One night in 1941, G.C. Bhattacharya walked in to a cowshed in Calcutta, India and saw a small figure struggling and twisting against one of the walls. It was a small bat fighting its way out from in between two of the bamboo strips that the shed's walls were made of.
As he got closer, Bhattacharya saw that the crevice was not the only thing the bat was struggling with. A large spider was holding the bat by the neck with its mandibles and biting it. The bat gasped and screamed and struggled against its attacker, but the spider would not let go. When Bhattacharya lit a torch to help him see better, the bat shrieked and flapped its wings, freeing itself from the crevice, but not from the spider.
After managing to crawl along the wall a little bit, the bat exhausted itself and stopped moving for around 20 minutes before flapping one wing a few last times and stretching it out, as if reaching out to Bhattacharya for help.
The battle's victor clearly decided, Bhattacharya captured both the bat and the spider in a glass jar and brought them home for closer observation. The next morning, he found the spider resting upside down at the top of the jar, and the bat lying stiff on the bottom with visible injuries to its neck. It had not survived the night.
Bats’ most prominent predators are owls, hawks, and snakes, but a study published earlier this year reveals that spiders are also a formidable enemy, and that plenty of other people have witnessed incidents of chiropterophagy like the one Bhattacharya did.
To see just how common spider predation on bats was, Martin Nyffeler from the University of Basel (Switzerland) and Mirjam Knörnschild from the University of Ulm (Germany), combed through published research, blog posts and Flickr photographs and interviewed scientists who studied spiders and bats and veterinarians who worked at bat hospitals.
Altogether, they were able to gather 52 reports of bats being caught by spiders (whether in webs, or being actively hunted by non-web-building spiders), 29 of which had never been published before. Reports came in from every continent except Antarctica, but more than three-quarters of the cases happened in tropical areas around the Equator, most often in Latin America and Southeast Asia.
In most of these incidents, the spiders were large (10–15 cm legspan, ~1–7 g weight) web builders, mainly from the genus Nephila. These spiders are nocturnal hunters that spin webs up to 1.5 m across, and occasionally get together to build several webs connected to each other. The victims, meanwhile, were overwhelmingly small (10–24 cm wingspan, 3–8 g weight) insect-eaters, mostly from the family Vespertilionidae.
It doesn’t seem like the spiders intended to catch a bat every time. In some of these incidents, the spider completely ignored the bat caught in its web. Other times, the bat was known to be dead already before a spider found and began eating it. In these cases, it appears that bats simply sometimes get stuck in spiderwebs and die of exposure or starvation without being preyed upon, and sometimes spiders simply scavenge the body of dead bats without having killed them. In many cases, though, it was clear that spiders were intentionally killing and eating bats, immobilizing ones that got caught in their webs with a silk wrap, biting them and later consuming them.
Fifty-two spider-on-bat battles doesn’t not seem like a whole lot, especially when you consider that these reports cover a timespan of 100+ years. Indeed, Nyffeler and Knörnschild were actually surprised that that was all they found, given the sheer number of spiderwebs that must stand in the way of bats’ flight paths throughout the tropical parts of the world. They suggest that bats may be very good at avoiding spiderwebs with the help of echolocation. While the silk threads that make up the web are likely too thin to echolocate, the denser decorations and barriers that spiders place on them—plus the spider itself sitting in the center of web—should be large enough for the bats to detect in flight.