Matt Walker

Editor, Earth News

Helix aspersa, a snail with a hidden advantage (NPL). Garden snails are evolving slower metabolisms. Natural selection is favouring snails with reduced metabolic rates, researchers in Chile have discovered. It is the first time that evolution has been shown to select for this trait in individuals of any species. Snails with lower metabolisms are at an advantage because they have more energy to spend on other activities such as growth or reproduction, the researchers say in the journal Evolution. Roberto Nespolo and Paulina Artacho of the Southern University of Chile in Valdivia examined a long standing biological hypothesis known as the "energetic definition of fitness". "This predicts that animals that spend less energy will have more surplus for survival and reproduction," says Nespolo. We could recover the dead because of their shells and because they did not move more than a couple of metres each year

Evolutionary biologist Roberto Nespolo Few studies have tested the idea, and three done on rodents could not find any evidence it was true. "Ours is the fourth and the first to demonstrate significant directional selection on metabolism," says Nespolo. Nespolo and Artacho measured the size of almost 100 garden snails (Helix aspersa). They also gauged their standard metabolic rate (SMR), by measuring how much carbon dioxide each animal produced while at rest. The standard metabolic rate is a measure of the minimal amount of energy an animal requires to stay alive. "Standard metabolic rate is the energy required for maintenance. In other words, having less maintenance permits you to have more energy for other activities, such as growth and reproduction. That's why less metabolism represents higher fitness," says Nespolo. After seven months, they recaptured the animals, collecting the empty shells of those which had died. Survival of the SMR They found size did not predict which animals survived. But metabolic rate did, with surviving snails having a metabolic rate 20% lower than that of the snails that didn't survive. And the lower each snail's metabolic rate, the greater its chance of survival. That means that nature is selecting for snails that are more energy efficient, says Nespolo. Nespolo's and Artacho's study worked in part because of the snails they chose to study. Previous research examined metabolism in wild mice. But it's impossible to know whether mice that disappear from a study have died, or simply moved away. So it's difficult to accurately measure how many mice survive year to year. By studying garden snails living in purpose-built enclosures, Nespolo and Artacho avoided this problem, as their snails did not move far and left behind empty shells when they died. "We could recover the dead because of their shells and because they did not move more than a couple of metres each year," says Nespolo. Snail's pace The researchers now plan to answer the ultimate question: is having a slow metabolism linked to moving slowly? If it is, that means that snails are not only evolving to use energy more slowly, but are increasingly moving at an even lower snail's pace.



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