At last – the final installment of this intriguing trilogy! The third reason that frogs and roads don’t mix is that the noise of road traffic (cars, trucks, motorcycles and other vehicles) can make it difficult for frogs to be heard by other frogs. Anyone who has listened to a group of frogs calling at a pond or stream can guess that acoustic communication (communication with sound) is very important for this group of animals. Male frogs call to attract females for mating, and also to stake a claim on a certain calling location. The advertisement call of a male frog says two things: 1) “Hey ladies, I’m over here”; and 2) “Back off chaps, this is my space”. Each species of frog has a unique call, so a female frog can recognise males of the correct species in the dark (although occasional mix-ups do occur).
Road-traffic noise can reduce the distance over which a male frog’s call can be heard by females of the same species by up to 90%, with bigger frogs being the biggest losers. This is because bigger frogs have lower-pitched calls which overlap more with the pitch (frequency) of traffic noise than do the calls of smaller frogs. The loss of communication distance in noise is known as masking or acoustic interference. Traffic noise is more than a nuisance for urban frogs – it may be interrupting froggy loving on a grand scale. Road-traffic noise also hinders acoustic communication in birds and insects, while boat-traffic noise interferes with acoustic communication in fish, dolphins and whales. Traffic noise may also make some animals more vulnerable to predation (because they can’t hear their predators coming) and reduce foraging success by animals such as bats that rely on sound to detect their prey. It’s a noisy world out there, and it’s not just humans who are experiencing the effects!
What can we do?
Some species of frogs are calling at a higher pitch in traffic noise, helping to reduce acoustic interference and get back some of the communication distance lost. For example, the southern brown tree frog Litoria ewingii from Australia calls at a higher pitch at noisy urban sites than at quiet rural sites. But this is not enough to overcome the effect of traffic noise completely. In many places around the world, we use sound barriers and other structures to protect people from high levels of traffic noise – perhaps it’s time to consider protecting frogs, birds, bats and other animals from traffic noise too?
Parris, K. M. (2015). Ecological impacts of road noise and options for mitigation. Pp. 151-158 in R. van der Ree, C. Grilo and D. Smith (eds), Ecology of Roads: A Practitioners’ Guide to Impacts and Mitigation (Wiley-Blackwell, Oxford).
Parris, K. M. (2013). Anthropogenic noise constrains acoustic communication in urban-dwelling frogs. Proceedings of Meetings on Acoustics 19: 010055.
Parris, K. M., Velik-Lord, M. and North, J. M. A. (2009). Frogs call at a higher pitch in traffic noise. Ecology and Society 14(1): 25. http://www.ecologyandsociety.org/vol14/iss1/art25/
Aussie frogs try new pickup line; All Things Considered, National Public Radio (USA) (11/10/2009)