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Energetic spiderman gets the girl

Size doesn't count Energy levels rather than size determines which male golden orb-web spiders are more likely to father offspring, new research shows.

Dr Michael Kasumovic of the University of New South Wales and Dr Frank Seebacher of the University of Sydney report their findings today in the journal Biology Letters.

"[The energetic males] are able to pump up their activity much higher than other males," says Kasumovic. "And that ability to use up more energy and produce more energy allows them to outcompete males in contests."

Being the first to mate with a female gives a male golden orb-web spider two advantages.

"Firstly, he can transfer more sperm and father more offspring," says Kasumovic.

A secondary benefit is that once a female has mated she is less likely to mate again.

It's not surprising then that males will do what they can to be first.

"Often there's quite a few males competing for a specific female and they fight quite voraciously with one another, knocking each other off the web and whoever settles closest to the female often gets to mate first with her," says Kasumovic.

In general, the larger the male is, the better chance he has of winning. But in about 10 per cent of cases, this isn't the case.

"Size never really explains all the variations in success," says Kasumovic.

"Sometimes smaller males win and the question is why? What gives them this competitive advantage?"

Kasumovic and colleagues wanted to investigate whether metabolic rate could affect a male's chance of success.

The researchers tagged the backs of 12 males with paint, and observed them on webs as they approached three different females.

They then took each male and placed them in a box, which measures the amount of oxygen consumed and the amount of CO2 produced.

This experiment revealed that while spiders used the same amount of energy when they were resting, those males that got closest to the females, had the ability to pump up their energy use to a higher level than those that didn't get as close.

Exercising spiders

As part of their experiment, the researchers had to get the spiders to exercise.

"We forced them to exercise at a very high level," says Kasumovic.

The researchers put each spider in a box with a magnetic stirrer. As the stirrer moved around, the spider ran around trying to wrap the stirrer up in its web.

"Males who could wrap faster were classed as having a higher active metabolic rate and we found that to be an excellent predictor of how they could perform," says Kasumovic.

To confirm this conclusion, the researchers also studied the activity of the mitochondrial enzyme citrate synthase.

"The males that could ramp up their metabolic rate had a greater density of mitochondria, the energy factories within the cell," says Kasumovic.

The researchers now plan to check their findings by manipulating energy levels in spiders using drugs and checking their mating success.