For over four decades, more than 500 bat fossils have been recovered from the Messel Pit UNESCO World Heritage Site in Germany, providing a fascinating glimpse into the past. These fossils, dating back to the Eocene, about 47 million years ago, are primarily from the bat species Palaeochiropteryx tupaiodon, a small bat species that shares many similarities with modern-day bats. The site, known for its exceptional preservation of animal fossils, has raised intriguing questions about the circumstances surrounding the high number of bat remains. A new study, published in Palaeobiodiversity and Palaeoenvironments, investigates whether the large number of fossils indicates an unusually high mortality rate of bats in Lake Messel and what may have caused their deaths.
Researchers, including PD Dr. Krister T. Smith and Dr. Renate Rabenstein from the Senckenberg Research Institute, and Prof. Dr. Joy O’Keefe from the University of Illinois Urbana-Champaign, explore two primary hypotheses to explain the bats’ mass deaths. One theory suggests that the bats may have succumbed to toxic gases, such as carbon dioxide, that accumulated above the lake, causing the bats to lose consciousness and drown. The other hypothesis proposes that cyanobacterial toxins in the lake’s surface water may have poisoned the bats after they drank from the lake. Both hypotheses imply that the mortality rate was much higher than that which would result from typical accidental drownings.
To test these hypotheses, the researchers posed several key questions: What is the natural mortality rate for bats in small bodies of water? How likely is it that drowned bats would be preserved as complete skeletons? And, is the number of bat skeletons at Messel higher than what would be expected based on these natural mortality rates?
One of the researchers’ most novel approaches involved surveying private swimming pool owners in the United States. Given that bats are known to seek out water sources like swimming pools, particularly during dry conditions, the researchers developed an online survey to collect data on bat drownings in pools. With 496 responses, they discovered that bats regularly drown in pools, with the highest recorded number being 14 bats per year. While this number is lower than the number of fossils found at Messel, the researchers suggest that the conditions at Lake Messel were different. The lake was much larger, had more extreme weather due to the Eocene greenhouse climate, and was a year-round water source, which may have led to even higher mortality rates for bats visiting the lake to drink.
In addition to the survey, the researchers conducted experiments with recently deceased bats from the Frankfurt Zoo to simulate drowning and sinking in water. By studying how quickly bat carcasses sank and how many could be preserved as skeletons, they aimed to better understand the preservation process at Messel. They found that bat carcasses with water-filled lungs sank more quickly than those with air-filled lungs. Additionally, animals that initially sank in shallow waters would float back to the surface due to inflation and eventually sink again after disintegration. These findings indicate that the lake at Messel must have been deep enough to allow the preservation of complete skeletons rather than just disarticulated remains.
When the researchers combined their results, they concluded that the annual bat mortality rate at Messel was likely comparable to the mortality rates observed in modern swimming pools. Their analysis did not support the idea of a “death trap” scenario where an unusually high number of bats perished due to environmental factors like toxic gases or toxins in the water. Instead, they suggest that the number of bat fossils found at Messel reflects a natural mortality rate that would be expected in a body of water where bats occasionally drown.
The study’s findings challenge the idea that Lake Messel was an unusually hazardous environment for bats and other animals. Rather, the researchers propose that the large number of fossils can be attributed to the lake’s favorable conditions for fossil preservation, such as its depth and the lack of significant disturbance to the carcasses. The exceptional preservation of the bats at Messel provides valuable insights into the ancient environment and the behavior of bats during the Eocene period, while also helping to refine our understanding of the natural mortality rates of these flying mammals.
Source: Senckenberg Research Institute and Natural History Museum