Evolution hid snake’s identity, professor says
“I tell my students that you don’t have to go to the Galapagos to see evolution in progress,” said Bulmer, who teaches biology at NVCC-Annandale. “It’s happening right in our back yards.”
Nature educators first discovered the snake, which is gray on top and orangish underneath, while giving a tour to a class off the Belmont Bay Coast in early April.
After consulting other snake enthusiasts, local environmentalist Kim Hosen declared that the snake was a red bellied water snake, the likes of which had never been documented this far north.
But after a photograph of the reptile ran in the Potomac News & Manassas Journal Messenger, several herpetologists came forward, claiming she was mistaken.
Friday morning, Bulmer did his best to put the debate to rest.
Bulmer clarified that the snake found was indeed a Nerodia sipedon sipedon, a northern water snake, but, over time its coloration has transformed to match its environment.
“We discovered a population of this type of water snake back in the mid-’70s,” Bulmer said. “You only find [northern water snakes] like this in very few places … including Pohick Bay and Belmont Bay.”
Normally, northern water snakes have a bonded pattern of dark and light colors on their top sides and orange blotches and spots underneath.
Yet the snake discovered last month did not match that description, prompting Hosen to believe it was a red bellied.
Instead, Bulmer said it is a perfect example of natural selection. Many northern water snakes in the Pohick and Belmont spend more time in the water because the bays are shallow, with warmer water and low visibility.
In order to best disguise themselves, the snakes have systematically “morphed” into looking like the snake in question.
A group from George Mason once opened up a pregnant northern water snake and found that of the 50 young inside her, 40 looked like the conventional northern water snake and 10 had evolved.
“So, 40 of those snakes were destined to be preyed upon,” Bulmer said.
This phenomenon is not unique to Northern Virginia.
In the islands of western Lake Erie, scientists discovered that northern water snakes had similarly evolved, according to a study posted on the Department of the Interior’s Web site. It was deduced that those snakes with unbanded or reduced patterns had a higher likelihood of survival than those that had normal patterns.
Their coloration change has been so uniform that scientists have classified them as a separate subspecies, Nerodia sipedon insularum, or Lake Erie water snakes.
“This has been a result of intense selection,” said Bulmer, who added that the northern water snakes in this area may be heading in that direction. “It’s still the same species here because the [different-colored snakes] still reproduce with [the typical northern water snakes]. There’s still more contact here.”
Hosen did not return phone calls placed to her Manassas office Friday afternoon.