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PUBLIC RELEASE DATE:
2-Aug-2010

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Contact: Graeme Baldwin
graeme.baldwin@biomedcentral.com
44-020-319-22165
BioMed Central
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Why are male spiders small while females are giant?

IMAGE: This is a bridging spider.

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'Bridging', an unusual mode of getting around frequently used by vegetation-inhabiting spiders to cross large gaps, may partly explain the tendency for male spiders to be much smaller than females. Researchers writing in the open access journal BMC Evolutionary Biology studied bridging, in which spiders use the wind to carry a strand of web to their destination and then clamber upside down along the resulting bridge, finding that small size was associated with a greater ability to carry out the maneuver.

Guadalupe Corcobado, from the Spanish National Research Council, worked with a team of researchers led by Jordi Moya-Laraño to investigate the spiders' behavior in a laboratory wind tunnel. She said, "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. This may lead to a selective pressure for smaller size."

The researchers investigated 204 spiders from 13 different species. They suggest that females do not feel the same pressure to be smaller as, for them, a larger body size confers and advantage in generating offspring. Corcobado emphasises that this 'bridging' theory to explain size differences between the sexes is not incompatible with other hypotheses. She said, "Previous studies have suggested that female fecundity was the main driver of extreme male and female size differences. 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".

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Notes to Editors

1. Introducing the refined gravity hypothesis of extreme sexual size dimorphism
Guadalupe Corcobado, Miguel A. Rodriguez-Girones, Eva De Mas and Jordi Moya-Larano
BMC Evolutionary Biology (in press)

During embargo, article available here: http://www.biomedcentral.com/imedia/1169183803596379_article.pdf?random=864693

After the embargo, article available at the journal website: http://www.biomedcentral.com/bmcevolbiol/

Please name the journal in any story you write. If you are writing for the web, please link to the article. All articles are available free of charge, according to BioMed Central's open access policy.

Article citation and URL available on request at press@biomedcentral.com on the day of publication.

2. A picture of a bridging spider is available here: http://www.biomedcentral.com/graphics/email/images/Spider03.jpg

3. BMC Evolutionary Biology is an open access journal publishing original peer-reviewed research articles in all aspects of molecular and non-molecular evolution of all organisms, as well as phylogenetics and palaeontology. BMC Evolutionary Biology (ISSN 1471-2148) is indexed/tracked/covered by PubMed, MEDLINE, BIOSIS, CAS, EMBASE, Scopus, Zoological Record, Current Contents, CABI, Thomson Reuters (ISI) and Google Scholar.

4. BioMed Central (http://www.biomedcentral.com/) is an STM (Science, Technology and Medicine) publisher which has pioneered the open access publishing model. All peer-reviewed research articles published by BioMed Central are made immediately and freely accessible online, and are licensed to allow redistribution and reuse. BioMed Central is part of Springer Science+Business Media, a leading global publisher in the STM sector.



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