As one might expect, a marathoner's ideal body type is very different from a Greco-Roman wrestler's physique. Long-distance runners might prefer long legs relative to their bodies, and wrestlers may gain a competitive advantage from a large body mass and low center of gravity.
Similar body archetypes can be found when comparing male and female Australian huntsman spiders, according to a study, "Effects on running speed of changes in sexual size dimorphism at maturity on in the cursorial huntsman spider, Delena cancerides," which published March 21 in the Journal of Comparative Physiology A.
Cornell researchers discovered that male huntsman spiders, who travel long distances to find mates, have small bodies relative to their long legs, while the females, who secure and defend their nests and don't stray far from it, have bigger abdomens compared to their leg lengths.
The study documents the body architectures of one of the few spider species known to have social behavior, sheds light on relationships between form and function, and invites comparisons with other species.
"This fits into a larger pattern that you see, not just in spiders, but in everything from reptiles to birds and human runners, a shift toward what we presume is higher levels of endurance being associated with longer legs and a lower body mass," said co-author Jacob Hurst '13, the Resnick Plant Health Lecturer at California State University, Fresno.
The paper was part of Hurst's undergraduate thesis, which he pursued while working in the lab of senior author Linda Rayor, senior research associate in the Department of Entomology in the College of Agriculture and Life Sciences.
Australian social huntsman spiders (Delena cancerides) are among the fastest spiders known and are fairly common throughout the southern half of Australia, living in large family groups with a single mother and multiple clutches of her offspring in retreats under the loose bark of dead trees. Males generally mate with unrelated females, a system that requires them to travel far distances, exposing them to predators.
In order to grow, spiders must molt, and at the molt to sexual maturity all male spiders dramatically increase their leg length. Rayor and Hurst found that this spurt increases the male's limb length by 37% between the penultimate and final molt, compared with only about 12% in the females. The males, with small bodies and long legs, are slightly smaller overall than the females.
"We predicted that the changes in leg length at the molt to maturity would instantaneously make the males faster than the females," Rayor said. They tested this hypothesis by making 1-meter see-through acrylic tubes with boxes at each end to catch the spiders and filming the spiders from above running through the tube. Using special software, Hurst calculated the spider's speed through the tube.
"Maximum velocities (centimeters per seconds squared) for all ages and sexes were similar," Rayor said. "All of these huntsman spiders are relatively fast, but where we found interesting differences was when we looked at how fast they run relative to their body lengths. Males are running far more body lengths per second than the females, in part because they don't increase much in size, except in their legs, when they reach maturity."
It turned out that mature males with long legs weigh less in comparison with the females, which makes them ideally built for longer-distance running.
Rayor and Hurst, along with other students are applying these approaches to a larger comparative study of running dynamics in 29 huntsman spider species that are both far faster and slower than D. cancerides to determine how morphology, overall body size, and evolutionary relationships influence running speed.
This study was funded by The Hunter R. Rawlings III Cornell Presidential Research Scholars program for Hurst.
