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New study uncovers how animals in Maryland use underground highway culverts
As traffic rumbles overhead, two eyes glow inside a 3-foot-wide storm drain beneath one of Maryland’s busy highways. A raccoon is making his way from one side of the street to the other in the relative safety of an underground tunnel. He is one of close to 60 diverse species of animals statewide that use the culverts as their own personal transportation system, according to a recent study by the University of Maryland Center for Envronmental Science.
“I was surprised at the sheer number of species using these culverts, from birds to reptiles to mammals,” said study author Ed Gates of the research center's Appalachian Laboratory. “If we can design culverts that would encourage animals to use them for crossing, we could minimize mortality on the roads for animals and improve the safety of the roads for us.”
Researchers set up infrared motion-detecting cameras to find out who is using the tunnels under Maryland’s highways. The results could impact how culverts are built and where they are placed to help wildlife connect to habitats bisected by highways, as well as to improve highway safety by reducing collisions.
Storm drains, known as culverts, were created to channel streams under roadways, but they are also used by wildlife to pass under the roads, making them an ideal way to link wildlife habitats interrupted by roadways. They have been known to be used by animals around the world to get from one side of the street to the other, including grizzly bears and moose in Canada and panthers and alligators in Florida.
In Maryland, it turns out that raccoons use the subterranean highway tunnels the most, but they not alone on their commute. Canada geese hurry goslings through the tunnels. Barn swallows build nests in them. Does lead their young through the conduits. And even the five-lined skink likes to use the concrete entrance for basking and foraging.
There are 3.85 million miles of paved roads in the United States, with the edges influencing the ecology of 15-20% of the land area, according to the study. Traffic on these roads alters the movement and home range of wildlife, affects their success in reproducing, limits their ability to escape predators, and causes stress.
“Habitat fragmentation by roads is one of the most pervasive forms of habitat destruction,” said Gates. “Roads are everywhere, and highway managers are seeking new understanding and methods to restore fragmented wildlife populations.”
Ranging from 2 feet to 15 feet around, culverts can be shaped like arches, concrete boxes and corrugated metal cylinders –- and it makes a difference, the study found. For instance, raccoons had little preference for which kind of culvert they chose for travel, but deer wouldn’t go near tunnels with cobbled floors. Eastern gray squirrels were not found in any arch-shaped culverts. Domestic cats seemed to take a liking to the cylinder-shaped tunnel. And great blue herons used the box shape more often than thought, most often when the bottom was sandy.
It also matters what kind of habitat exists at the end of the tunnel. Woodchuck and white-footed or deer mouse more often used sites near pasture, hay fields and woody wetlands. Eastern gray squirrels traveled near forests, and humans (we were spotted using the tunnels, too) were most frequently found in areas with maintained lawns. And much like us, an unobstructed view is an important factor in an animal’s use of the tunnel.
Few studies have investigated wildlife use of culverts in such a broad geographic region. This is the first statewide study in the Mid-Atlantic region, and it reveals a wide range of commuters. In the Piedmont area of central Maryland you’re most likely to run into raccoon, red fox, white-tailed deer and Norway rat making their way under the highway. In the Appalachian Mountains, prepare to greet Virginia opossum and domestic cats. The study found 57 species in all used 265 culverts to cross the road or build a nest in Maryland, confirming that these drainage structures are carrying more than just streams.
To see more pictures of animals caught using the culverts, visit our Facebook page and click on Photos.