Red foxes have long adapted to life in our big cities. Compared to their rural counterparts, the city foxes show measurable differences – both in behavior and in their genes, as a study now shows: The Berlin city foxes and their counterparts in the surrounding area represent two genetically defined populations. It acts as a separator between the two especially the greater shyness of the land foxes against man-made structures such as roads and railroad tracks.
The red fox has the largest distribution area of all wild predators and occurs in almost all habitats – from dense forests to open landscapes to semi-deserts. Its success is based, among other things, on the fact that it is a rather undemanding omnivore that can adapt flexibly to currently available food sources. The fox does not even spurn fruit. It is therefore hardly surprising that he also feels at home in cities.
City foxes are a separate genetic population
Berlin is an example of a city that had been “conquered” by foxes for a long time: the first red foxes were observed in the urban area as early as the 1950s, and have been distributed throughout the city since the early 1990s. Sophia Kimmig from the Leibniz Institute for Zoo and Wildlife Research (Leibniz-IZW) and her colleagues have now investigated whether and how these Berlin city foxes differ from their counterparts in the Brandenburg area. To do this, they collected tissue samples from 188 urban foxes and 186 rural foxes from the surrounding area and analyzed the genetic material contained therein. In addition, the researchers used a model to determine which factors influence the distribution and exchange of the foxes.
The DNA analyzes showed that the fox populations within Berlin and the surrounding area are genetically different. “Our results confirm that there is a genetic differentiation between urban and rural areas in these animals – and thus the hypothesis of the so-called urban island,” report Kimmig and her colleagues. According to their genetic data, there is no unimpeded influx of land foxes into the city and vice versa. There is still a gene exchange between the two populations, but this is clearly limited. As a result, the urban Foxes’ genetic diversity compared to the rural population is poor, as the researchers found.
City highway as a distribution corridor
But what prevents urban and rural foxes from freely changing their living space? One factor for this are physical barriers such as rivers, large streets or railroad tracks. But in the case of the Berlin Foxes, these obstacles alone are not enough to explain the demarcation of the populations. Because the dividing line between urban and rural foxes did not run along these barriers, but rather pretty precisely along the administrative city limits. “There are only a few physical barriers here,” says Kimmig. “However, along this line, human activity and density of house populations are increasing by leaps and bounds.” The scientists therefore suspect that behavioral adaptations separate the populations from one another. “It doesn’t seem to be freedom of movement, but the fear that keeps rural foxes from entering the city island,” they say.
This is supported by an observation by the Berlin city foxes: contrary to expectations, they do not use parks, gardens and other green corridors to navigate within the city. Instead, they prefer to move along the large railroad tracks and the busy city motorways – although there they are in acute danger of being run over. Despite their risk, these traffic routes have one thing in common: there are many vehicles, but hardly any pedestrians or cyclists. Kimmig and her team therefore suspect that the city foxes are more afraid of people than of their buildings and traffic routes. But that seems to be different with their rural counterparts: “Our results show that land foxes, unlike their urban relatives, do not use artificial structures as distribution corridors,” said the researchers. This probably prevents them from immigrating to the city in large numbers.
Source: Forschungsverbund Berlin e.V .; Technical article: Molecular Ecology, doi: 10.1111 / mec.15345