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Urban swans might be born with a daredevil gene that makes them less fearful of humans, compared with their rural colleagues.
A study has found swans living in cities tend to be bolder and it is at least partly determined by a gene called DRD4. Birds with this gene could therefore be better adapted to settling in more populated areas, while more timid（胆怯的）swans would escape to less inhabited regions.
It could help to explain why swans in public parks and on urban rivers have a reputation for being so aggressive towards humans - they are simply not afraid of us.
Biologists from some universities in Australia, tracked and collected blood samples from black swans living in two wetlands near cities. One group of swans lived in a recreational urban park, while the other occupied a waste-water processing site out of town. The two areas were less than 20 miles apart. Later, from 2011 through 2013, the researchers started systematically scaring the swans at both sites. The researchers approached the birds and calculated how close they could get before they flew away. As expected, they noticed that while rural swans started flying away if a human was closer than 119 meters, the scientists could come as close to urban swans as 39 meters before taking off.
Analyses of the blood samples, in fact, showed that 83 per cent of the less cautious swans presented the same genetic DRD4 variation（变异)while rural swans have different genetic makeups resulting in more wariness(谨慎，小心）.
The researchers conclude that swans with the bold variation of the gene might choose to live in urban environments, as they tend to be less bothered by human presence.
