Except in a very few cases, spiders rank in the lower echelons of the food chain, eating insects and themselves being eaten by birds, mammals and reptiles of various sorts, right?
Actually not, says a new study in PLOS One. After reviewing 100 years of reports about bat-catching spiders, scientists have discovered that bat-eating spiders are almost everywhere in the world. The only place you don’t have to worry about seeing an arachnid constructing a web to snare a bat or eating one is Antarctica.
(If you have even a mild case of arachnophobia, I recommend you stop reading!)
About one-fifth of species of mammals are bats, making them among the most successful groups of all mammals, says Charles Choi on Yahoo. All told, there are some 1,200 species of bats. The winged mammals have few natural enemies besides hawks, owl and snakes — and, apparently, spiders, in a case of the backboneless (i.e., invertebrates) eating the vertebrates.
Hunting spiders, including tarantulas in Peru, have been seen eating small bats in Peru’s and Ecuador’s tropical rainforests. In northeastern Brazil, a large tarantula was actually observed eating a bat on the floor of a forest. It’s not clear how the bat was caught there and if it was already dead when the spider started its meal but there have been other instances of spiders actually hunting and killing bats in India, in Kerala and in Kolkata (where a very appropriately named hunstman spider was seen capturing and killing a small bat in a shed).
In most (19) of the cases of spiders eating bats, the latter were caught via webs. Indeed, in Costa Rica, spiders had actually made their webs directly outside of or very near to buildings where bat colonies dwelt. Some giant tropical orb-weaving spiders have a legspan of 4 to 6 inches and weight up to 7 grams (about 0.02 of a pound) and have been seen catching bats in webs that are up to five feet wide. In some cases, webs are woven by groups of female spiders.
The bats do not, it seems, just let themselves become prey for spiders. Some species known to eat bats have been missing legs, possibly the result of what the scientists call “aggressive encounters between spiders and bats trying to defend themselves after being entangled in spider webs.”
90 percent of species of bat-catching spiders live in the warmer parts of the world, including Central and South America, eastern and southeastern Asia, Australia and Papua New Guinea. Microbats (including some who are the smallest bat species on earth) and bat pups are most commonly caught by spiders. The researchers note that “it is plausible that only the lightest individuals of any given bat species get entangled in spider webs” and bats who are captured are usually juveniles or subadults.
Nonetheless, scientists also suggest that, in some cases, bats get caught accidentally in webs. In particular, bats who echolocate at lower frequencies (and who fly fast) may simply not detect the webs that they get stuck in.
So How Likely Is It You’ll Encounter a Bat-Eating Spider?
Though there have certainly been more instances of spiders eating bats than we might have thought, the scientists conclude their study by writing that “the examples of bat-eating orb-weavers reported in this paper are consistent with the ‘rare, large prey’ hypothesis” — that, given the relative size of bats to spiders, and the physical effort they would need to exert, regular eating of small flying mammals by spiders is still unlikely.
The clear advantage for spiders catching bats is, as the researchers note, something of the same reason that we humans seek out ways to acquire a large amount of food at a time: “The catch of a ~2 g bat yields a[n orb-wearing] Nephila pilipes a potential prey biomass that is about 10 times the average daily prey catch.” Mega-webs do have their purposes.
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