Death on the road

Bill Bateman & Lauren Gilson.  Perhaps the most fundamental impact we can have on wildlife is killing it.  We can be very opinionated on the rights and wrongs of killing animals; for instance, hunting is a very emotive issue.  One cause of death of wildlife that we might not think about that much but which has attracted attention from researchers is road traffic (Jaeger et al. 2005, Coffin 2007, Fahrig & Rytwinski 2009).  Road kill is one of the commonest causes of death for urban wildlife (Bateman & Fleming 2012) but is also a major cause of death for animal populations in rural or relatively undeveloped areas.

How a Bennett's wallaby may start the day
How a Bennett’s wallaby may start the day
How a Bennett's wallaby may end the day
How a Bennett’s wallaby may end the day

If life gives you lemons, make lemonade.  What can road kill tell us about evolution , animal populations, and behaviour?

Evolution:  If roads act as a selective pressure, then evolution of populations will be the result.  In a provocative paper entitled ‘Where has all the road kill gone?’ Brown & Bomberger Brown (2013) suggested that a declining number of cliff swallows (Petrochelidon pyrrhonota) recorded as road kill in Nebraska despite no major change in traffic levels or swallow numbers over 30 years suggests that selection has winnowed out the physically slower or behaviourally inept members of that population (road killed birds had longer wings than the population average), resulting in birds that are better able to avoid cars.

Even birds can get it wrong
Even birds can get it wrong
A quenda (Isoodon obesulus) bites the dust
A quenda (Isoodon obesulus) bites the dust

Presence or absence of animals: If you visit Tasmania one thing that strikes you is that there is a lot of road kill.  Pademelons (Thylogale billardieri) and Bennett’s wallabies (Macropus rufogriseus) are common victims, and this probably reflects the healthy populations of these animals in Tasmania.

Sometimes even scarce animals turn up on roads: the Tasmanian devil (Sarcophilus harrisi) population has plummeted across the island, with losses of 95% and more in some areas.  When populations of an animal drop to these minuscule levels they are effectively invisible, and the chance of seeing one becomes vanishingly small.  On a recent trip to Tasmania we found a young male devil dead on the roadside near Lake St Clair National Park.  We reported this to Park officials who were extremely interested as they had not recorded any sightings of devils in the area for many months.  The death of the devil is a tragedy, and it would be better if it had survived its attempted road crossing, but at least it is evidence of some devils hanging on in the area.


The end of a devil
The end of a devil

Behaviour:  red foxes (Vulpes vulpes) in Bristol (Baker et al., 2007) seem to have behaviourally adapted to traffic in their urban environment.  They avoid roads when traffic is heavy and shift their activity peaks to when the roads are quiet later at night, choosing to cross roads only then.  This makes complete biological sense, we know that animals will change their activity to avoid predators and competitors, cars are just another example of that.  It also means that although roads are deadly, they are not ‘efficient killers’: enough foxes have survived a terrifying encounter with a road to learn to avoid it in the future, and perhaps pass this information on through social learning to their offspring.

We increase the ‘inefficiency’ of killer cars by changing our own behaviour: we put up signs to warn drivers of the presence of wildlife in the hope of reducing road kill (and car accidents).

Downtown Perth warns drivers of road hazards
Downtown Perth warns drivers of road hazards.

One aspect of road kill that is of great interest and is likely to a  productive research area is why some species rarely get hit on roads while others seem to actively throw themselves under the wheels of passing cars?

In the case of small macropods it may be a combination of ‘inappropriate’ escape behaviour and the road acting as an ecological trap.   Cleared road verges sprout tender grass, drawing pademelons and wallabies, while the open vista of the road looks like a good escape route when borne down on by a predator car.

Pademelon pondering the appropriateness of its escape behaviour
Pademelon pondering the appropriateness of its escape behaviour
An oblong turtle crushed by a car

Driving on Tasmanian roads at night can be a horrible experience for a conservationist as macropods appear in the lights, look stunned and then head inexorably to their doom.  Other ‘inappropriate’ escape behaviour includes organisms such as European hedgehogs (Erinaceus europaeus) or echidnas (Tachyglossus aculeatus) and gopher tortoises (Gopherus polyphemus) or oblong turtles (Chelodina colliei) responding to approaching cars by rolling up into a spikey ball or withdrawing into their shell – against the tyranny of the car, this is no defence.

Roads then, and the traffic on them, are undoubtedly huge selective pressures on the evolution of animal behaviour.

Move slow, and die
Move slow, and die


Baker, Philip J., Claire V. Dowding, Susie E. Molony, Piran CL White, and Stephen Harris. “Activity patterns of urban red foxes (Vulpes vulpes) reduce the risk of traffic-induced mortality.” Behavioral Ecology 18, no. 4 (2007): 716-724.

Bateman, P. W., and P. A. Fleming. “Big city life: carnivores in urban environments.” Journal of Zoology 287, no. 1 (2012): 1-23.

Brown, Charles R., and Mary Bomberger Brown. “Where has all the road kill gone?.” Current Biology 23, no. 6 (2013): R233-R234.

Coffin, Alisa W. “From roadkill to road ecology: A review of the ecological effects of roads.” Journal of transport Geography 15, no. 5 (2007): 396-406.

Fahrig, Lenore, and Trina Rytwinski. “Effects of roads on animal abundance: an empirical review and synthesis.” Ecology and Society 14, no. 1 (2009): 21.

Jaeger, Jochen AG, Jeff Bowman, Julie Brennan, Lenore Fahrig, Dan Bert, Julie Bouchard, Neil Charbonneau, Karin Frank, Bernd Gruber, and Katharina Tluk von Toschanowitz. “Predicting when animal populations are at risk from roads: an interactive model of road avoidance behavior.” Ecological Modelling185, no. 2 (2005): 329-348.

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