How Is A Snail Like A Finch?
Grove snails are a widespread and varied species that, by one measure, are one of the fastest animals we’ve measured. Find out how a dutch researcher used a natural experiment to reveal the impressive adaptability of an unassuming gastropod.
EPISODE NOTES
Menno Schilthuizen, in addition to the research on snails featured in this episode, has written two great books that take on animal biology, behaviour and evolution from odd angles. One, Nature’s Nether Regions, is all about animal sex lives and reproductive organs. The other, Darwin Comes To Town, is on a personal favourite subject of mine: how animals evolve to deal with increasingly urban environments.
TRANSCRIPT
Today, we’re doing two things. We’re gonna answer an age old riddle I just made up: ‘how is a snail like a finch’, and we’re going to see how, by one definition, snails can be one of the fastest animals around.
Specifically, grove snails. Also called brown-lipped snails or lemon snails. They have done pretty well for themselves. They’re the most common snail in Europe and they’ve made serious inroads in North America since arriving in the 1800s. They’re just as at home in open meadows as they are in a forest.
And their success is even more impressive when you think about just how vulnerable they are. Which might sound strange. After all, isn’t the whole point of being a snail that you’ve got a protective shell so you’re not vulnerable? Well… turns out that armoured back kinda backfired on them. Because of these.
Female birds in nesting season need to ingest a ton of calcium so they can produce strong eggs. And snail shells are mostly calcium carbonate - making them a slow-moving meal of exactly what the birds are looking for. Snail shells can be the primary source of calcium for some bird species.
So - we know two things about grove snails. They’re prolific and they’re valuable prey. There’s one more thing you need to know about them. Their colouration can be all over the place. The shade of the shell itself, how many bands from zero to five, how thick, how dark, seems to be no rhyme or reason to it - until evolutionary pressure comes into play.
Remember, these snails can live in open meadows or dense forests. And what works in one environment does not work in the other. Darker snails absorb more heat and if they don’t cook in the sun they’ll still be more visible against the lighter meadow. Just like a bright yellow snail will stick out under a shaded forest canopy.
Combine that with a lot of calcium-hungry birds and snails can start to look a lot like finches, because to evolutionary biologists, grove snails are held up on par with Darwin’s famous Galapagos finches, showing clear results of selection pressure in their specific environments. Grove snails are darker with more bands in the forest and lighter with fewer bands in the meadow.
So we knew grove snails were adapting locally and quickly. But it was only recently we learned just how fast these changes occur. Dutch biologist Menno Schilthuizen found an ingenious way to track them using a natural experiment.
He focused on former bodies of water that had been drained, called ‘polders’, that create brand new environments. They gave him an exact starting point for when the grove snails moved in, from 40 to 80 years ago. And that let him see how quickly the population shifted to more advantageous colouring.
What did he find? After 50 years in a new forest, the snails exhibited four times higher frequency of beneficial colouring than in a comparable meadow. Put more simply, that is one of the fastest evolutionary adaptations ever seen in the wild.
So - snails may be a timeless symbol for ‘slow’. But it turns out, when it comes to evolution, they happen to be olympic level sprinters.