Colour polymorphism in spiny spiders (Gasteracantha fornicata): testing the adaptive significance of a geographically clinal sensory lure

Kemp, Darrell J., Holmes, Claire, Congdon, Bradley C., and Edwards, Will (2013) Colour polymorphism in spiny spiders (Gasteracantha fornicata): testing the adaptive significance of a geographically clinal sensory lure. Ethology, 119 (12). pp. 1126-1137.

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Many sit-and-wait predators use conspicuous displays of color to attract prey. These displays sometimes express discrete polymorphisms; however, the adaptive drivers of such variation are not well understood. Here, we explore a previously unknown color polymorphism in the orb-web spider Gasteracantha fornicata. We discovered that in North Queensland, Australia, female spiders exhibit dorsal bands appearing (to the human observer) either white or yellow and characterized by sigmoidal spectral curves centered on approx. 447 and 496 nm, respectively. Based on sensory drive theory, we hypothesized that morphs may be alternatively favored by the switch in ambient viewing conditions engendered by sunny vs. cloudy skies. We addressed this hypothesis indirectly by studying morph frequencies across a approx. 200 km geographic gradient of solar exposure (a surrogate for cloudiness), and by investigating the phenotypic signature of catch success via a resource stress experiment and in wild spiders. Our data indicate substantial geographic variation in morph frequency, with white morphs dominating at more cloudy northern locations. Rather than a gradual cline, morph frequency inverted mid-way along this range and was closely fit by a logistic relationship with latitude. Experimentally restricted access to larger prey caused spiders to lose more mass than size and to exhibit less bright dorsal markings. Wild-sampled spiders from two localities of divergent morph frequency indicated no differences in residual mass, but intriguingly, the white morph was larger and heavier (than the yellow morph) where it was relatively rare. Our data hint at negative frequency dependence, but remain broadly consistent with a sensory drive explanation based on cloudiness, and we suggest these as worthy avenues for closer investigation.

Item ID: 29569
Item Type: Article (Research - C1)
ISSN: 1439-0310
Funders: Australian Research Council (ARC), Macquarie University, Australia-Pacific Science Foundation, James Cook University (JCU)
Projects and Grants: ARC grant DP0557190, New Staff Grant 9200900977, APSF 10-09
Date Deposited: 21 Oct 2013 01:30
FoR Codes: 06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060304 Ethology and Sociobiology @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960806 Forest and Woodlands Flora, Fauna and Biodiversity @ 100%
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