Big big love in wetas?

Weta is not just the name of a special effects studio; it’s the common name for one very large insect (pictured) that is found in New Zealand. Like many animals on New Zealand, it’s under a bit of pressure from introduced mammals, so there are definitely conservation implications if you can understand the mating system of the animal.

Wetas make an interesting case study for studying body size and mating, because they are large for their lineage, and there’s also a big size difference between the males and the females. In this particular species of weta (Deinacrida rugosa), females are about twice as heavy as males.

There’s a lot of permutations and combinations to figure out what factors are important in mating in these species. Having a large body can have a lot of advantages in the mating game. It can help you compete with other members of the same sex in a physical contest. But another way being large can help you compete is that bigger bodies can often mean bigger reproductive organs, and more gametes, which for males translates to more sperm.
Is having a large body size advantageous in either sex?

Four years of chasing wetas in New Zealand, radio-tagging them (yes, they’re big enough for radio tags), letting them copulate in buckets and burrows, and authors Kelly and company are getting closer to some answers.

Somewhat unexpectedly, they did not find any statistically significant relationship between body size of mating wetas. Big boys didn’t mate with big girls or small with small.

Second, heavy females received significantly less sperm than light females. This is again a bit unexpected, given that in invertebrates, large females almost always are able to produce more offspring. Typically, you find males will expend as much sperm on the highest quality females as possible. Additionally, male body size doesn’t limit how much sperm the males can produce, so there was no evidence of “the spirit is willing but the flesh is weak.”

Why? One possibility is that weta males just don’t adjust their sperm allocation for mate quality. If so, this would be very surprising, given how often males in how many different species have been shown to be able to do this. Maybe female size isn’t an indicator of fecundity, though that also seems a bit hard to believe.

The authors also float the idea that large females are so sexually attractive, that males act as if they “assume” large, high quality females will be mated multiple times, and they produce less sperm so as not to “over-invest” in a competetive situation. The authors admit themselves that their data don’t support that; there isn’t a statistically significant relationship.

Reference

Kelly, C., Bussière, L., & Gwynne, D. (2010). Pairing and insemination patterns in a giant weta (Deinacrida rugosa: Orthoptera; Anostostomatidae) Journal of Ethology DOI: 10.1007/s10164-010-0211-7

Photo by mollivan_laura on Flickr. Used under a Creative Commons license.