How spiders distinguish living from non-living using motion-based visual cues
Ability to identify other animals from relative positioning of the joints not unique to vertebrates
IMAGE: UNKNOWN JUMPING SPIDER, AUGUST 2012, BELTSVILLE, MARYLAND, BENJAMIN A COULTER HELPED NARROW THIS DOWN TO PERHAPS AN IMMATURE THIODINA SYLVANA view more
CREDIT: SAM DROEGE, USGS BEE INVENTORY AND MONITORING LAB, FLICKR
Jumping spiders can distinguish living from non-living objects in their peripheral vision using the same cues used by humans and other vertebrate animals, according to a study publishing 15th July 2021 in the open-access journal PLOS Biology by Massimo De Agrò of Harvard University in the United States.
The ability to detect other living creatures in your surroundings is a key skill for any animal - it is crucial for finding mates, avoiding predators, and catching prey. The movements of vertebrates and invertebrates are distinct from inanimate objects because their rigid, jointed bones and exoskeletons constrain the relative positioning of certain body parts. Most vertebrates can recognize this biological pattern of movement from very limited visual information, such as a point-light display, which shows dots representing the positions of the main joints.
To investigate this phenomenon in invertebrates for the first time, researchers partially restrained 60 wild-caught jumping spiders (Menemerus semilimbatus) on a spherical treadmill and used a computer screen to show point-light displays on each side of their peripheral vision (only visible to their lateral eyes). They found that spiders were more likely to try to turn and face displays that showed random movements, compared to those that moved in a more biological way, with the distances between joints constrained.
The result seems contrary to the expectation that spiders should focus their attention on objects in their surroundings that appear to be living - potential prey, mate or predator. However, the authors suggest that this behavior may allow the spiders to focus their forward-facing primary eyes on unidentifiable objects to get a better look. Complex vision evolved independently in vertebrates and arthropods and so the ability to distinguish living from non-living motion using the relative positioning of the joints has most likely arisen convergently in the two groups of animals.
"Jumping spiders' secondary eyes confirm themselves to be a marvelous tool," the researchers add. "In this experiment, we observed how they alone can tell apart living from non-living organisms, using the semi-rigid pattern of motion that characterize the formers and without the aid of any shape cue. Finding the presence of this skill, previously known only in vertebrates, opens up new and exciting perspectives on the evolution of visual perception. My co-authors and I can't wait to see what other visual cues can be perceived and understood by these tiny creatures."
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Research Article
Peer-reviewed; Experimental study; Animals
In your coverage please use these URLs to provide access to the freely available articles in PLOS Biology: http://journals.
Citation: De Agrò M, Rößler DC, Kim K, Shamble PS (2021) Perception of biological motion by jumping spiders. PLoS Biol 19(7): e3001172. https:/
Funding: PSS was supported by the John Harvard Distinguished Science Fellows Program within the FAS Division of Science, Harvard University. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist
A new spidey sense
Harvard study shows jumping spiders can distinguish living objects from non-living objects based on their movement
Add this to the list of real-life spidey senses: Harvard researchers have shown that jumping spiders are able to tell the difference between animate objects and inanimate objects -- an ability previously known only in vertebrates, including humans.
Using a specialized treadmill system and a point-light display animation, the team of scientists found that these spiders are able to recognize biological motion. This type of motion refers to the visual movements that come from living organisms when they are moving. The visual cue is how people, even babies, can tell someone is another person just by the way their bodies move. Many animals can do this, too.
The ability, which is critical for survival, is evolutionarily ancient since it is so widespread across vertebrates. The study from the Harvard team is believed to be the first demonstration of biological motion recognition in an invertebrate. The findings pose crucial questions about the evolutionary history of the ability and complex visual processing in non-vertebrates.
"[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, which published in PLOS Biology on Thursday.
The study was authored by a team of researchers who were John Harvard Distinguished Science Fellows during the time of the study or are current fellows. Massimo De Agrò, now a researcher at an animal comparative economics lab in Regensburg, Germany, led the work. Paul Shamble, a current fellow, and Daniela C. Rößler, and Kris Kim, former fellows, co-authored the study.
The researchers chose jumping spiders to test biological motion cues because the animals are among the most visually adept of all arthropods. With eight eyes, for example, vision plays a central role in a wide range of behaviors.
They placed the jumping spiders, a species called Menemerus semilimbatus, into a forced choice experiment. They suspended the spiders above a spherical treadmill so their legs could make contact with it. The spiders were kept in a fixed position so only its legs could move, transferring its intended direction to the sphere which spun freely because of a constant stream of compressed air shooting up below it.
(Friendly disclaimer: No spiders were harmed during the experiment and all were freed in the same place they were captured afterwards.)
Once in position, the spiders were presented two animations as stimuli. The animations were called point-light displays, each consisting of a dozen or so small lights (or points) that were attached to key joints of another spider so they could record its movements. The body itself is not visible, but the digital points give a body-plan outline and impression of a living organism. In humans, for example, it only takes about eleven dots on the main joints of the body for observers to correctly identify it as another person.
For the spiders, the displays followed the motion of another spider walking. Most of the displays gave the impression of seeing a living animal. Some of the displays were less real than others and one, called a random display, did not give the impression it was living.
The researchers then observed how the spiders reacted and which light display they turned toward on the treadmill. They found the spiders reacted to the different point-light displays by pivoting and facing them directly, which indicated that the spiders were able to recognize biological motion.
Curiously, the team found the spiders preferred rotating towards the more artificial displays and always toward the random one when it was part of the choice. They initially thought they would turn more toward the displays simulating another spider and possible danger, but the behavior made sense in the context of jumping spiders and how their secondary set of eyes work to decode information.
"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 said.
The researchers hope to look into biological motion recognition in other invertebrates such as other insects or mollusks. The findings could lead to greater understanding of how these creatures perceive the world, De Agrò said.
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