Image caption The "bridging" behaviour of male spiders may be key to their small size

In some species of spider, males are far smaller than females; now, scientists think they know why.

A group of Spanish researchers says evolution favours small, light males because they can more easily traverse thin strands of silk.

But large females are favoured because they reproduce more abundantly.

Writing in the journal BMC Evolutionary Biology, the researchers say their finding helps to explain why this trait is so widespread among spiders.

A selective pressure against large male body size has been searched for by researchers since Darwin Guadalupe Corcobado

In some species of orb-weaving spider, females are more than 12 times longer than males.

Other research has shown that in many of these species, the females have grown progressively larger over evolutionary time, probably because of the reproductive advantages.

But the males have not followed suit, leading to the huge size differences - a trait known as extreme sexual size dimorphism.

The key behaviour that the Spanish team investigated is known as "bridging".

Here, the plant-dwelling spider produces a strand of silk and allows the wind to carry one end of it.

When it lands on a far leaf or stem and sticks, the spider pulls it tight, secures the near end and crawls across hanging upside down from the strand.

Highwire act

The research team, headed by Guadalupe Corcobado from the Spanish National Research Council's Arid Zones Research Station in Almeria, used a wind tunnel to aid their investigations.

Image caption Golden silk spiders (Nephila clavipes) are among the dimorphic species

They looked at how males and females behaved - whether they tried to bridge, and how succesful they were.

Across 13 species that show extreme dimorphism and use bridging, the smaller ones attempted it more often and were more succesful.

"In species where bridging is a very common mode of locomotion, small males, by being more efficient bridgers, will enjoy more mating opportunities and thus will be better at competition to reach receptive females," said Dr Corcobado.

"This may lead to a selective pressure for smaller size."

The idea that gravity is involved in selection for small size is not new - researchers have previously suggested small males could flee from predators faster, or gain easier access to prospective mates.

But the link to bridging is new, said Dr Corcobado.

"[Female] fecundity alone could not explain why males may grow as large as giant females in some species but remain extremely small in others," she said.

"A selective pressure against large male body size has been searched for by researchers since Darwin; the constraint on bridging seems to be such a selective pressure."

Richard.Black-INTERNET@bbc.co.uk