Sperm and sex peptide stimulate aggression in female Drosophila

Abstract

Female aggression towards other females is associated with reproduction in many taxa, and traditionally thought to be related to the protection or provisioning of offspring, such as through increased resource acquisition. However, the underlying reproductive factors causing aggressive behaviour in females remain unknown. Here we show that female aggression in the fruit fly Drosophila melanogaster is strongly stimulated by the receipt of sperm at mating, and in part by an associated seminal fluid protein, the sex peptide. We further show that the post-mating increase in female aggression is decoupled from the costs of egg production and from post-mating decreases in sexual receptivity. Our results suggest that male ejaculates can have a surprisingly direct influence on aggression in recipient females. Male ejaculate traits thus influence the female social competitive environment with potentially far-reaching ecological and evolutionary consequences.

Keywords: behaviour; female-female aggression; mating costs; sex peptide; trade-off.

Figure 1. Two proposed pathways for mating-induced female aggression

a) The transfer of male ejaculates stimulates increased egg production, which could in turn stimulate increased female aggression b) Alternatively, the transfer of male ejaculates could stimulate increased female aggression directly, without requiring elevated egg production.

Figure 2. Mated females spend more time fighting than virgins but do not win more fights.

(a) Contest duration in mated, virgin and mixed mating-status female dyads. MM = two mated females (N = 29), MV = one mated female and one virgin female (N = 22), VV = two virgin females (N = 26). (b) Proportion of encounters won by mated females = black bar (n = 45) and virgin females = grey bar (n = 45) in the mixed treatment (MV in 2.a). Model estimate means and standard errors are shown.

Figure 3. Effects of male ejaculate components, and female egg-production and sex peptide receptor on female total contest duration.

a) Total contest duration of mated sterile females (ovo^D1) and control females. N pairs: Mated sterile (ovo^D1) = 33, Virgin sterile (ovo^D1) = 31, Mated control = 31, Virgin control = 33. b) Effect of sperm transfer on female contest duration. N pairs: Control = 26, Spermless = 25, Virgin = 25. The experiment was carried out in two blocks, but results are pooled in this figure. c) Effect of sex peptide transfer on female contest duration. N pairs: SP+ = 39, SP null = 42, Virgin = 49. d) Effect of SPR on female contest duration. N pairs: Mated control = 25, Virgin control = 24, Mated SPR deficiency = 29, Virgin SPR deficiency = 25. Bars represent means, while error bars indicate standard errors.