Size counts for mating male wetas

File photo / NZPA

Big is better - well, for male weta anyway.

For them, size really is everything when it come to the mating game, says Massey University post-doctoral researcher Dr Cilla Wehi.

Six of the seven species of male tree weta in New Zealand all sport a distinctly large head.

And the bigger the head, the better the chance of mating, she says.

Dr Wehi has been studying whether there is a downside to this evolutionary feature. But she found that, despite their size, males are at no greater risk of being spotted and attacked by predators than the female weta are.

"Having the big head is a plus in terms of getting and guarding females," she says.

"The big head means a male has a larger mandible that helps win the battles with other males for control of the females."

It also means, however, that adult males are conspicuous when they come out at night to feed and fight.

According to theory, the downside is that there should be some disadvantage such as higher predation because the males are out in the open being more active and visible.

"So are they more likely to be snapped up by a morepork or a rat for example?" Dr Wehi asks.

She and her colleagues wanted to test whether the theory of sex-biased predation was evident.

If there were more females than males in different populations of weta "then we would know there was a cost to having that big head."

But theory was turned upside down when they gathered data from 58 weta populations around the country. There was not a higher predation rate in males.

Still, there must be some cost, says Dr Wehi.

"Otherwise the size of male weta heads would just keep getting bigger and bigger."

More work was needed to identify what that cost was.

Working with colleagues in Massey's Ecology group, including Dr Mary Morgan-Richards and Dr Steve Trewick, Dr Wehi has published a paper in the Journal of Evolutionary Biology.