It is splendid news that jumping spiders can tell if you’re alive or dead. Harvard scientists have discovered that these tiniest and prettiest of arachnids possess an ability thought to be the fief of vertebrates, i.e., higher animals: noticing “biological motion” – meaning, movement by living beings as opposed to inanimate objects. Ergo, they can tell if you are alive and don’t have to waste their time hunting leaves blown by the wind and whatnot.
And how did Massimo De Agrò, Daniela C. Rösler, Kris Kim and Paul Shamble discover this unexpectedly advanced “spidey sense”? By putting the spiders on a special spidey treadmill and using an animated point-light display, they explained on Thursday in PLOS Biology.
Spiders are among the arachnids, in the order Araneae, which are members of the arthropod crowd, and vertebrates they are not. They seem to have originated almost 400 million years ago – note ye, humans split off the greater ape tree less than a million years ago.
From that perspective one could see spiders as a sort of living fossil, though like the shark and the coelacanth and the bacterium, obviously they have evolved a great deal in this time. They just look a lot like they used to, with the caveat that the earliest known proto-spider, Attercopus fimbriungus, had a long furry tail and didn’t have spinnerets to weave a web. It could however weave silk sheets using modified hairs called spigots.
Unlike some of their primordial ancestors, jumping spiders are tiny, ranging from 1 to 25 millimeters (0.04-0.98 inches). The family, Salticidae, includes over 6,000 species and as spiders go, they can be splendidly colored, and cute as a puppy that happens to have eight eyes and eight legs.
It had already been clear that they were among the best sighted in spider circles. Now the Harvard team has shown they too, like your puppy and you, can identify live beings as opposed to inanimate objects, by the way the body moves.
The fact that detection of movement is so widespread is a clue that this ability is extremely ancient; and also that it improves survival. It also begs the question of where and when such complex visual processing first arose.
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“[It] opens the possibility that such mechanisms might be widespread across the animal kingdom and not necessarily related to sociality,” the researchers wrote in the paper.
They didn’t happen on the discovery by chance. Having eight eyes, the team observes, indicates that vision plays a central role in a wide range of spider behaviors. The question was which behaviors. The team chose to test the ability to identify biological motion cues in jumping spiders because they have better visual capabilities than most other spiders: they hunt by sneaking up on prey and leaping.
And thus the researchers put the test spiders, members of the species Menemerus semilimbatus, into a “forced choice” experiment. The spiders were tethered in a fixed position, suspended above a spherical treadmill propelled by a stream of air, so their little legs could make contact with the wheel. The mechanism could thus transfer their intended direction of movement to the wheel.
Then the spiders were shown two animated “movies,” each consisting of a dozen or so small lights (“points”) that were attached to key joints of a simulated spider so they could detect its movements. They could not see the other simulated spider, but the points of light created a body-plan outline and the impression of a living organism.
Some displays looked just like spiders walking; some displays were “less real” than others and one, a random display, did not convey the impression of biological motion.
Long story short, the suspended tethered spiders reacted by turning toward the displays they thought were not alive, or were enigmatic. The random one attracted the most attention. They did not pivot and turn toward the displays they thought were spiders on the move.
And why does that indicate these creatures can discern biological motion?
Because they had already figured out what they were: spiders (never mind that they were wrong), the scientists deduced. They had figured the spiders would pivot more toward the displays simulating another spider and potential danger, but it was not so. They pivoted toward the weird display that they couldn’t understand and they wanted to understand it, the researchers suggest.
So there we have it. Jumping spiders are the only non-vertebrates known to possess this ability, but likely others do too, and the team says it may expand its search to insects and mollusks. And if the jumping spider’s insane cuteness has captured your heart, you should: a) be pleased to know the researchers disclaimed that “no spiders were harmed during the experiment and all were freed in the same place they were captured,” and b) be grateful you are not a male jumping spider.