Crafty Peruvian spider builds its own decoy

December 19, 2012 • 11:57 am

Don’t expect deep thoughts today, for I have writing to do!

For some reason I’ve been posting a lot of arachnids lately.  But this one is special: it weaves a web that contains a “fake spider” used as a decoy, so it’s a case of one of my favorite evolutionary phenomena—mimicry. Have a look:

A decoy spider hangs below its much smaller builder, suspected to be a new species in the genus Cyclosa. Photo: Phil Torres.
A decoy spider hangs below its much smaller builder, suspected to be a new species in the genus Cyclosa. Photo: Phil Torres.

That big “spidery” thing in the middle isn’t a real spider, even though it has the requisite eight legs. It’s a dummy, woven (and with stuff added to it) by the much smaller real spider, which you can spot right above the dummy.

A piece by Nadia Drake at Wired Science  explains (although leaves out some of the science):

A spider that builds elaborate, fake spiders and hangs them in its web has been discovered in the Peruvian Amazon.

Believed to be a new species in the genus Cyclosa, the arachnid crafts the larger spider from leaves, debris and dead insects. Though Cyclosa includes other sculpting arachnids, this is the first one observed to build a replica with multiple, spidery legs.

Scientists suspect the fake spiders serve as decoys, part of a defense mechanism meant to confuse or distract predators. “It seems like a really well evolved and very specialized behavior,” said Phil Torres, who described the find in a blog entry written for Rainforest Expeditions. Torres, a biologist and science educator, divides his time between Southern California and Peru, where he’s involved in research and education projects.

“Considering that spiders can already make really impressive geometric designs with their webs, it’s no surprise that they can take that leap to make an impressive design with debris and other things,” he said.

I teach about Cyclosa in my favorite lecture (on mimicry) for my introductory evolutionary biology class, but the species I show makes only a crude, spider-shaped figure in its web.  I’ve never seen anything like this, and will be including it in my future lectures. Drake notes:

Though Cyclosa are known for building decoys, most of the described spiders’ constructions are clumpy, made out of multiple little balls built from egg sacs, debris or prey, rather than something resembling an actual spider. “Known Cyclosa don’t have that spider-with-leg looking thing, which is why we think it’s a new species,” Torres said.

As Drake does note, this dummy (an “extended phenotype” using Dawkins’s argot) gives the real spider protection from predators.  Birds often eat spiders, and do so by striking the web. If there’s a big, juicy-looking dummy there, it’s much more likely to be struck by predators than the real spider, which can then escape being nommed. Ergo, the behavior is adaptive.

Now that’s a hypothesis, of course, and I’m not sure it’s been tested. One way to do that would be simply to monitor webs, seeing how often birds strike the dummy versus the real spider, and compare that to successful strikes of spiders that lack dummies. That would be hard work given the infrequency of predation events, but it’s the only way to buttress the story.  But I suspect the story is real, simply because I can’t think of an alternative (a “Jerry-of-the-gaps” argument!).

Here’s another photo. It’s a remarkable likeness, attesting to the efficacy of natural selection, and I’m sure it would fool most human observers at first glance (and a glance is all that birds get). The fact that the real spider jiggles the dummy adds to the illusion that it’s real.

Photo by Phil Torres
Photo by Phil Torres

A bit more on this amazing case of mimicry:

In September, Torres was leading visitors into a floodplain surrounding Peru’s Tambopata Research Center, located near the western edge of the Amazon. From a distance, they saw what resembled a smallish, dead spider in a web. It looked kind of flaky, like the fungus-covered corpse of an arthropod.

But then the flaky spider started moving.

A closer looked revealed the illusion. Above the 1-inch-long decoy sat a much smaller spider. Striped, and less than a quarter-inch long, the spider was shaking the web. It was unlike anything Torres had ever seen. “It blew my mind,” he said.

So Torres got in touch with arachnologist Linda Rayor of Cornell University who confirmed the find was unusual. “The odds are that this [species] is unidentified,” she said, “and even if it has been named, that this behavior hasn’t previously been reported.” Rayor notes that while more observations are necessary to confirm a new species, decoys with legs — and the web-shaking behavior — aren’t common in known Cyclosa. “That’s really kind of cool,” she said.


h/t: Gregory

31 thoughts on “Crafty Peruvian spider builds its own decoy

  1. Are there any other cases known of organisms constructing decoys? Apart from humans, this is the first time I’ve heard of any organism actually building a false representation of a creature.

