Evolutionary pressures shape the organisms on which they act over time. Predators, disease, climate and a host of other stresses are thrown at species in an endless barrage against which there is only one defense: adaptation. When we think of these evolutionary battlegrounds we tend to conjure up primeval images of predators and prey locked in a never ending battle to gain the upper hand. We picture a gazelle whose agility improves just enough to evade capture by a cheetah, or an owl developing an auditory sense that is just a little more sensitive to the sound of rodents scurrying in the undergrowth. We rarely, if ever, envision birds struggling to gain the upper hand against a piece of glass traveling at 60 miles per hour. Well, apparently they have been. A recent study in Current Biology suggests that some populations of cliff swallows(Petrochelidon pyrrhonota) in Nebraska may be developing shorter wings to outmaneuver traffic near their nesting sites on bridges and overpasses.
A husband and wife team analysed nearly 30 years worth of data on road-killed swallows and found that the numbers dying on the road year after year are decreasing, despite traffic rates remaining steady and swallow numbers increasing. The researchers found that in recent years, road-kill swallows tend to have slightly longer wings than the rest of the population. They also found that the average wing-length for swallows had decreased significantly over the past 30 years.
So why the change? Well, shorter wings lead to an increase in maneuverability as well as allowing the birds to take off at a tighter vertical angle. Imagine a bird on the ground that needs to reach a height of 2 metres to safely avoid an oncoming truck. If the bird leaves the ground at an angle of 30 degrees, it has to travel a total of 3.46 metres at that angle before it reaches a vertical height of 2 metres. If the same bird left the ground at an angle of 60 degrees, it would reach the same height after traveling just 2.31 metres. Clearly the bird that can get out of the way faster is less likely to become a feathery hood ornament and he/she will live long enough to reproduce, giving rise to more short-winged avian daredevils. Over time, the population starts to lean towards shorter wings as the long winged individuals are weeded out.
The researchers make it clear that they cannot completely discount other factors contributing to this observed phenomenon, but traffic avoidance is almost certainly playing a part. It is a fascinating study, and it is truly rare that we can observe such a marked change in an animal’s morphology over such a short time frame. That being said, there are some anecdotal reports of animals such as rattlesnakes adopting rapid changes in behaviour to counter novel anthropocentric threats. There have been suggestions that rattlers in some parts of North America have stopped “sounding off” in areas where the infamous rattlesnake roundups are commonplace. A rattlesnake’s iconic rattle serves as a warning to other animals to stay away, but now it is being used as a tracking beacon by those who are out to capture and kill it. A silent rattlesnake is less likely to be discovered by humans (or snake-eating, feral pigs) and is thus less likely to be killed.
The potential influence of traffic on evolution leaves us with an interesting question. Are other animals adopting strategies to avoid hitching a ride with the grim reaper? How long will it be before the phrase “like a deer in headlights” becomes redundant? Will we ever see a rubber-resistant rabbit? Will the day ever come when one of those cliff swallows pauses, mid-air, just long enough to antagonise the oncoming driver with the time honoured, avian taunt: “Meep Meep!”
Images: Credits as per captions
Ingrid Taylar’s Website
internets_dairy’s Flickr stream