Insect rodeo – sexual selection in water striders

Sometimes it seems that men and women are completely different. A lot of the times they are. We all know the physical differences between men and women in humans, but also the peacock is a good example in which the male has a large bright tail which he shows off to all the ladies, while the female is plain brown with a normally sized tail (see the picture below). These differences between male and female peacocks are caused by sexual selection.

Male Peacock displaying

The differences in the peacock are very clear, but when we look closely we can also find differences between the sexes in insects. More specifically, when a structure evolves to further the advantage of one sex at the expense of the other sex we call it “sexual antagonism”.  Recently, Khila et al. (2012) published about just such a trait in the waterstrider (Rheumatobates rileyi). In this species it is very hard for males to get females to mate (isn’t it always?). The females struggle vigorously to reject male mating attempts. But these males are no quitters; they evolved very special antennae to overcome the female resistance. Contrary to the female antennae, male antennae evolved to be shaped wrench like, able to grasp something. Furthermore, in their wrench like antennae they have 3 more structures adapted for grasping; a hook, a spike, and a pad (see the awesome figure below!).


So what are these special antennae supposed to grab? Well, they are used to grab a female by her eyes! (see picture below, male is on top) The struggle to mate involves an extensive behavioral sequence in which the males jumps the female, grabs her eyes with his antennae and holds on for dear life (like a rodeo ride). If he succeeds in holding on the female he will eventually be able to mate with her.


But this is not the complete story; they also found the gene responsible for the differences in antennae structures. For developmental biologists it is a very well-known gene; distal-less (Dll).Which is known to be involved in appendage development. Khila et al. were able to knock down this gene by RNA interference (see my post on RNAi) and found a graded response in the sex-specific characters of male antennae. When the effect of the knock-down was severe, the male antennae looked like female antennae. However, when the effect was moderate, the wrench like shape was still present but the hook, spike and pad were missing. They tested the effect of the lack of these male characters on the mating success and found that it reduces their chance of mating significantly. Although the mating success for males of this species is generally only 12% (though luck guys), males that lacked the hook, spike and patch had a mating success near to 0%! So at least for water striders it pays to have a big hook, spike and pad!

I want to thank Abderrahman Khila for providing me with these cool pictures to add to my post!
You can find the full story in Science here.