Sorry, booty. Black widows have surprisingly good memories
Black Widows must despise Clint Sergi. While working on his Ph.D. in biology at the University of Wisconsin-Milwaukee, Sergi spent his time devising little challenges for the spiders—which often involved rewarding them with tasty dead crickets or confusing them by stealing crickets. “The big question that motivated the work was just wanting to know what was going on in the minds of animals,” he says.
Biologists already know that spider brains are not like human brains. Their sensory world is adapted to life in nets and dark corners. “Humans are very visual animals,” says Sergi. “These web building spiders have almost no vision. They have eyes, but they are mostly good at sensing light and movement.” Instead, he says, black widow perception comes mostly from vibrations, like hearing. “Their legs are like ears that receive vibrations through the web.”
And in terms of cognition, biologists know that these spiders remember when they caught prey. Some scientists, including Sergius, believe they even form mental representations of their networks. However, not much is known about how detailed their memories are or how past events influence their future decisions. So Sergi and his advisor, expert on spider cognition Rafa Rodríguez, decided to put the black widow’s memory to the test. As you can guess, Sergi would offer the spiders dead crickets and then steal them back.
The result, they he wrote in the journal Ethology, shows that black widows have better memories than previously known. When their prey is taken away, spiders keep looking for it in the right place. In some cases, they seem to remember the size of the prey – asking more for the biggest stolen snacks. “They don’t just respond to a certain stimulus using certain behavior patterns,” says Sergi. “They have the capacity to make decisions.”
This work serves as a reminder that complex cognitive computations are widespread in the animal kingdom—that internal navigational systems appear in both large and miniature brains, including those that depend on vastly different sensory inputs. “It shows that arthropods are capable of encoding complex memories that people often associate with vertebrates,” said Andrew Gordus, a behavioral neuroscientist at Johns Hopkins University who was not involved in the work. “Invertebrates are much more sophisticated than we give them credit for.”
Sergio’s results add to growing evidence that insects and spiders form—and act on—detailed memories, much like humans do, but with very different machinery. We orient with “place cells” in the hippocampus, which arthropods lack. Yet, says Gordus, “they have regions of the brain that have evolved to perform the same function.”
Your central nervous system contains a spinal cord and a 3-pound brain. Spiders have two clusters of neurons called ganglia: one above the esophagus, one below it. Critical input to this brain comes from thousands of sensors along the spider’s exoskeleton called slit sensilla. Each looks like a tiny crack, which deforms as vibrations pass through the spider’s body. (Some evidence suggests that widows can tune into different frequencies adjusting their posture.) Spiders are so well-wired to sense vibrations that it’s even debated is the spider’s web part of his brain.
Compared to humanity’s vast lump of gray matter, this might seem like a radically different memory-processing computer. But to Sergio, what the animal brain looks like is less important than the behavior it produces. For example, birds, as a biological class, share a common brain structure. However, some excel at cognitive tasks that others do not. Crows count and use zero. Cockatoos solve logic puzzles. Blue jays hide food in the summer and fall, then remember where to find it in the winter. Even among mammals, another class with similarities in brain structure, some animals are better than others at locating hidden food. Squirrels, of course, are great at this. “They have a standard mammalian brain, but they’re much better even than humans at remembering where they stuck things,” says Sergi. “But you wouldn’t necessarily get that just by looking at the anatomy of the brain or looking at what they’re doing on an MRI.”
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