Sorry, prey. 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 designing small challenges for spiders, often rewarding them with tasty dead crickets or confusing them by stealing the crickets away. “The big question that motivated the work was just wanting to know what goes on in the minds of animals,” he says.
Biologists already know that spider brains are not like human brains. Their sensory world is attuned to living in webs and dark corners. “Humans are very visual animals,” Sergi says. “These web-building spiders have almost no vision. They have eyes, but they are especially good for perceiving light and movement.” Instead, he says, a black widow’s perception comes primarily from vibrations, a bit like hearing. “Their legs are like ears that pick up the vibrations through the web.”
And in terms of cognition, biologists know that these spiders remember when they’ve caught prey. Some scientists, including Sergi, believe they are even mental representations of their webs. Still, not much is known about how detailed their memories are, or how past events influence their future decisions. So Sergi and his advisor, spider knowledge expert Rafa Rodríguez, decided to put the black widow’s memory to the test. As you might guess, Sergi would offer spiders dead crickets and then steal them back.
The result, she wrote in the journal Ethology, it appears that black widows have better memories than previously known. When their prey is chased away, the spiders repeatedly look for it in the right place. In some cases, they seem to remember the size of the prey – they look more for the largest stolen snacks. “They don’t just respond to a particular stimulus using set patterns of behavior,” Sergi says. “They have the capacity to make decisions.”
This work reminds us that complex cognitive computation is rife in the animal kingdom — that internal navigation systems are emerging in both large and minuscule brains, including systems that rely on vastly different sensory inputs. “It shows that arthropods are able to encode complex memories that humans 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.”
Sergi’s results add to the mounting evidence that insects and spiders form and act upon detailed memories, in the same way humans do, but with very different machines. We orient ourselves with “place cells” in the hippocampus, which arthropods lack. But, Gordus says, “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. This brain’s critical input comes from thousands of sensors along the spider’s exoskeleton, called rift sensilla. Each looks like a small crack, which distorts as vibrations sweep through the spider’s body. (Some evidence suggests that widows can tune into different frequencies by: adjust their attitude.) Spiders are so well wired to perceive vibrations that there is even a discussion about it whether the spider’s web is part of his brain.
Compared to humanity’s giant chunk of gray matter, this may seem like a radically different computer for processing memories. But to Sergi, what an animal’s brain looks like is less important than the behavior it produces. Birds, for example, have a common brain structure as a biological class. Still, some excel at cognitive tasks that others don’t. 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 stored food. Squirrels, of course, are great at it. “They have a standard mammalian brain, but they’re much better than even humans at remembering where they’ve pinned things,” Sergi says. “But you wouldn’t necessarily pick that up just by looking at the brain anatomy or looking at what they’re doing in an MRI.”
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