highly female biased (0.87). The observed year-to-year difference, occurring across two populations and showing similar changes, was putatively caused by weather variation. ^ Overall these results show that while sex ratio expression in ants can at times agree with predicted theoretical values, we also commonly find substantial variation in expression within a species when we examine multiple years and/or multiple populations. Understanding the causes and evolutionary implications of this variation in sex ratio expression remains as an important challenge for sex ratio researchers. ^