Black Widows should despise Clint Sergey. While working on his Ph.D. At Biology at the University of Wisconsin-Milwaukee, Sergey has spent his time designing little challenges for spiders — which often involve rewarding them with tasty dead crickets or confusing them by stealing the crickets away. “The big question that motivated the work was just the desire to know what was going on inside the minds of the animals,” he says.
Biologists already know that spider brains are not the same as human brains. Their sensory world is oriented to life in networks and dark corners. “Humans are very visual animals,” says Sergey. “These web-building spiders almost lack vision. They have eyes, but they are mostly good for sensing light and movement.” Instead, he says, the perception of the black widow comes primarily from vibrations, such as hearing. “Their legs are kind of like ears picking up vibrations through the web.”
And in terms of perception, biologists know that these spiders remember when they caught prey. Some scientists, including Sergey, believe that they even form mental representations of their networks. However, little is known about how detailed their memories are, or how past events affect their future decisions. So Sergey and his advisor, spider identification expert Rafa Rodriguez, decided to put the black widow’s memory to the test. As you might imagine, Sergey was showing spiders dead cockroaches and then stealing them again.
They wrote in the journal Ethology that the result shows that black widows have better memories than previously known. When their prey is turned away, spiders frequently search for it in the right place. In some cases, they seem to remember the size of the prey – looking more for the biggest stolen snack. “They don’t just react to a specific stimulus using specific patterns of behaviour,” says Sergey. “They have the ability to make decisions.”
This work serves as a reminder that complex cognitive computations are widespread in the animal kingdom – and that internal navigation systems appear in both large and small brains, including those that rely on vastly different sensory inputs. “This shows that arthropods are able to encode the complex memories that people often associate with vertebrates,” says Andrew Gordus, a behavioral neuroscientist at Johns Hopkins University who was not involved in the work. “Invertebrates are much more complex than we take credit for.”
Sergey’s findings add to mounting evidence that insects and spiders form and act upon detailed memories similar to the way humans do, but with very different mechanisms. We orient ourselves with “place cells” in the hippocampus, which arthropods lack. However, Gordes says, “they have brain regions that have evolved to perform the same function.”
Your central nervous system contains the spinal cord and a brain that weighs 3 pounds. Spiders contain two sets of neurons called ganglia: one above the esophagus, and one below. Critical input to this brain comes from the thousands of sensors located along the spider’s exoskeleton called the sensory cleft. Each is like a small crack that deforms when vibrations sweep across the spider’s body. (Some evidence suggests that widows can tune to different frequencies by adjusting their posture.) Spiders are so well wired to sense vibrations that there is debate over whether a spider’s web is part of their brain.
Compared to a human’s gigantic mass of gray matter, this might look like a radically different computer for processing memories. But for Sergey, what the animal’s brain looks like is less important than the behaviors it produces. For example, birds, as a biological class, have a common brain structure. However, some excel at cognitive tasks that others do not. Crows are prepared and used zero. A cockatoo solves logical puzzles. Blue Jays hide food in the summer and fall, then remember where to find them in the winter. Even among mammals, another class that is similar in brain structure, some animals are better than others at locating hidden food. Squirrels, of course, are great at that. “They have a normal mammalian brain, but they are much better than humans at remembering where things got stuck,” says Sergey. “But you wouldn’t necessarily pick that up from just looking at the anatomy of the brain or watching what they’re doing on an MRI.”