Black Widows must despise Clint Sergi. While working on his Ph.D. In biology at the University of Wisconsin-Milwaukee, Sergi spent his time designing little challenges for spiders, which often involved rewarding them with delicious dead crickets or confusing them into stealing the crickets. “The big question that drove the work was just what’s going on in the minds of animals,” he says.
Biologists already know that the brain of spiders is not like the human brain. Their sensory world is adapted to living in webs and dark corners. “Humans are very visual animals,” says Sergi. “These web-building spiders have almost no vision. They do have eyes, but they’re mostly good at detecting light and motion. Instead, he says, a black widow’s perception comes mainly from vibrations, a bit like hearing. “Their legs are a bit like ears that pick up vibrations through the canvas.”
And in terms of cognition, biologists know that these spiders remember when they have caught prey. Some scientists, including Sergi, believe they even form mental representations of their webs. Yet, not much is known about how detailed their memories are or how past events affect their future decisions. So Sergi and his advisor, spider cognition expert Rafa Rodríguez, set out to test the Black Widow’s memory. As you might guess, Sergi would offer dead crickets to the spiders and then steal them.
The result, they wrote in the journal Ethology, shows that black widow spiders have better memories than previously known. When their prey is removed, the spiders repeatedly search for it in the right place. In some cases, they seem to remember the size of the prey, looking more for the larger stolen snacks. “They don’t just react to a particular stimulus using set behavioral patterns,” Sergi explains. “They have the ability to make decisions.”
This work serves as a reminder that complex cognitive computations are prevalent in the animal kingdom, that internal navigational systems appear in large and tiny brains, including those that depend on vastly different sensory inputs. “This shows that arthropods are able to encode 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 sophisticated than we think.”
Sergi’s findings add to growing evidence that insects and spiders form – and act on – detailed memories, similar to humans, but with very different mechanisms. We orient ourselves with “place cells” in the hippocampus, which arthropods lack. Yet, says Gordus, “they have brain regions 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. The critical input to this brain comes from thousands of sensors located 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 widow spiders can tune into different frequencies by adjusting their posture.) Spiders are so well wired to sense vibrations that there is even some debate over whether the spider web is part of her. brain.
Compared to humanity’s giant mass of gray matter, this might seem like a radically different computer for processing memories. But for Sergi, the appearance of an animal’s brain matters less than the behaviors it produces. For example, birds, as a biological class, have a common brain structure. Yet some excel at cognitive tasks that others do not. Crows count and use zero. Cockatoos solve logic puzzles. Blue jays hide food in summer and fall, then remember where to find it in 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 excellent at this. “They have a standard mammalian brain, but they’re much better than humans at remembering where they stuck things,” Sergi explains. “But you wouldn’t necessarily notice that just looking at brain anatomy or looking at what they’re doing in an MRI.”
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