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The Bold and the Beautiful

The Bold and the Beautiful

By Wouter van Dongen

The discovery that a gene partly determines which swans are bold and which are wary of people could assist captive breeding programs in cities.

A common pastime for many people in parks is to feed native wildlife such as ducks, swans and other birds. These birds often come into very close contact with humans and will even sometimes snatch the food out of our hands. However, if you spot the same species of birds farther away from the cities, such as in lakes where fewer people occur, the birds are often much more wary and won’t let people approach closely at all. What causes this stark difference in wildlife fear of humans?

One explanation for the greater tameness of city animals is a process known as habituation. This involves an animal repeatedly experiencing a harmless stimulus and eventually learning to stop responding to that stimulus. This could be useful, as persistently responding to a harmless stimulus could waste a lot of the animal’s time and energy. Thus city birds may learn that the constant stream of people walking by poses no threat and therefore cease to respond to approaching humans.

Habituation also occurs in humans. When people move into a new house on a busy road, the noise may be unbearable at first. However, they eventually become used to the sound and may no longer consciously detect it.

Habitation may be one explanation for the tameness of our city swans, but we were interested in exploring a different idea: that wariness of humans differs among individual swans, and those that can’t tolerate humans don’t settle in our cities.

Past research on other animals has indeed revealed that individual animals differ consistently in certain behaviours, such as fear, aggressiveness or novelty-seeking. Therefore, similar to individual personalities in humans, some swans may be more tolerant of potentially-threatening stimuli such as people while others are much more wary, avoiding such stimuli wherever possible. If this turned out to be the case, we guessed that these individual differences may partly be under genetic control and that swans hatched with a “wary” personality are more likely to settle in areas where fewer people occur.

Our first task in finding out whether fearful swans flee our cities was to test the idea that city swans are indeed tamer than their country-dwelling counterparts. We therefore measured wariness towards humans among swans at a city lake in Melbourne, and another at a sewage treatment plant where there are few people.

We measured wariness using a metric known as the flight initiation distance: the distance at which an animal escapes an approaching threat. We found that city swans are indeed less wary of humans, typically waddling away from our approaches when we came to within 13 metres. In contrast, the country swans could only tolerate us when we stayed an average of 96 metres away.

This difference could either arise due to habituation of the swans at the city lake or a preference of wary swans for quieter wetlands. If the latter is true, we’d expect swan wariness to be at least partly under genetic control, as occurs in both humans and other animals.

We therefore asked whether variation in the wariness of swans was related to individual differences in their genes. A likely candidate gene was DRD4, which synthesises a neural receptor for dopamine, a neurotransmitter involved in a wide array of behaviours including fear, novelty-seeking and addiction. Previous research has shown that a certain variant of the DRD4 gene is related to the tendency of some people to be extroverted and the need to constantly seek new experiences. Research has shown that this gene is involved in influencing diverse processes in animals as well, such as fear in dogs and exploration of new environments in birds.

The DRD4 gene was therefore an obvious first place for us to look. During our research we had regularly captured the swans at both sites, which meant that we were able to take small samples of blood containing the DNA of the swans. We could therefore run some genetic analyses to isolate the DRD4 gene in each swan and read its DNA sequence.

We found that the vast majority of swans possessed one particular variant of the DRD4 gene. Swans with this variant had much shorter flight initiation distances (8 metres) than those with rarer variants (14 metres). Thus swans with the common DRD4 variant were more tolerant of humans than other swans. This suggested that the wariness of swans towards humans is indeed at least partly under genetic control.

Although these results were exciting, the icing on the cake was our discovery that swans carrying the wary variant were much more common at the sewage treatment plant. This supports our contention that wary swans are preferentially settling in quieter areas.

So a possible scenario may run like this. Two swans hatch at a city lake, one with the bold DRD4 variant and the other with the wary variant. It would pay the bold swan to stay where it is: city birds are often fed by humans, so food may be abundant here. The survival and breeding success of birds is also often greater in cities due to the lower number of predators found there.

However, the wary swan may rapidly become stressed if it remains within the city. It therefore flies off to find a quieter area to live in. The observed difference in wariness between city and country swans may therefore arise.

But things are never simple. Our comparison of flight initiation distances between the city and country sites showed a difference of 83 metres, yet swans with the wary DRD4 variant escaped our approaches only 6 metres sooner than more tolerant swans. A lot of variation in wariness therefore remains unaccounted for.

One likely explanation is that genes rarely function alone, and multiple genes are likely to influence a swan’s wariness towards humans. In addition, habituation is probably also playing a large role in reducing the wariness of city swans to people.

Can this information aid in the conservation of the black swan and other species? It probably can. Our research suggests that some individual birds are better able to cope with high levels of human disturbance, and that this is partly under genetic control.

This information could be very useful for captive breeding programs. Captive-bred individuals earmarked for release in areas close to higher densities of humans could be selected based on their genetic makeup and ability to survive in such environments. This is beneficial from both a conservation and animal welfare perspective.

So next time you feed the birds at your local park, take some time to consider their individual personalities and the journey they may have been through to arrive at your park.

Wouter van Dongen is a research scientist at Victoria University in Melbourne.