In our cities, many bats hunt for insects at night. However, large bats have less success finding food there than on land, a new study shows. This could drive the animals out of the cities in the long term or they might need a different strategy to still get enough to eat there. According to the study, urban bats have already adapted their behavior and hunt alone, while their rural counterparts hunt together.
Some wildlife, including many bats, do relatively well in urban habitats; they adapt and find enough food. However, foraging there poses challenges for other species. “Although many bat species live in urban areas, not all of them are doing equally well,” says senior author Christian Voigt from the Leibniz Institute for Zoo and Wildlife Research (IZW) in Berlin. Due to the dense development, the food supply in the city is rather poor, says Voigt. The bats only find prey in parks, cemeteries and around floodlights. This can be a problem, especially for larger bat species such as the noctule (Nyctalus noctula), which weighs around 30 grams.
A lot of effort, little reward
A research team led by first author Laura Stidsholt from the IZW in Berlin has now examined how the animals deal with this situation. To do this, the scientists repeatedly equipped large noctules in the urban area of Berlin and in a rural area around Rostock with small GPS trackers (24 animals) and sensors (22 animals). The sensors included ultrasonic microphones, accelerometers, and meters for magnetic field, temperature, and barometric pressure. This enabled the researchers to track the movements and aging of the animals as they hunted at night. From this data, Stidsholt and her colleagues analyzed how many animals were traveling in the areas at the same time, how long, how far and how energy efficiently they flew, and how many insects they attacked. Using chewing noises, the researchers estimated how many of the attacks were successful and how large the prey were.
The results showed that the noctules hunted similarly sized prey in both environments, but with different success: in the city they captured significantly fewer insects than in the countryside, both overall and per flight time. For example, bats attacked two insects per minute in the city and six in the countryside, as the movement data showed. In addition, the bats in the city had to fly a good 100 meters higher and significantly longer distances in order to find enough food.
Overall, bats have to use more energy to search for food in the city and still catch less prey than in the countryside, as the researchers report. In addition, the bats have apparently already adapted their behavior to the reduced urban food supply. “In the city, noctules were less social, so they hunted less often with their own species,” reports Stidsholt. “Group hunting in the city was probably unnecessary, as it is difficult for a city bat to determine which green spaces contain prey insects. In rural areas, they need the support of their peers to do this,” says Stidsholt.
Save energy with sleep mode?
In the long term, however, the bats need another strategy to survive in the city. In order to compensate for the overall lower energy intake, larger species such as the noctule bat could specifically lower their body temperature when resting or sleeping and thereby reduce their energy consumption, the researchers suspect. Although they found no evidence of this in their data, since these were recorded during the warm summer months, the situation could be different in colder months, the study says.
It is known from previous studies that bats can generally resort to this strategy when food is scarce. But we also know that the energy saving mode can slow down the growth of animals, including bat offspring. The research team suspects that the animals will therefore not resort to this strategy in the long term and will instead leave the cities. In order to preserve large bat species in the city, habitats with many insects such as large parks are crucial.
Bat as an example of losers of urbanization
According to Stidsholt and her colleagues, the observed situation of large bat species in urban areas is a good example of the negative consequences of urbanization for wildlife. The majority of wild animal species avoid urban landscapes because they find less food there and have higher competition. Species that cannot adapt to the urban environment are thus pushed out. Urbanization is therefore increasingly reducing the habitats of wild animals, leading to a general decline in biodiversity, the researchers report.
Source: Laura Stidsholt (Leibniz Institute for Zoo and Wildlife Research Berlin) et al., Global Change Biology, doi: 10.1111/gcb.17063