A couple of years ago, while walking at night down a jungle path in Peru, I stumbled upon a spider that had constructed an elaborate web only a couple of feet off the ground. It sat absolutely still in the very center of the web, with each spindly appendage (and there were many!) casually resting against one of the main spokes of the web.
My husband and I quickly took up positions nearby, and aimed our headlamps at the beautiful hunter. In the few seconds that it took for us to examine the nest, we were enveloped in a mass of small insects diving at our faces. Several were in the advanced stages of exploration of our nasal and auditory canals when a terrible thing happened. My frantic swatting disturbed a beautiful grasshopper from its perch and it took a wild leap forward. Had it been a less flamboyant of a display, perhaps its fate would not have been sealed so tragically. As it was, it landed in that spider’s parlour and a violent shudder rippled across the surface of the web.
Seconds later, the dance began. The spider approached the insect without any hesitation at all – clearly, this event didn’t bear thinking about. It systematically cocooned the still-struggling hopper in a fine web that it delicately, but expeditiously, drew out from the spinnerets attached to its rear. In a matter of minutes, the struggle was deemed pointless and the insect settled down to being slowly liquified by the digestive enzymes injected into it. The spider then did the most remarkable thing – it created a small loop on the end of the ovipositor, or the long appendage or “tail” of the grasshopper and, by hooking one foot into the loop, seemingly effortlessly hoisted the whole insect back to the center of the web.
In all of ten minutes, everything was back to the way we first found it (minus one grasshopper). Of all the kill sequences on BBC documentaries I have watched in my lifetime, I had never seen anything quite so elegantly efficient. No fuss, no mess, just execution of an ingrained plan developed over millenia.
Thus, it was with some trepidation then that I began to read an article recently published in the journal PlosOne, on spiders that eat, of all things, bats.
The Prey – Bats
Bats are mammals of the Order Chiroptera, and they leave all their competitors, such as flying squirrels and colugos, miles behind them when it comes to flight. There are approximately 1240 known bat species, which is a staggering number when you look at other large mammal groups, such as the primates, that have only 425+ species. In fact, only one other group of mammals, the rodents, have more species than the chiroptera (2277 species). All bats are either megabats (fruit-eating flying foxes) or microbats (that echolocate, fun!)
Left: The Fijian monkey-faced bat, a megabat; Right: The greater bull-dog bat, a microbat
Despite their widespread distribution and convenient size, bats are not preyed upon by very many predators possibly due to their adaptation for flight. Owls, hawks and snakes likely give your average bat the most scares, although some other nightmarish species have evolved to prey solely on bats, such as the giant centipedes (Scolopendridae) of Venezuelan caves.
The Hunters – Spiders
Spiders are the seventh most diverse group of organisms, and are found on every continent on the planet save for Antarctica (arachnophobes, please note, there is yet hope). They have fine-tuned hunting into an elegant science, with creative solutions far more complex than the weaving of a sticky web in which to wait for prey. Here are a few of the more creative arachnids that we know of today:
The water spider (Argyroneta aquatica) lives within an under water globe of air trapped by a web from which they capture their prey. Spiders of the family Deinopidae weave small webs, stretch them between their first two pair of legs, and lunge and capture prey that could be up to twice their size. The bolas spiders (genus Mastophora) sit on a single silk trapeze line with a large “bolas” made of sticky silk. They then swing this bolas at passing prey and reel them in. Simple. And stylish.
The Current Study
The authors of the present study, Martin Nyffleler and Mirjam Knörnschild, were inspired by two recent publications on spiders that were observed to prey on a small bat. These candidates, the web-building spider Argiope savignyi, and a tarantula-like spider Poecilotheria rufilata, are so very good at hunting bats that it seems quite likely that this behaviour is more common than we think. So they set out to scour the literature for evidence for such behaviour among other spiders.
And were they right!
Their hard work, that even included contacting bat hospitals for the victims of just such a crime, revealed a startling array of evidence for bat predation by spiders. Most cases occurred in the tropics, and the fact that these bat-eating spiders are found everywhere but in Antarctica has been exclaimed upon by a great many news reports, often as headlines. This is not an indubitably astonishing thing in itself, primarily because it reflects the distribution of spiders in general (and a very large chunk of the rest of the living world to boot!)
If you watched Arachnophobia at a tender age, as I did, the only spider that comes to mind when anyone so much as mentions eight-legged creatures is a) black, b) hairy, c) HUGE, and d) out to kill you. As it turns out, you are right in one regard: only the very large spiders possess chelicera, or pointy mouthparts, that are long enough to penetrate human skin. Thankfully, almost everything else you think you know about spiders is wrong, beginning with the fact that only two hundred of over forty thousand species are known to have venom that can seriously harm you. Nyffeler and Knörnschild did confirm, however, that this archetypical, makes-me-pee-my-pants spider is indeed more involved in bat predation than other smaller models.
Tarantulas, orb-weaving spiders, golden silk orb-weavers and the huntsman spiders are the four, very guilty, culprits of bat murders (that we know of). Interestingly, a lot of the data on neotropical bat predation came from scientists working at my field site, the Los Amigos Biological Field Station, which brings this threat a little closer to home. All I can say is, I’m rather glad I’m not a bat.
Practically anywhere seems to be a bad place to be a microbat, and good looks don’t help either. Plain-faced bats (awww), sheath-tailed bats, horseshoe bats (lousy luck!), Old World and New World leaf-nosed bats have all been, unfortunately, eaten by spiders at some point. In a disturbing number of cases (31%),the identity of the bat species in question remains unknown, which is possibly due to the lack of this sort of skill on the part of the observer, or worse, the partial liquefaction of the bat by the spider before the observers arrived on the scene. Some of the bats that fell prey to spiders were the smallest of the microbats (nanobats?), who were presumably unable to escape from the spider webs once ensconced in them.
The Hunted becomes the Hunter
In a satisfying twist of fate, there also exist bats for whom spiders make up over 75% of prey items. These rock stars, the Golden-tipped Bat and Geffroy’s Myotis, are adept at manoeuvering at low speeds, using high frequency echolocation to locate their prey and avoid the webs. They particularly focus on web-building spiders and have evolved hunting strategies that allow them to glean spiders right out of their webs.
Just when this is beginning to look like some serious payback, data out of Australia and Germany seem to suggest otherwise:
First, these bats when captured in mist nets have evidence of sticky web on their fur. If that was not a dead give away, someone then proceeded to look at pellets of bat poo and found that they were preying on orb-weaving spiders of a disarmingly slight stature (cowards!).
Luckily, these bats that rage against the machine do not co-occur in areas with the massive spiders that could eat them right back. In Europe and North America, that is.
Australia, as Bill Bryson would agree, is an entirely different story. In some tropical areas of that island country, there lurk both the golden-tipped bat AND golden-silk orb-weaving spiders. Coincidental monikers? I think not. Only time (and some intrepid scientists) will tell which will be the true huntsman.
Nyffeler M., Knörnschild M. & Bilde T. (2013). Bat Predation by Spiders, PLoS ONE, 8 (3) e58120. DOI: 10.1371/journal.pone.0058120.s001
More on Spiders
The BBC films net-casting spiders hunting in the wild for the first time
Understand the strength of a spider’s web
Silk from more than a million golden-orb spiders is woven into the most fantastic garments
Spider silk is so coveted that some goat’s have been bred to produce extra protein in their milk that can be spun into silk. How did they do this? Add a spider’s genes to the goat’s genome, of course.