“Spider-vision” study shows how arachnids can detect biological motion
Inside a dark laboratory, a little spider hangs in front of a computer screen. Rather than a silk line suspending it from above, a complex cylindrical apparatus seems to hold it in place. The spider’s feet touch a plastic ball covered with small dark patches. As images move across the computer monitor, the spider tries to turn and its moving legs rotate the ball, capturing the motion precisely.
This curious setup allowed researchers led by Massimo De Agrò and Paul S. Shamble from Harvard University, USA, to make an interesting discovery. They found that jumping spiders can distinguish living creatures from non-living ones by their motion, an ability that was hitherto only known to be possessed by vertebrates (backboned creatures, e.g., mammals and birds).
For creatures living in the wild, it is important to be able to differentiate between the movement of inanimate objects (e.g., a leaf falling) and animate ones (e.g., a predator pouncing). One way for our brains to achieve this would be to store the exact movement patterns of all the animals one might encounter and then check every new movement against this database. However, this would be both impractical and energetically costly. Instead, our visual systems have come up with an ingenious solution to this problem.
Many animals, particularly vertebrates, have mobile joints connected by rigid body parts. For example, our forearms connect our wrists and elbows, while our thigh bones stretch between our knees and hips. As a result, when animals move, the relative distance between pairs of joints remains constant. We can use this specific pattern of movement, called “biological motion”, to quickly distinguish between living and non-living objects, without needing to process their features in complete detail.
To get a sense of this, take a look at the video below. It shows 12 white dots moving against a dark background. Yet, our brain has no trouble interpreting them as a person walking. This ability to distinguish biological vs non-biological motion has been observed not only in humans, but also in birds, new world monkeys, and fish, suggesting that this ability may be widespread among vertebrates.
Shamble and his colleagues wanted to check whether some invertebrates (creatures without a backbone, e.g., insects, worms, or spiders) might have the same skill. The reason for picking the jumping spider was simple – it is known to have an extremely well-developed visual system. With their four pairs of eyes, these spiders have a nearly 360° view of the world, which they use to hunt their prey. (Unlike their cousins, jumping spiders do not usually build webs, instead using their eponymous jumping ability to pounce upon their prey.)
The eight eyes are not all the same – the large front eyes are believed to have the strongest capacity for distinguishing details while the lateral (side) eyes are more suited to detecting motion. When the lateral eyes send the signal that something interesting is happening in its visual field, the spider pivots quickly to face the new object with the front eyes.
The researchers decided to use this rapid pivoting motion to check how the spiders responded to visual stimuli. They collected five dozen spiders from a garden in Italy, glued a magnet to the head of each, and fed them a mealworm before mounting them to the apparatus described earlier. As they hung in front of the computer monitor, two different signals appeared on the screen, moving in from opposite sides. The spider could choose to turn towards one or the other, and in trying to do so, their legs rotated the ball they were suspended over. A camera recorded the movement.
The signals consisted of patterns of dots, representing both biological and random motion. For biological motion, the researchers used dots corresponding to various points on a spider’s body, moving as they would if a spider were walking across the screen. As a control, they presented a signal where the same number of dots were present on the screen but were all moving in random directions.
The researchers found that the jumping spider could easily discriminate between biological and random motion. The decision to pivot one way or the other also happened really fast – the speed indicating that vision from the side-eyes was sufficient for this process, without the front eyes being involved at all.
Surprisingly, the spiders preferred to turn away from the perceived biological signal and towards the random one. This is the opposite of what has been seen in most vertebrates so far. One reason for this could be that the spiders interpreted the biological signal as a predator or a threat.
However, the researchers think that this is unlikely. Instead, they suggest that the spiders might have chosen to take a closer look at the signal that they could not interpret readily. They speculate that this might also be linked to the fact that biological motion perception has so far been primarily studied in social species (like humans or chickens), while spiders tend to be solitary creatures.
The eyes of spiders and the eyes of vertebrates have very different structures, and are believed to be an example of convergent evolution – a process wherein similar features evolve independently in distant branches of the living tree in order to serve a similar function. Whether this study represents an example of the same, or whether this is a behavior evolutionarily conserved and passed down from a distant ancestor of both spiders and humans, remains to be seen.
Cover Image: Paraphidippus aurantius jumping spider (Credit: Thomas Shahan, CC BY 2.0 , via Wikimedia Commons)