A common trope in science fiction: the hero is presented with two people of identical appearance, one a loyal ally and the other a dastardly villain. “Shoot him!” they both cry, “I’m the real one!”
It seems that this scenario is also played out in the natural world. A species of orb spider called Cyclosa mulmeinensis constructs decoy models of itself, in an effort to confuse predators. Ling Tseng and I-Min Tso of Tunghai University in Taiwan decided to investigate this behaviour, which they call a “Darwinian puzzle”, in a paper published in the journal Animal Behaviour.
For some animals, drawing attention to yourself works a defence, as long as you have the muscle to back it up. Wasps, the natural predator of the orb spider, advertise their stinger through their distinctive yellow and black markings. Anything that wants to feed on them know not to get involved, unless they want a nasty jab.
This explanation doesn’t make sense for C. mulmeinensis though, because it has no defences. Why encourage predator attention by offering the prospect of multiple meals if you can’t fight off an attack?
The obvious answer is that whilst having decoys increases the number of attacks on a spider’s web, less of them end up directed towards the actual spider itself. Biologists suspect this to be the case, but until now no-one had verified it in the wild. In July and August of 2003, 2004 and 2005 Tseng and Tso went to Orchid Island, 90km off the coast of Taiwan, to do just that.
C. mulmeinensis are found in abundance on the island. They build their webs hanging vertically, using webbing, eggsacs and remains of their prey to create decoy models of themselves. The image to the left shows decoys made from a) prey remains and b) eggsacs, with the spider marked by an arrow.
The researchers found the size of the decoys to be very closely related to the length of the spider who made them, implying that they were intentionally built to confuse predators. To investigate this further, they also looked at how light is reflected by both the spiders and their decoys. The results were surprisingly similar, so to the limited visual systems of the predatory wasps the decoys appear almost indistinguishable to the spiders.
Both spider and decoys show up in sharp contrast to background vegetation however, making them an easy target for the wasps. The scientists set up video cameras and found that the number of attacks on webs with two or more decoys was twice that of webs with one or no decoy.
This might make decoys sound like a poor strategy as they led to increased attacks, but most of these were made against the decoys and not the spider. Whilst the arachnid might be attracting more attention, it can often redirect it and escape without harm, leaving the unfortunate wasp empty-handed and with an empty stomach.
Tseng, L., & I-Min Tso, . (2009). A risky defence by a spider using conspicuous decoys resembling itself in appearance Animal Behaviour DOI: 10.1016/j.anbehav.2009.05.017