Blog written by Robyn Grant & Kirsty Shaw. Read the full paper here. Featured image of a common pipistrelle bat copyright Hugh Clark (bats.org.uk)
More than a quarter of all mammal species in the United Kingdom (UK) are bats, they also make up around 20% of all mammal species worldwide. Bats play important roles in many ecosystems, being pest controllers, pollinators and seed dispersers. Urbanisation is one of the most dramatic forms of land-use change and many bats, such as the Common pipistrelle (P. pipistrellus), exploit urban environments for roosting, water, and foraging under artificial lights. However, this also exposes them to urban risks, including collisions with man-made structures and predation from species that are concentrated in urban areas, such as cats.
In the UK, thousands of bats are found and rehabilitated by specialist bat carers every year, many of them for wing tear injuries. Indeed, when we surveyed bat carers around the UK, more than 2000 bats with wing tear injuries were reported to be taken to rescue centres annually. This is not a problem that is specific to the UK, wing tears are commonly found in bat populations worldwide. Tears are considered significant and severe injuries. Despite bat wings having resilient fibre structures and a good blood supply to encourage healing, rehabilitation in captivity can take a long time, which can significantly affect a bat’s health and welfare.
The causes of wing tears are not always clear, but may include collisions, fungal infections and predator attacks. We spoke to many bat carers around the UK, and they believed that the main cause of wing tears were cat attacks; however, positively identifying a cat attack can be difficult. In some cases cats can present a bat to their owners, or the tears “appear” to be made by claws. Previous studies have identified that 20-68% of bats admitted to rescue centres may be as a result of cat attacks; however, there is not yet an objective method to corroborate this.
We applied an objective, forensic DNA analysis method to identify the presence of cat DNA on bat wing tears. We asked bat carers to swab bats with wing tears and send us the swabs. We also asked them to take a photograph of the tear, and tell us what they think caused it. Overall, we collected 72 samples from bat carers, including 40 Common pipistrelles, 18 Soprano pipistrelles, 4 Whiskered bats, 4 brown long-eared bats, 2 Natterer’s bats and one Serotine, as well as 3 swabs from unknown species.
Our results showed that 48 out of 72 (67%) samples had cat DNA present. The presence of cat DNA appeared relatively equally across different bat genders, ages, and species, indicating that all bats may be targeted equally. While our method is a very sensitive technique for the detection of cat DNA, this value of two-thirds could still be an underestimation, due to bats not always being brought to carers, low amounts of DNA being transferred from the cat to the bat during the attack and variability in swabbing and storage techniques.
Bat carers tended to receive bats from a small working area within a 20 mile radius. The same bat carers sent us many samples, especially in Kent and East Dorset, so we also looked at using forensic DNA profiling to identify individual cats within these areas. We did not identify any of the same individuals. Other studies have suggested that there are likely to be “super predator cats” that repeatedly target bat roosts, so identifying any individuals that repeatedly predate on bats within a small area will be a useful thing to do in the future.
Photographs of the tears showed that when cat DNA was present, these tears were often large, running from the internal membrane to the trailing edge, and tended to appear in the more proximal wing sections, close to the body. When bat carers supplied the suspected cause of the tear, they successfully identified a cat attack in all but one sample (in 93% of all cases). This confirms that bat carers are able to make strong, positive identifications of cat attacks.
Free-roaming domestic cats cause a huge number of bird and mammal fatalities and, with the number of cats increasing annually, the effect of cat predation on wildlife is only likely to rise. Unfortunately, this means that the number of injured bats from cat attacks is also likely to increase in the future. As well as causing wing tears, cat attacks can also lead to bacterial diseases in bats. Cats may even receive a viral infection from the bats, such as Nipah virus and European bat lyssaviruses, which can lead to cat mortality. We would suggest that night-time curfews for cats, as well as anti-predator collars, will have beneficial impacts on the local bats as well as other nearby wildlife.
This is the first time that cat attacks on bats have been objectively identified using forensic DNA analysis techniques. Our results suggest that cat predation on bats, at least in the UK, is likely to be much higher than previously estimated. A better understanding of cat and bat interactions has implications for both cat and bat populations, as well as their health and welfare.