A well-known cognitive property of vertebrates is the ability to distinguish between moving and stationary objects; an essential ability when it comes to escaping a predator or, on the contrary, tracking prey. But vertebrates are not ultimately the only ones to master this skill. Recently, researchers have indeed shown that jumping spiders (Menemerus semilimbatus) were also able to make this distinction.
In a test during this new study, jumping spiders behaved differently when confronted with simulated animate and inanimate objects, in a way that indicated an ability to discern them. The study not only suggests that this ability can be found more widely in the animal kingdom, but also 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 presence of a motion-based biological detection system in jumping spiders deepens questions regarding the evolutionary origins of this visual processing strategy and brings the possibility that such mechanisms may be prevalent in the animal kingdom. », Write the researchers.
Testing the distinguishing ability of spiders
It might make sense to think that animals 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 certain whether or not small invertebrate creatures depend 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 areas of clear black on the front, which perhaps gives them tetrachromatic color vision. A team of researchers led by biologist Massimo De Agrò collected 60 specimens of M. semilimbatus, common throughout the northern hemisphere. These spiders were then subjected to a specially designed point light test.
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 stationary, will not convey the same meaning – they are only 11 points. De Agrò and his team designed a similar point system 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.
A distinction involving the secondary eyes of spiders
To show the animation to the spider, the team held the spider’s body fixed on a spherical conveyor belt placed over 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. They then showed each of the 60 spiders the spot light displays and their reactions were carefully recorded.
Interestingly, the jumping spiders rotated their bodies to look with their large eyes at the screens which were less realistic. The effect was more pronounced with the random display of point light, which moved the least like a living organism. The team realized this was related to how spider eyes work.
The secondary eyes on the side of the head may not have the visual acuity of the two large frontal eyes, but they do give spiders almost 360-degree vision. If the spider spots something it can recognize, but also something it doesn’t recognize, it will prioritize the unknown, since what is recognizable will remain in its field of vision.
” The secondary eyes are watching this one-time display of biological movement and they can already figure it out, while the other random movement is weird and they don’t understand what’s in it. », Explains De Agrò. 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.