A quickie by James Kho
Say hello to Portia labiata, a species of jumping spider mostly found in Southeast Asia. Or, don't, because it's an evil genius hunter and why introduce yourself to something that would and could murder you if it were just a little bit bigger? Frequently called an "eight-legged cat" due to the cleverness and ingenuity of its hunts, Portia can improvise hunting techniques and then, by trial and error, remember which are most effective against which kind of prey. And its techniques often demonstrate a fairly high level of intelligence, or at least craftiness—it mostly hunts spiders, which are generally clever hunters, so its intelligence is all the more impressive. It'll take detours around dangerous prey to find the best attack angle, even if it requires hours of work and means the spider loses sight of the prey. Perhaps its scariest technique is the "pluck." All web-weaving spiders use the web as a sort of extension of its senses—it can feel the vibrations of all kinds of possible prey and recognize what might be tasty and what might be dangerous. But Portia is a talented mimic—it can "pluck" a web with near-limitless variance, able to make a sound that can lure a spider out into the open. But what that poor spider thinks is a delicious fly is actually Portia, which pounces.
...I shall return to Portia later in the page but, because this week I'm getting fixated on spiders anyway, let's talk about their intelligence:
Some spiders are so clever their brains extend down into their legs
The smaller the spider, the bigger the brain. Huge brains ensure that even the tiniest spiders can weave webs
(from dailymail, Dec. 2011)
(from dailymail, Dec. 2011)
If you've ever thrown a spider off the premises, then turned round to see your eight-legged foe creep back into the house and thought, 'How did it get back in here?', cease wondering.
Spiders aren't just clever – some have brains so huge they extend down into their legs.
Researchers at the Smithsonian found that in some species, the brain occupies up to 80 per cent of the body – and 25 per cent of their legs.
Spiders need fairly complex brains – not only do they have to manipulate eight limbs, they also have to weave webs, a pretty complex task.
The researchers found that smaller spiders tended to have bigger brains, proportional to their size – even tiny spiders have to weave webs, and those tend to be the ones with brains spreading down into their limbs.
'The smaller the animal, the more it has to invest in its brain, which means even very tiny spiders are able to weave a web and perform other fairly complex behaviours,' said William Wcislo, staff scientist at the Smithsonian Tropical Research Institute in Panama.
'We discovered that the central nervous systems of the smallest spiders fill up almost 80 percent of their total body cavity, including about 25 percent of their legs.'
Some of the tiniest, immature spiderlings even have deformed, bulging bodies. The bulge contains excess brain.
The diameter of the nerve fibers also cannot be made smaller because if they are too thin, the flow of nerve signals is disrupted, and the signals are not transferred properly. One option is to devote more space to the nervous system.
'We suspected that the spiderlings might be mostly brain because there is a general rule for all animals, called Haller's rule, that says that as body size goes down, the proportion of the body taken up by the brain increases,' said Wcislo.
'Human brains only represent about 2-3 percent of our body mass. Some of the tiniest ant brains that we've measured represent about 15 percent of their biomass – and some of these spiders are much smaller.'
The enormous biodiversity of spiders in Panama and Costa Rica made it possible for researchers to measure brain extension in spiders with a huge range of body sizes.
Nephila clavipes, a rainforest giant weighs 400,000 times more than the smallest spiders in the study, nymphs of spiders in the genus Mysmena.
...This much for spidery intelligence, just to keep it brief. But let's return to Portia now:
The tiny arachnids possess an abstract working memory—a capability usually seen in larger animals, a new study says
With brains the size of a sesame seed, jumping spiders may seem like mental lightweights.
But a new study shows that many species plan out intricate detours to reach their prey—smarts usually associated with far bigger creatures.
The arachnids, already well known for their colors and elaborate mating rituals, have sharp vision and an impressive awareness of three-dimensional space. (See article "Surprise: Jumping Spiders Can See More Colors Than You Can.")
Jumping spiders of the subfamily Spartaeinae (spar-TAY-in-ay) are particularly ambitious—they eat other spiders. Researchers suspect that preying on other predators requires extra intelligence and cunning.
In the 1980s and 1990s, Robert Jackson of New Zealand’s University of Canterbury demonstrated that Portia fimbriata, a member of this spider-snacking subfamily, methodically plans winding detours to sneak up on prey spiders. Portia can even find hidden prey, suggesting that the predator can visualize its prey's location and a path to get there.
OBSTACLE COURSE
Demonstrating in the lab that other spider species also planned detours, however, proved challenging: Other spiders ignored the experimental walkways that Portia used.
So Jackson and Cross decided to take advantage of jumping spiders’ dislike of getting wet.
To reach the box containing the meal, the spider would have to crawl down the tower and onto the platform, which also had two pillars leading to separate suspended walkways—one to the food, one to the leaves.
But once the spider started its descent from the tower, the researchers emptied out the boxes, preventing it from getting visual reminders of the meal’s location.
In their lab at Kenya’s International Centre for Insect Physiology and Ecology, Cross and Jackson placed individuals from 14 different Spartaeinae species in this tortuous obstacle course.
“We have to hide ourselves, so that the spider isn’t distracted,” says Cross. “They’re attracted to blinking.”
It turned out that each species was overwhelmingly successful at finding its way to the box containing food—despite the fact that none of the subjects could see the food mid-detour, according to the study, published Tuesday in the Journal of the Experimental Analysis of Behavior.
What’s more, spiders that chose the wrong path often paused, seemingly confused.
“Their expectations for what they were going to weren’t met,” says Cross. “It wasn’t part of their plan.”
The finding hammers home that, like Portia, many related jumping spiders must have an abstract sense of the food’s location and a working plan for how to navigate the walkways.
“A lot of times, when papers are published about a particular organism, broad generalizations are made about the group,” says Berkeley's Elias.
The study authors have "done a really good job of showing [planning] in this very large group.” (Read about other colorful new jumping spiders, Sparklemuffin and Skeletorus.)
Researchers still have a long way to go to understand how spiders think, but for now they're left to wonder what’s going on in those tiny little heads.
"What they do just astounds me,” says Cross.
Greshko has a master’s degree in science writing from the Massachusetts Institute of Technology and a bachelor’s degree in biology from Vanderbilt University. In his spare time, he enjoys musical theater, magic, and a good book. He lives in Washington, D.C.
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☞ A couple of articles (with photo galleries) that may be of sure interest to arachnophiles:
