Here’s a neat bit of research that I was alerted to while reading the newspaper: a team of scientists studying the Cook Strait giant weta (Deinacrida rugosa) found that smaller males with longer legs are much more successful in gaining copulations (Kelly et al. 2008). (There’s a lot of information & pictures on NZ soil invertebrates – not just weta – on this Massey University site.)
In many species there are quite noticeable differences between the sexes – in colour or size, for example. This is called sexual dimorphism. Charles Darwin came up with the concept of sexual selection to explain these differences, but 150 years later biologists are still interested in teasing out the underlying mechanisms that lead to their evolution.
One familiar pattern of sexual dimorphism is seen in the size differences between male & female. In mammals & birds this is often male-biased – males are bigger than females. The size difference is explained in terms of reproductive success – larger males leave more offspring. But we do also see the reverse case: female-biased dimorphism in size. And giant weta are a good example of this.
Kelly et al. were interested in finding out how this size difference might have evolved in Cook Strait giant weta (Deinacrida rugosa). This species is strongly size dimorphic: males weigh around 10g while females are twice that weight. The team recognised three possible scenarios: 1) strong selection for larger female body size (if larger females have more or higher-quality offspring) & week selection for smaller males; 2) weak selection on females but strong selection for smaller body size in males eg smaller males might be better at getting around & finding mates; & 3) strong directional selection on both sexes.
D. rugosa males are quite mobile. Instead of guarding a harem (like tree weta), or resources the females might want, they wander round at night looking for mates. When one’s found, the male stays with her – touching her with his legs or his antennae – until she finds somewhere to rest during daylight. Then they mate repeatedly. Kelly & his co-workers decided to test the hypothesis that smaller, longer-legged males are more mobile & will do better in scramble competition for mates. They did this by capturing, measuring, & radio-tagging weta, then following their movements & estimating how far they’d travelled using a 50m measuring tape (or a GPS unit, where the distance travelled exceeded the length of the tape). Each individual was recaptured 3 times over 72 hours, at which point the team also noted whether the animals had spent the night with a Significant Other.
But just how do you tell how successful a male giant weta is at mating? This is actually fairly straightforward, because each time a male mates he produces a package of sperm – a spermatophore – which he places in the female’s reproductive tract. The next time he mates, he first pushes out the (now empty) spermatophore before pushing a fresh one in. The empty spermatophores aren’t eaten, so researchers can collect & count them. (Just in case they were underestimating male success – after all, spermatophores aren’t that big & could be missed in the overnight refuge – they also mated animals in the lab, but found no difference in the number produced by males of a given size.
It turned out that males moved around much more than females, travelling an average 18.87m compared to just under 7m. One male managed 88m! And, as predicted, this was related to body size: smaller, longer-legged males travelled significantly further than their larger brethren. What’s more, this was positively correlated with mating success: males that travelled further left more spermatophores with their partners. This suggests strong sexual selection for body size in males – in giant weta, it’s the smaller guys that get more girls. However, the jury’s still out on whether there’s also selection for larger, more fecund females – fertile ground for future research 🙂
C.D. Kelly, L.F. Bussiere & D.T Gwynne (2008) Sexual selection for male mobility in a giant insect with female-biased size dimorphism. The American Naturalist 172(3): 417-423