Frogs are small animals that hop or walk along the ground. Roads are long stretches of gravel, concrete or bitumen that allow vehicles like cars, trucks and motorbikes to go from one place to another. These vehicles are often large and heavy, and travel very fast – much faster than frogs can hop or walk. This means that it is very difficult for a frog to cross a busy road safely, and many thousands of frogs (maybe even millions) are run over and killed on roads around the world every year.
Why did the frog (try to) cross the road?
So why do frogs try to cross roads? To get to the other side, of course! Frogs are busy animals and often have places to go. Frogs that breed in ponds may spend part of their time at a pond and part of their time in another kind of habitat. And at certain times of the year – such as the start and end of the breeding season – they need to move between the two. This type of movement is known as migration. Young frogs (juveniles) may also leave the pond where they were born and move to a new one. This type of movement is called dispersal.
Roads are a problem for frogs because they can turn a regular journey into a very dangerous one. A number of years ago, researchers in Denmark measured how quickly amphibians (frogs and newts) moved when they crossed a road (Hels & Buchwald 2001). The fastest species in their study was the moor frog (Rana arvalis), which moved at 2 metres per minute. The slowest was the common newt (Triturus vulgaris), which moved at only 0.5 metres per minute. On Danish motorways, cars and other vehicles can travel at 130 kilometres per hour, which equals 2,167 metres per minute. That is more than 1,000 times faster than moor frogs can hop, and more than 4,000 times faster than common newts can walk!
Road kill
Tove Hels and Erik Buchwald used this information to work out the chance of a frog or newt getting safely across roads with different amounts of traffic. The news wasn’t very good. The moor frogs had about a 60% chance of safely crossing a road carrying 100 vehicles per hour (2,400 per day). This means that for every 100 frogs trying to cross the road, 60 would make it and 40 would be run over. The slower-moving newts only had about a 10% chance of crossing safely, meaning that 90 out of every 100 newts trying to cross a road with this much traffic would be run over. On busy roads with more than 15,000 vehicles per day, the speed of the different amphibians didn’t make much difference to their chance of a safe crossing – no matter how quickly they walked or hopped, they were almost certain to be run over.
What can we do?
In Europe and North America, where lots of frogs tend to move at once, tunnels under roads have been quite useful in reducing road kill. Fences along each side of the road direct the frogs to the tunnels, where they cross underneath the road before going on their way. At this stage, we don’t know whether these kinds of tunnels work in other places such as Australia, where frog movements are more spread out over time.
Some conservation groups (such as Froglife in the UK) help frogs and other amphibians to cross roads – this is known as patrolling.
Otherwise, if you are driving at night – particularly if the weather is warm and wet – slow down and watch out for small animals crossing!
Unfortunately roads are the main causes of amphibian mortality, most of which goes unnoticed.