Researchers have discovered that jumping spiders can hear sounds three meters away. It was first believed that they could only sense nearby air vibrations.
Paul Shamble and Gil Menda learned about the jumping spiders’ unexpected sense of hearing while they were testing the spiders’ brain activity linked to its field of sight. When Menda dragged a chair across the room causing a screeching noise, the brain sensor fired up, indicating activity in the spider’s brain.
“The sensory world of the tiny jumping spider was thought to be dominated by sight and tactile touch. Surprisingly, we found that they also possess an acute sense of hearing. They can hear sounds at distances much farther away than previously thought, even though they lack ears with the eardrums typical of most animals with long-distance hearing,” stated lead researcher Paul Shamble.
Jumping spiders can hear you across the room
Scientists noted that jumping spiders, the largest family of spiders, have behaviors primarily influenced by their sight, as they have the best vision out of any arthropods. They use eyesight for mating, hunting, and landing precise jumps. The jumping spider family, or Salticidae, is characterized by always having four pairs of eyes.
Researchers suggested that these spiders respond to low-frequency sounds by freezing in place, in an attempt to lure away predators. It was also determined that the hairs present on the spiders’ forelegs also had a role in the acoustic perception of stimuli.
Testing how much can spiders hear
The spiders were recorded on video to see how they reacted to specific types of acoustic stimuli. But one of the main inconveniences is that spiders were already known to be sensitive to air vibrations.
The test subjects were surrounded by a wired mesh, and a speaker was set 2-and-a-half meters away. The speaker emitted sounds equivalent to 80 Hz and 2,000 Hz. The responses to low-frequency sounds were more noticeable, as the spiders froze in place most of the times that the sound was perceived, which leads to the conclusion that freezing in place is one of the jumping spiders’ anti-predatorial mechanisms.
The brain activity of the spiders was also measured. The subjects were anesthetized and held in place by using a 3D-printed spider holder and wax. Then researchers inserted a microprobe onto the spiders’ brain to record neural activity.
Then the spiders were submitted to different variations of acoustic stimuli. The sounds were played 2, 3, and 3.3 meters away. The sound was measured with a microphone, and the spider was also recorded. Researchers also took into account the vibrations that may have been perceived by the spiders, which would not count as acoustic stimuli.
Spiders may hear through the hair on their legs
The spiders appeared to react more forcibly to low frequencies, and it seems that their filiform hairs were particularly sensitive and did not react to acoustic stimuli but rather to air vibrations. Spiders are capable of capturing prey without any visual assistance, guiding themselves with the vibrations perceived by the hairs on their legs. These hairs are also present on cockroaches and crickets, mainly to escape danger by sensing the air vibration that surges when a predator approaches from behind.
But in the acoustic spectrum, spiders do not have timpani to help them perceive air pressure. Researchers state that airborne sound is comprised of air pressure and particle velocity, but the neurological structures activated by hairs appear to respond to both vibrations and particle velocity.
The study suggests that acoustic startle responses may help spiders determine their focus of attention. Whenever the spider picks up an airborne stimulus, it would use its sense of vision to enable the safest subsequent behavior. After perceiving a sound, the spider would be more attentive to visual cues. Because the spiders lie in place, it is likely that they may be on the look for predators, being ready to jump if anything appears on its field of sight.
Acoustic signals are also emitted by enemy flying insects, such as wasps and flies, which often are regarded as a cause of spider mortality. This is where the hairs on the spiders’ legs come in, which allow them to detect predators. This mechanism now appears to be somehow related to a spider’s hearing sense.
The experiments showed that jumping spiders have the best sense of hearing of all spiders so far. Neurological responses were present even when the distance between the spider and the source of stimulus exceeded 3 meters, allowing for new questions about the true acoustic spectrum of specimens that do not possess an eardrum.
“The ability to detect the presence of threats—even before they become visually apparent—could provide these animals with an important fitness advantage,” said researchers in the paper titled “Airborne Acoustic Perception by a Jumping Spider.”