Black widows must despise Clint Sergi. While working on his PhD in biology at the University of Wisconsin-Milwaukee, Sergi spent his time designing little challenges for spiders—which often involved rewarding them with tasty dead crickets, or confounding them by stealing the crickets away. “The big question that motivated the work was just wanting to know what is going on inside the minds of animals,” he says.
Biologists already know spider brains aren’t like human brains. Their sensory world is geared for life in webs and dark corners. “Humans are very visual animals,” says Sergi. “These web-building spiders have almost no vision. They have eyes, but they're mostly good for sensing light and motion.” Instead, he says, a black widow’s perception comes mainly from vibrations, kind of like hearing. “Their legs are sort of 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 that 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 adviser, spider cognition expert Rafa Rodríguez, decided to put black widow memory to the test. As you might guess, Sergi would offer spiders dead crickets and then steal them back.
The result, they wrote in the journal Ethology, shows that black widows have better memories than previously known. When their prey is spirited away, the spiders search for it repeatedly in the right place. In some cases, they appear to recall the prey’s size—searching more for the biggest stolen snacks. “They're not just reacting to a particular stimulus using set patterns of behavior,” 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 navigation systems turn up in both big and minuscule brains, including ones that depend on vastly different sensory inputs. “It shows that arthropods are capable of encoding complex memories that people oftentimes associate with vertebrates,” says Andrew Gordus, a behavioral neuroscientist with Johns Hopkins University who was not involved in the work. “Invertebrates are a lot more sophisticated than we give them credit for.”
Sergi’s results add to mounting evidence that insects and spiders form—and act on—detailed memories, similarly to the way humans do, but with very different machinery. We orient ourselves with “place cells” in the hippocampus, which arthropods lack. Yet, Gordus says, “they have brain regions that 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. This brain’s critical input comes from thousands of sensors along the spider’s exoskeleton called slit sensilla. Each looks like a tiny crack, which deforms as vibrations sweep through the spider’s body. (Some evidence suggests that widows can tune into different frequencies by adjusting their posture.) Spiders are so well-wired to sense vibrations that there is even a debate about whether the spiderweb is part of its brain.
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Compared with humanity’s giant lump of gray matter, this might seem like a radically different computer for processing memories. But to Sergi, what an animal’s brain looks like 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 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 stashed food. Squirrels, of course, are great at it. “They have a standard mammal brain, but they're way better than even humans at remembering where they've stuck things,” says Sergi. “But you wouldn't necessarily pick up on that from just looking at the brain anatomy or watching what they do in an MRI.”
Some spider experts think that arthropods have been underestimated thanks to bias towards big-brained animals. “My background and psychology had initially made me think that only animals with big brains could do anything of interest,” says Fiona Cross, a spider cognition expert from University of Canterbury in Christchurch, New Zealand, who was not involved in the research. “For the longest time, the very suggestion that you could consider spider cognition would just be seen as a joke.”
Yet last year, Sergi published evidence that black widows are capable of path integration, which means that a roaming individual can remember the distance and direction to their retreat, a corner of the web where they rest and eat. He found that they can move around the web without retracing their steps, and even take shortcuts. This time, based on Rodríguez’s previous evidence from banana spiders, Sergi wanted to see if his black widows could search the web for stolen prey—a sign that they can change their behavior when prompted by a memory, rather than just in immediate reaction to an event.
His team’s experiment began with empty plastic boxes, each about a foot wide and deep and 4 inches tall. Sergi would let a black widow build its web inside for one week—“probably a little overkill, but also to make sure that they're hungry and motivated to attack crickets,” he says. In arachnology parlance, each web has two main sections: an upper sheet, which looks like dense net of silk, and a forest of “gumfooted” lines that connect the sheet to a base, like a windowsill or a branch. Gumfooted lines nab crawly creatures like beetles or caterpillars, and sheets catch creatures flying by.
Once the web was ready, Sergi would place a dead cricket into either the sheet or gumfooted lines. Black widows sense that they’ve snagged a meal based on motion and tension in their lines. They approach and touch the prey, then quickly flick out sticky silk and begin wrapping it to immobilize it. Under normal circumstances, the spiders would lug their prey back on a line of silk to their retreat. (“Think of a rock climber's chalk bag, suspended from their waist by a short cord,” Sergi says.) After that, the widows feast: “They’ll suck the juices out of the exoskeleton, then they'll chuck the exoskeleton back out.”
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But this time, Sergi stole the feast before they got the chance. He’d snip that line of silk with scissors and yank the cricket back with forceps.
As the black widows went in search of their purloined prey, Sergi’s team would count how many bouts of searching each spider performed. “Each new bout of searching is a decision by the spider to continue searching,” he says.
From these observations, the team made two conclusions: The spiders searched the part of the web where the cricket had been—the sheet or the lines—which indicated a memory of prey location. And when Sergi stole prey from the gumfooted lines, the spiders made more searches for prey that was especially large relative to themselves. To Sergi, it’s an indicator that the spiders are more responsive to this land-dwelling prey, which is often a more reliable meal.
Gordus says the evidence is clear that black widows are searching in an intentional way. "Oftentimes, people think of invertebrates as being very reflexive organisms, that their output is a function of input, whereas we have more complex memories,” he says. But, he continues, it turns out that arthropods “are also capable of performing pretty sophisticated cognitive calculations. Which, if you think about it, isn't too surprising—they navigate the environment, they can find out where they are in space and time. It's a very valuable ability to evolve."
In future work, we may ultimately learn something cool by testing how long the spiders can retain this memory of prey, says Cross. “We need to have an understanding of what brains are capable of doing,” she says. “The spiders that I work with have brains that would fit on a pinhead. And yet, we see this remarkable behavior—that just utterly intrigues me.”