With their eight eyes, terrifying ability to kill, and tendency to occasionally transform whole cities into hellscapes, spiders aren’t typically known for being one of nature’s cuter offerings.
There is an exception though: the jumping spider, which even some spider haters have to agree is just the most adorable little bouncy fuzzball.
And a new study, published yesterday in the journal PLOS Biology, has given us a new reason to love the clever creepy-crawlies. They possess an ability previously unknown among invertebrates: biological motion perception.
To illustrate what biological motion perception is, just take a quick look at the video below.
What did you see? Presumably, a bunch of human figures doing various things: walking, crawling, jumping, and so on.
Except what you actually saw was groups of dots moving around the screen almost randomly. What made your brain think they were people was the fact that the visual cues – the dots placed at the locations of various joints in the “body” – were strategically placed and, crucially, kept at fixed distances from each other. These kinds of semi-rigid patterns are called biological motion because they mimic the movement of living creatures.
Animals, including humans, instinctively interpret this kind of motion as belonging to something alive – an ability that has been crucial for our survival throughout history. But despite being an evolutionarily ancient trait, it has so far only been demonstrated in vertebrates. That is, until now.
“The presence of a biological motion-based detection system in jumping spiders deepens questions regarding the evolutionary origins of this visual processing strategy,” the researchers wrote. “[It] opens the possibility that such mechanisms might be widespread across the animal kingdom and not necessarily related to sociality.”
To test the spidey senses, the team captured 60 jumping spiders and showed them pairs of point-light display animations – similar to the video above, but with other spiders instead of humans. Some animations were realistic, some were “scrambled” – artificially constructed, and not based on anything living, but nevertheless maintaining this semi-rigid movement – and one showed a random display that wouldn’t mimic biological movement at all. The brave participants were then suspended with their feet on a tiny spherical treadmill, and the scientists observed their reactions.
"We simultaneously presented 2 point-light displays with specific dynamic traits and registered their preference by observing which pattern they turned toward,” describes the paper. “Spiders clearly demonstrated the ability to discriminate between biological motion and random stimuli […] However, they showed no preference between biological and scrambled displays, results that match responses produced by vertebrates.”
Strangely, the spiders seemed to show a strong preference for random motion over biological motion – something which took the researchers by surprise. When faced with a "spider" on one side and a bunch of random dots moving around on the other, surely, the team reasoned, the spiders would keep your attention on the spider. After all, spiders can be creepy and, you know, cannibalistic. But the key, they realized, was in how jumping spiders see the world.
“These animals are among the most visually adept of all arthropods, with vision playing a central role in a wide range of behaviors,” the paper explains. “The visual system of these animals is also unique, with a total of 8 eyes and with different pairs thought to be specialized for different visual tasks.”
So basically, the team figured, the spiders were getting confused.
“The secondary eyes are looking at this point-light display of biological motion and it can already understand it,” said lead author Massimo De Agró in a statement. “The other random motion is weird and they don't understand what's there.”
Having proven the ability exists outside the vertebrate world, the team now hopes to investigate biological motion perception in animals like mollusks and insects. This research could give us a new insight into how the world looks to nature’s creepy-crawliest inhabitants.
And for anybody concerned about the study’s spidey subjects – don’t worry. The team released them back into the same place they were captured, unharmed (and presumably a little bit fitter, after all that time on the treadmill.)