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Tiny little jumping spiders, with their gorgeous eyes, seem capable of doing something we’ve never seen before in vertebrates: distinguish between animate and inanimate objects.
In a new test, wild jumping spiders (Menemerus semilimbatus) behaved differently when presented with simulated objects of both types, in a way that indicated an ability to discern between them.
The research not only suggests that this ability can be found more widely in the animal kingdom than we knew, it demonstrates that the team’s experimental setup can be used to test other invertebrates in the same way.
“These results clearly demonstrate the ability of jumping spiders to distinguish biological movement signals,” the researchers wrote in their article.
“The presence of a motion-based biological detection system in jumping spiders deepens questions regarding the evolutionary origins of this visual processing strategy and opens the possibility that such mechanisms may be prevalent in the animal kingdom.”
When you think about it, it makes sense that creatures are able to distinguish between living and non-living things. It could literally be a matter of life and death – escaping predators or hunting for prey. Nonetheless, it was not clear whether or not tiny invertebrate creatures relied on the ability to distinguish between movement and non-movement, or animate and inanimate objects.
Jumping spiders appeared to be an excellent candidate for testing, due to their spectacularly good vision. Like all spiders, they have eight eyes; but the eyes of jumping spiders include two large, glistening pools of limpid black on the front of their small faces, which perhaps gives them tetrachromatic color vision.
A team of researchers led by biologist Massimo De Agrò, formerly of Harvard University, collected 60 specimens of M. semilimbatus, common throughout the northern hemisphere. These spiders were then subjected to a specially designed point light test.
Here is how it works. When presented with 11 moving points corresponding to the positions of major joints on the human body, human test subjects can recognize the movement pattern as belonging to a human. These 11 points, when they are still, will not convey the same meaning – they are only 11 points.
(From Agrò et al., PLOS Biol., 2021)
De Agrò and his team designed similar point lighting based on the joints of a spider. They also designed other point-light displays, including a moving ellipse, and blurred random movement that didn’t resemble the movements of any living creature.
To show the animation to the spider, the team held the spider’s body in place on a spherical “conveyor belt” that rolled on a stream of compressed air. The way the spider tried to walk on the treadmill was seen as an indicator of its response to point animations. Each of the 60 spiders then saw the spot light displays and their carefully recorded reactions.
Interestingly, the jumping spiders swiveled their bodies to look with their large eyes at the screens that were less realistic. The effect was most pronounced with the random display of point light, which moved the least like a living organism.
This, the team realized, has to do with how spider eyes work. The secondary eyes on the side of the head may not have the visual acuity of the two large eyes, but they do give spiders almost 360-degree vision. If the spider spots something with these eyes that it can recognize, but also something that it doesn’t recognize, it will prioritize the strange thing, since the recognizable thing will remain in its field of vision.
“The secondary eyes are looking at this point-light display of biological movement and it can already understand it, while the other random movement is weird and they don’t understand what is there,” De Agrò explained.
The team hopes their system could be used to apply their test to other invertebrates, such as insects and snails, to try to learn more about the evolution of this ability.
The 60 spiders were all returned to the wild unharmed… although maybe a little confused.
The research was published in PLOS Biology.
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