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Bridging Holds Key To Dimorphism In Spiders

August 3, 2010

It’s a question that has puzzled scientists for years: why, in some species of spiders, are the females so much larger than their male counterparts? Now, investigators from the Spanish National Research Council (CSIC) believe they have found the answer.

According to their findings, which are published in Wednesday’s edition of the open access journal BMC Evolutionary Biology, the cause is likely tied to bridging, a technique of transportation used by some spiders to cross large gaps. When bridging, the spider uses wind to carry a strand of web to a destination, creating a sort of bridge that it then crosses upside-down.

“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,” research head Guadalupe Corcobado said in a press release dated August 2. “This may lead to a selective pressure for smaller size.”

Using a wind-tunnel, the research team studied a total of 204 spiders from 13 different species and found that the effect did not apply to female spiders–”for them, a larger body size confers and advantage in generating offspring,” the press release notes.

“Previous studies have suggested that female fecundity was the main driver of extreme male and female size differences,” added Corcobado. “However, fecundity alone could not explain why males may grow as large as giant females in some species but remain extremely small in others. 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.”

The difference between the size of the males and the females is known as sexual size dimorphism (SSD), and the phenomenon has also been observed in creatures such as deep-sea anglerfish and marine barnacles, according to Dartmouth College researcher Robert Cox.

According to a March 2010 study by Cox, entitled “Evolution of sexual size dimorphism”, “Charles Darwin hypothesized that males are the larger sex when large size gives them an advantage in competition for mates (sexual selection). He also proposed that females are the larger sex when large size gives them an advantage in the number of offspring they can produce (fecundity selection).”

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