Photo by Margarita Ramos

Spider locomotion: Overcoming a sexual burden

An on-going line of inquiry in the laboratory concerns the extrinsic and intrinsic factors that affect locomotion in spiders, in collaboration with assistant researcher Margarita Ramos and Dr. Terry Christensen from the Dept. of Psychology at Tulane University.  Males of the spider genus Tidarren (shown above) have a unique behavior of removing one of their pedipalps directly after their pentultimate molt.  These spiders remove one of their two enlarged pedipalps by securing it to a silk threat, and then turning in circles.  Remarkably, the removal of this pedipalp dramatically improves both maximum speed and endurance in these spiders.  Thus, it is likely that these spiders have evolved the behavior of removing their pedipalp to enhance their locomotor capacities which, in turn, may enable them to more easily find females.  Perhaps most interestingly, this novel behavior may have been a consequence of the extreme sexual size dimorphism (SSD) observed in this spider, as males in this genus have become extremely small, which results in very large pedipalps in order to fit with the much larger female.  Thus, this work provides a compelling example of how an evolutionary constraint arising from SSD can lead to the evolution of a novel behavior.

Relevant literature:

Ramos M, Irschick DJ, Christenson T.  2004.  Overcoming an evolutionary conflict: Removal of a reproductive organ greatly enhances locomotor performance.  Proceedings of the National Academy of Sciences. 101:4883-4887.

Ramos M, Coddington JA, Christenson TE, Irschick DJ.  2005.  Have male and female genitalia co-evolved?  A phylogenetic analysis of genitalic morphology and sexual size dimorphism in web-building spiders (Araneae: Orbiculariae).  Evolution (Cover).  59:1989-1999.

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