Strange, but interesting, fact from the natural world

Need for Speed

Here’s a consummately unequal partnership for you: male cobweb spiders (genus Tidarren) weigh in at only about 1 percent the weight of the females. Not surprisingly, that creates a few problems.

The male has two copulatory organs known as pedipalps, and either one can make him a dad. To do the job properly, each pedipalp has to be enormous, relative to the rest of his body. Sure enough, the two make up about 20 percent of his body weight – more, apparently, than he’s willing to carry around. So, just before molting into his adult form, the male Tidarren half-emasculates himself. He spins a silk structure, ties it around one – just one – of his pedipalps, and then twists off the unwanted organ.

Working in the laboratory of Duncan J. Irschick, an ecologist at Tulane University in New Orleans, undergrad Margarita Ramos and a colleague quantified how much the pedipalps drag males down. By chasing spiders, the investigators found that males with a single pedipalp move 44 percent faster, have 63 percent more endurance, and travel 300 percent farther before pooping out than males with both pedipalps intact. Evolution didn’t come up with a way to grow just one pedipalp so as to increase mobility, but it did find a behavioral work-around. (“Overcoming an evolutionary conflict: Removal of a reproductive organ greatly increases locomotor performance,” Proceedings of the National Academy of Sciences 101: 4883 – 87, April 6, 2004)

[Reprinted from Natural History, June 2004, page 17]

Actually they probably evolved to start with two on purpose -- that way they can choose to keep the one that will function better. Sort of like why mammals have two testicles -- need a backup.