Following months of relative inactivity, Christmas Island red crabs run a marathon. In only 5 to 6 days, they migrate from their rainforest homes to the ocean—a trek of about 5 kilometers--to mate. How do they get in shape so quickly? To find out, researchers traveled to Christmas Island and collected leg muscles from the crabs during their migration and 6 months later, when the crabs were back in the rainforest taking it easy. Genetic analysis revealed significant differences in expression in 14 genes between the two samples, the team reports online today in May issue of The Journal of Experimental Biology. Those changes help the crabs prepare their muscles for the long, arduous journey by altering the way in which they produce energy. Gene activity in the non-trekking crabs helps muscle fibers engage in anaerobic metabolism, which doesn't require oxygen and is suited to short-duration exercise. Gene expression in the migrating crabs, on the other hand, fosters aerobic metabolism, which requires oxygen and is best suited to high-endurance activities. Meanwhile, we humans still have to rely on good old fashioned training to tone up for a race.