By following trails, periwinkles save slime.

A seaside snail crawling along the gooey streak left by another snail is saving a lot of energy, say researchers, because it doesn't have to ooze so much slime itself.

Scientists have observed various kinds of snails following each others' paths, says Mark S. Davies of the University of Sunderland in England. Now, he proposes that followers are economizing on mucus. Davies and his colleague Janine Blackwell have measured the thickness of new and reused trails of a common periwinkle (Littorina littorea), which creeps along rocky Atlantic shores. Following a fresh trail, it secretes much less slime than it expends when laying a new trail, the researchers report in a paper non, online for an upcoming Proceedings of the Royal Society B.

"It's much, much more expensive to go around on a carpet of mucus than to run, walk, swim, or fit" Davies says. He has calculated that a periwinkle uses more than 35 times as much energy making mucus as it does crawling along it. And he finds that a limpet creeping along seashore rocks spends roughly a third of its total energy intake producing that mucus.

To study snail mucus, Davies and Blackwell permitted a periwinkle to crawl over microscope slides in the lab and measured the thickness of its slime. A second periwinkle following a trailblazer secreted, on average, only 27 percent of the mucus typically laid down in a new trail. After a slide had been exposed to one or more tide cycles along the shore, the streaks deteriorated, and a snail coming along later had to do more resurfacing.

Saving energy by following trails "sounds plausible," says mathematician Eric Lauga of the Massachusetts Institute of Technology. Last year, he and a colleague made a mathematical model of a snail crawling on mucus. The substance is 95 percent water, with a dash of various salts and a small dose of the glycoproteins that give the mucus cohesiveness. The researchers concluded that the properties of the slime minimize the amount of mucus needed for crawling. Nevertheless, Lauga agrees that producing the trail is more expensive than moving along it.

If mucus saving is such a big issue, then "land snails should be even more given to trail following" because they have a more difficult time replacing water, muses snail neurobiologist Melissa Harrington of Delaware State University in Dover. "However, we see it pretty rarely," she says, noting that land and shore snails may make different energy trade-offs.

The periwinkle work, she says, makes "an interesting observation."