Jon Luly Gurungada, Mount Surround Qld.
The old saying that “dog bites man” is not news whereas “man bites dog” is news, comes to mind when it comes to interactions between spiders and bats. Bats eat spiders. They may be plucked haplessly from their webs (the Golden-Tipped Bat specialises in this) or foraged from the ground, from branches and foliage, and even water surfaces. But sometimes, the tables are turned and a spider eats a bat.
A 2013 review of bats being killed by spiders indicated that the phenomenon is worldwide, though comparatively rare. Although most bats that fall foul of spiders are caught in webs, some are also actively hunted by ground-dwellers, such as huntsman spiders and Old World tarantulas. Most bats caught in spider webs die of dehydration and exhaustion but some are most definitely subjected to arachnid chiropterophagy (if that is really a word), including the unfortunate juvenile eastern horseshoe bat (Rhinolopus megaphyllus) caught and eaten before our very eyes by a huge golden orb spider.
The bat had been spending daylight hours roosting on a rafter of our verandah, and was presumably finding its way in the bat world after leaving parental supervision. It was an active flyer during the evening and all seemed to be well until one morning, when a spider web, built by a golden orb spider across the verandah rafters some days before, was distinctly over loaded with a dead bat and assorted insects. The insects were wrapped in neat silken shrouds, as is the usual practice for golden orb spiders. The bat was not wrapped, beyond the entanglement that trapped it.
On discovery, the spider was actively feeding on the bat, and continued to do so over the course of the day. While spiders famously liquefy prey and sip at their externally digested food, the bat seemed to also be subjected to increasingly severe mastication. By day’s end, the bat was almost gone. The web was re-built overnight, the spider resumed hunting and the bat became spider shit.
According to the review of spider predation on bats, bats caught by spiders tend to be the most common small bats in any given area. The eastern horseshoe bat is common in our yard, but on the face of it, seems a less likely victim than many of the other species present. Spider-eating bats have distinctive echolocation calls that allow them to identify and avoid spider webs whilst extracting the web owner for consumption. These calls are typically broad sweep, closely spaced pulses which can discriminate fine detail of potential targets whist also allowing the bat to navigate cluttered spaces in dense vegetation.
The eastern horseshoe bat uses a different strategy to negotiate its densely cluttered habitat. Calls are constant frequency at around 70-80 kHz and allow the bat to pick out small flying and crawling insects, negotiate obstacles and finely estimate the relative distance between the bat and a target. Their ability to acoustically navigate is comparable to, or better than, other clutter dwellers. I suspect the demise of our bat was a consequence of youthful inexperience – it is hard to imagine an adult eastern horseshoe bat falling for the snare of a spider.
Nyffeler, M. and Knörnschild, M. (2013) Bat Predation by Spiders. PLoS ONE 8(3): e58120. doi:10.1371/journal.pone.0058120