Tiny little jumping spiders, with their magnificent eyes, seem to be able to do something we'd only ever seen before in vertebrates: distinguishing between animate and inanimate objects.
In a new test, wild jumping spiders (Menemerus semilimbatus) behaved differently when presented with simulated objects of both kinds, in ways that indicated an ability to discern between them.
The research doesn't just suggest 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 discriminate between biological motion cues," the researchers wrote in their paper.
"The presence of a biological motion-based detection system in jumping spiders deepens questions regarding the evolutionary origins of this visual processing strategy and opens the possibility that such mechanisms might be widespread across the animal kingdom."
When you think about it, it makes sense that creatures ought to be able to distinguish between living and nonliving things. It could literally be a matter of life or death - evading predators, or chasing prey. Nevertheless, it was unclear whether or not tiny invertebrate critters rely on the ability to distinguish between motion and non-motion, or animate and inanimate objects.
Jumping spiders seemed to be an excellent candidate for testing, because of their spectacularly good vision. Like most spiders, they have eight eyes; but the eyes of jumping spiders include two large, sparkling pools of limpid black on the fronts of their little faces, which possibly give 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's how it works. When presented with 11 moving dots corresponding to the positions of the main joints on the human body, human test subjects can recognize the pattern of motion as belonging to a human. Those 11 dots, when still, won't convey the same meaning - they're just 11 dots.
(De Agrò et al., PLOS Biol., 2021)
De Agrò and his team designed a similar point-light display based on the joints of a spider. They also designed other point-light displays, including a moving ellipse, and scrambled random motion that didn't resemble the movements of any living creature.
To show the spider the animation, the team held the spider's body fixed in place over a spherical "treadmill" that rolled over a stream of compressed air. The way the spider tried to walk over the treadmill was considered an indicator of its response to the point-light animations. Each of the 60 spiders was then shown the point-light displays, and their reactions carefully recorded.
Interestingly, the jumping spiders swiveled their bodies around to stare with their big eyes at the displays that were less lifelike. The effect was most pronounced with the randomized point-light display, which moved the least like a living organism.
This, the team realized, has to do with how the spiders' 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 the spiders almost 360-degree vision. If the spider spots something with those 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 view.
"The secondary eyes are looking at this point-light display of biological motion and it can already understand it, whereas the other random motion is weird and they don't understand what's there," De Agrò explained.
The team hopes that their system could be used to apply their test to other invertebrates, such as insects and snails, in order to try and learn more about how this ability was evolved.
All 60 spiders were returned to the wild unharmed… although maybe a little confused.
The research has been published in PLOS Biology.