what spiders could teach us about the evolution of brains and intelligence
Spiders seem like completely alien creatures. Instead of bones that grow with them, they have exoskeletons they gruesomely molt as they get bigger. They have eight eyes rather than two, although some have six or even four. Instead of eating their prey, they dissolve it in silk cocoons and drink it after injecting it with venom and digestive enzymes. Their eight-legged body plan seldom changed over some 400 million years. And they hunt by either lighting fast jumping or by laying elaborate, hypersensitive traps. So, it’s no wonder what when we need an eldritch, ancient, inhuman evil for a horror movie, we turn to spiders either as inspirations or to fill the role as impossibly supersized versions of themselves.
But could spiders be weird and alien enough to have a cognitive process that extends outside of their bodies and into their webs? That’s a question asked by researchers who watched how spiders would react to having their webs strategically severed or some of their strings quickly tightened. If you guessed that their arachnid subjects were extremely attuned to the changes in tension and vibrations, you’re correct. You may have also guessed that the stimuli also changed how they build future webs, making areas that trigger more vibration, and therefore likely catch more food, larger and denser. (Funny enough, giving spiders coffee or drugs can also alter how their webs are built in bizarre and interesting ways.)
Now then, if spiders are so reliant on their webs to process what’s going on around them, with many web-weaving spiders even suffering from extremely poor eyesight, and the shapes of the webs they create being an accurate barometer of the spiders’ stress levels and basic cognition, would it be fair to say that those webs are part of their consciousness, ask the researchers. To back up their hypothesis, they cited the fact that what we refer to as cognition isn’t limited to their brain. Octopi, cicadas, and even humans have fairly clever reactions to stimuli without involving the brain for reasons that are still being hotly debated. Some posit that it’s an energy saving measure, while others say that brains are just parts of a much larger system.
It’s that second camp that Hilton Japyassú and Kevin Laland, the researchers behind the spider study wanted to advance when trying to label tools used by living organisms to understand and navigate the world extensions of their cognitive processes. But there’s a reason why plenty of researchers find this idea rather problematic. While pop culture based on old timey hunches paints intelligence as something we can accurately measure and define, the reality is a lot more fluid and complicated. When we stop looking at humans as the only intelligent life on Earth and start thinking about how creatures like octopi can be so clever, it’s quickly apparent that not all intelligent creatures evolved and operate the same way.
Brains are likely very useful in coordinating higher order cognitive tasks as we scan the world and its treats and opportunities, and generating a body map, while neurons are useful in reflex level processing and detection, with the two operating in concert. Exactly how they’re laid out, the relative size of all the nodes in a nervous system, and how they interact comes down to the organisms’ evolutionary history and developmental quirks. Through this lens, there’s nothing outside a spider’s body being used to outsource its cognitive function. It’s simply using a tool to help alert its peripheral nervous system to a stimulus, kind of how we’d use a walking stick to make sure the way forward is safe and clear.
In short, this is a philosophical debate more than a scientific one. Yes, spiders can change webs on the fly and adjust how they’re used based on how effective it is at catching prey, just like we can fix our tools when we need to finish a job and modify finished designs for anything we want to build to optimize desired results. But many experts don’t see advanced tool use as cognition, just a manifestation of it. The critics of Japyassú and Laland’s approach say that they’re losing a key distinction between information, which is trivial to collect and act on, and knowledge, the ability to process the information, accumulate it alongside other useful examples, and use it in more strategic ways.
However, all that said, one thing is clear. We’re still a long way away from truly understanding intelligence and the role that neurons and brains play in it. Nature is full of exceptions, blurred lines, and opportunities for long and passionate philosophical debates about how to classify already broad, vague, but nevertheless, very important concepts, and it’s not different when we try to study intelligence and cognition. Just consider that next time you read a story which grades living things by intellect and try not to think too hard about the heavily implied fact we danced around through this entire article: that spiders are actually pretty clever little bastards despite being so small and bizarre.
See: Japyassú, H.F., Laland, K.N. (2017) Extended spider cognition. Anim Cogn 20, 375–395 DOI: 10.1007/s10071-017-1069-7