    Amazing stuff!

  2. That’s amazing!!!! One alternative explanation is that the dummy scares away some predators. Big spiders do catch birds in their webs sometimes. And big spiders are just scarier than little ones.

    If this were a decoy, I’d expect the real spider would stay away from it. If it is scary, the real spider should stick close by.

    1. If there were no other reasons for the spider to be close to the decoy, those would be fair predictions; however, I can see three advantages to being right next to the fake:
      (1) jiggling to increase verisimilitude of the decoy,
      (2) location of both at the centre of the web, to make the decoy visually obvious while the spider efficiently monitors vibrations in all radial strands, and
      (3) the spider actually being part of the decoy – specifically the head end, which a predator is least likely to attack.

  3. Wow, I’m impressed.

    The spider even has the right number of legs. Does this mean they can count, possibly?

  4. I’m really looking forward to finding out more about this! Web decoration is common among orb weavers, and it seems probable that there is more than one purpose. And a number of spiders shake their webs when they feel threatened, including spiders that belong to branches of the spider evolutionary tree that evolved before the orb weavers. It will be interesting to see whether the main predator of this spider is birds or wasps, because I suspect the two might perceive this decoration differently. But I don’t really know–it will be great to find out more as someone starts observing!

  5. A few other hypotheses:
    – to deter other animals from damaging the web accidentally? (ie. they see the big spider, realize a web is there, and avoid the web)
    – territorial display? To fool other spiders into thinking a larger spider has made the web, so they are detered from attacking or building their own webs too close.
    – sexual display? Similar to bowerbirds, it is a construct to show the spider’s genetic superiority to the opposite sex.

    The thing that bothers me is that usually a spider web is designed to be almost invisible in order to catch prey. By putting a huge spider in it, wouldn’t that scare off potential prey?

  6. As soon as the spider discovers how to power the construct it will strap on the extra exoskeleton and rule the universe!

  7. Michael–

    Maybe you’re an arachnologist and already know this, but those are all current hypotheses in the study of web decoration. My guess is that different hypotheses will turn out to be true for different species. I personally am skeptical that this construction deters predators, especially if this spider’s main predator is wasps, which see things differently from us and birds. But it could be true: we’ll only know when someone runs some controlled experiments. One hypothesis that has been shown as probably true in some Argiope species is that various decorations, which are visible to insects, actually attract prey, and I wonder whether there’s something about this decoration that attracts prey.

    1. Leslie – these arachnid posts are perfect timing! My local bookseller left a message for me today that a copy of your evolution of spider silk book is ready and waiting for me. I’m eager to dig in!

  8. Fascinating. There may be some (admittedly far-fetched) parallels in human creativity, such as gods imagined in the shapes of humans that serve as deterrents and protectors of humanity. Or writer and poets who write in part from a sense that their works will deflect harm away from them personally, like amulets.

  9. So that explains the Sphinx, The Statue of Liberty, The Easter Island Faces and a whole host of other monuments. When the aliens come they’ll attack them and give us all a chance to escape? Kewl!!

  10. Really interesting comments and hypotheses, and happy to see the spider make it onto one of my favorite blogs. As I return to Tambopata next year I’ll be monitoring the spiders to see which of the potential theories seem to be matching up with observations and work an experiment based upon that. Any other thoughts or suggestions are always welcome!

    1. Phil–

      I highly recommend you read my co-author’s book: “Spiderwebs and Silk: Tracing Evolution from Molecules to Genes to Phenotypes,” by Catherine L. Craig (Oxford Univ. Press), and her various papers on experiments concerning web decoration. They were the foundation for this kind of research, and you can get good ideas for experiment design from them. Then if you follow the citation track, you’ll see how other researchers have followed up looking at other hypotheses and other species. If you have any questions, please contact me.
      Congratulations on this exciting discovery!

  11. Greetings,
    This is slightly off topic but I think I may find some answers here. How does one become less /attractive/ to spiders? They just seem to enjoy/want to be around or actually on either my mother, my son, or myself. It doesn’t matter which species of spider. Could be a common brown or black widow or recluse. They all take great strides to be with us. It’s unnerving. Seriously, I do not want to kill them. Just make them stay away. Help?

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