Changing Planet

Ladders for Voles—Plus 5 Man-made Wildlife Crossings

The old joke asks, “Why did the chicken cross the road?” But in the world of wildlife conservation, the big question is, “How did the animal cross the road?” And the answer is often: With the help of a bridge or tunnel so there are no worries about animal-vehicle collisions.

A vole ladder is installed. Photograph courtesy Canal & River Trust

One conservation group is testing, for the first time ever, wooden ladders to expand the habitat of endangered water voles in London. The Canal and River Trust hopes that the “superhighways”—designed to guide the voles over the tall edges of the Grand Union Canal—will help an isolated group venture beyond the small pond they live in and into the canal, where they can nest in and feed on new man-made islands. If successful, more ladders will be installed throughout London to encourage the voles to make more use of the canals. Then the city’s fragmented vole populations can connect, mate, and improve their genetic diversity.

Here are five other ways that man-made structures help animals avoid obstacles to get to the other side.

—Linda Poon



Crabs cross a road using a "crab bridge" on Christmas Island. Photograph courtesy Max Orchard, Parks Australia
Crabs cross a road using a “crab bridge” on Christmas Island. Photograph courtesy Max Orchard, Parks Australia

Crab Bridges and Tunnels

Christmas Island

On Australia’s Christmas Island, around 50 million red crabs make their way out of the rain forest each year during the migration season in mid-October, marching slowly across the island’s roads to reach their breeding ground near the sea. But heavy traffic crushes an estimated 500,000 adult and young crabs every year. Since 1995, the government has built bridges and installed nearly 40 tunnels, in addition to closing certain parts of the road, to reduce the alarming death toll, according to Parks Australia. Seven and a half miles (12 kilometers) of aluminum wall line the edge of the roads to funnel the crabs into the tunnels and fenced overpasses during their migration. Not only have they saved thousands of crabs each year, but the overpasses have also become a popular tourist attraction. (Watch millions of red crab babies hatch and move inland.)



Elephants using an underpass
Elephants exit Africa’s first dedicated elephant underpass near Mt. Kenya. Photograph by Jason Straziuso, AP

Elephant Underpass


As part of the larger effort to restore the historic migration route used by African elephants in northern Kenya, conservationists built an underpass beneath the busy Nanyuki-Meru road in 2010 so the large mammals could safely move between isolated areas without angering locals by walking through the fences and destroying their crops. The project, the first of its kind in Kenya, has been successful. Hundreds of elephants have been spotted walking through it, chief conservation officer Geoffrey Chege of Lewa Wildlife Conservancy told National Geographic in 2012.



Storm Drains


What was originally meant to channel water under the highway has often become a passageway for wildlife throughout the country. Professor J. Edward Gates and his team at theUniversity of Maryland’s Center for Environmental Science placed infrared cameras inside nearly 300 storm drains and tunnels in Maryland; measurements range from 2 feet (3.2 kilometers) to 15 feet (24 kilometers) around. The goal was to learn what species use the drains to cross highways. The cameras revealed that raccoons, Virginia opossums, and larger animals like white-tailed deer have waded through the water to get to the other side of the road. Gates believed that many of the animals may have discovered the culverts while looking for food.



A wildlife crossing over a highway.
A wildlife crossing over Highway A50. Photograph from Photontrappist/Alamy

Highway A50


Among the most impressive and visually stunning overpasses made just for animals is the one built over Highway A50—a lush green bridge covered with vegetation to help endangered wildlife like the badger, deer, and wild boar cross the road from one part of its habitat to another. The bridge is one of 600 wildlife crossings in the Netherlands, home to the world’s largest overpass, which runs half a mile (800 meters) across a highway, railroad, and golf course.



Dead salamander on road
A dead salamander on a road (file image). Photograph by Roger Eritja, Alamy

Salamander Tunnels


In 1988 in Amherst, Massachusetts, 50 people came out in the rain to watch the opening of a salamander tunnel. One salamander showed up, ignored the tunnel, and headed straight for the road. Today, that same tunnel, built under Henry Street, has become an important passageway in the spring, when spotted salamanders make their annual trek across the road to mate in the vernal pools on the other side. Across the country, conservation groups in Santa Rosa, California, employed the same method to give the endangered tiger salamander a safe path to its breeding grounds.



  • Bill Cornelius

    “2 feet (3.2 kilometers) to 15 feet (24 kilometers)” is somewhat incorrect.

  • Adaora

    Atleast now,no ‘visitor’ can say that we Earthlings aren’t doing our best to preserve our environment……

  • Rick

    This kind of construction may trigger a more vast effort to save the world’s wildlife, for the generations of mankind yet to come, to enjoy! This makes me very happy!!! In Genesis, God gave mankind dominion over the animals, and I’m quite certain this is part of what we’re supposed to do as caretakers of the animals.

  • Jasmin Tremblay

    Special tunnels works for the moose in the Laurentides Park in Quebec!

  • Thomas Hafen

    Storm Drains Maryland”
    “… nearly 300 storm drains and tunnels in Maryland; measurements range from 2 feet (3.2 kilometers) to 15 feet (24 kilometers) around. …”

    If this was in Texas I could almost believe that they have storm drains the size of 3.2 kilometer (2 miles) to 24 km (15 miles) round!
    It is mind boggling that a professional writer would be so careless not to have a publication proofread when making such hasty conversions and that in today’s time and age someone would not realize how way off such measurements are before making it public.

    2 ft is about 60 centimeter and 15 ft comes to 4.58 m
    It is high time that such a complicated and archaic measurement system, that is still in wide use here in the US, is abolished. Especially considering that even England, the country where it originated from, abolished it decades ago. All industries where it really counts (aerospace, electronics, automotive, military to name a few) have long made the switch for simplicity and safety reasons.
    This puts the US on a very small and not quite reputable list of countries still not metric:
    United States of America, Burma (Myanmar) and Liberia

    When will the rest of industries and the general population finally follow suit and when doing so, please stick to symbols and abbreviations commonly used worldwide already (e.g. 2 grams is 2g as opposed to 2gm or 2gms) ?

  • laurence

    @ Rick, sorry I can’t cope with religious nutts..1) God did not give dominion over the animals,the black death nearly killed mankind off, Go eyeball to eyeball with a shark and tell them you’ve got ‘dominion over them’ 2) remove your tinted glasses – this is saving a PERCENTAGE of creatures slaughtered for nothing.

  • John Paul

    i think there should be wildlife awareness programmes held at school levels
    in my country people are always looking a way for food and survival just paving ways for there basic of human instincts, but in that way they almost have forgot their instinct more ancient, that is a concern and kindness for our environment
    at school levels an initiative has been taken but it is of no use
    what i mean if you wanna make someone to be concerned about somthing in india at a primary level then examinations and special drives have to be held
    hail natgeo

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