On St. Patrick's Day, here's the real reason Ireland has no snakes

St. Patrick's Day celebrates the patron saint of Ireland, who, according to legend, banished snakes from the Emerald Isle.

Science Notebook

Tuesday marks St. Patrick's Day, named after the 5th-century missionary who was famous for banishing all of the snakes from Ireland. With staff in hand, the Christian preacher cast the slithering critters into the sea, never to return – at least that's the story.

The Emerald Isle is indeed one of very few places in the world without snakes, not counting the few serpents kept in zoos or as pets. But can an ancient saint really take credit for Ireland's curious lack of snakes?

The National Museum of Ireland in Dublin has combed through the country's fossil records. As far as its researchers can tell, there's no evidence that snakes ever lived in Ireland. In other words, St. Patrick couldn't have banished many snakes, because there weren't any native snakes to begin with. 

Rather than trusting a 1,000-year-old legend, scientists think the real answer reaches back more than 10,000 years. 

Long before St. Patrick, an ice age gripped Europe. At the time, Ireland was too cold for reptiles. Yet, as the ice thawed, some animals migrated from continental Europe over a land bridge to Britain and then west to Ireland. Boars, brown bears, and lynxes arrived in Ireland around this time.

Snakes, however, moved north much more slowly. Three species slithered into Britain – the adder, the grass snake, and the smooth snake – but they arrived too late to reach Ireland. Around 8,500 years ago, melting glaciers caused ocean levels to rise, cutting off the Emerald Isle. Some animals could still swim over, but scientists have never found a snake species that could migrate across open ocean. (For this reason, several other large islands don't have snakes either, including Greenland, Iceland, and New Zealand.)

The rising waters kept away more than just snakes. In fact, Ireland has only one species of native reptile, the viviparous lizard. 

If Ireland never had snakes, why make such a big deal about St. Patrick? Many think the snake legend is symbolic. Several pagan religions in and around Ireland used serpent imagery. So when stories say that St. Patrick cast out the snakes, they actually mean that Christians cast out the pagans.

Snakes have become a favorite pet among rich Irish, who enjoyed defying the legend by importing high-end species, according to a The New York Times report from 2013. But as Ireland's economy turned several years ago, some snake owners couldn't afford their scaled pets. Many snakes were set loose. "A California king snake was found [in 2012] in a vacant store in Dublin," reports the Times, "a 15-foot python turned up in a garden in Mullingar, a corn snake was found in a trash bin in Clondalkin in South Dublin, and an aggressive rat snake was kept in a shed in County Meath."

Perhaps Ireland could use a modern-day St. Patrick, one who could make the legend a reality.