Jumping spiders (Salticidae) are renowned dancers, but until recently, the music of their courtship was thought to fall on deaf ears. No one had heard of spiders being able to perceive sounds until a recent serendipitous finding published in Current Biology by Ronald Hoy and colleagues from Cornell University, USA. Having previously shown that the spiders rely heavily on vision as they strut their stuff, Hoy's colleagues, Paul Shamble and Gil Menda, were intrigued when they noticed that the spiders also reacted to sounds made by a creaky lab chair: the noises triggered nerve signals in the spider's brain. This led to subsequent experiments where clapping elicited more brain activity, and eventually the researchers designed a more detailed series of experiments designed to test the spiders’ hearing more thoroughly.
First, Menda carefully inserted an electrode into a jumping spider's (Phidippus audax) main brain area, which is thought to be important for interpreting the senses. Once the researchers inserted the electrode, they played a series of tones ranging in frequency (50–400 Hz) and amplitude (volume) to the spiders. Importantly, the scientists ensured that the sounds were being transmitted through the air, and not the surface that the spiders were standing on, by placing the spiders on a vibration-free air table. Brain recordings made by Shamble and Menda revealed that single neurons were sensitive for a narrow range of frequencies, similar to tuning curves in other species. Moreover, they realised that neurons fired more in response to frequencies important to the spider at lower amplitudes, suggesting that the brain can hear salient sounds from longer distances.
While the neural responses suggest that sounds are perceived, it was unclear whether spiders would respond to the tones. To investigate this, the team played notes either at 80 or 2000 Hz while filming the spider's behavioural responses. Interestingly, the spiders froze almost instantaneously when they presented low-frequency tones, but didn't react to high-frequency sounds. Shamble and Menda explain that common spider predators, such as wasps, produce sounds around 100 Hz, which are well within the hearing range of these jumping spiders and so the deeper tones could possibly be interpreted as a threat.
However, how a spider actually ‘hears’ sound was still a mystery. Spider legs are covered with air-flow-detecting hairs, which may translate airborne sounds into neural activity and subsequent behavioural responses. To test whether these sensory hairs encoded airborne sounds, Shamble and Menda stimulated individual hairs on the spider's foreleg using a microshaker. Vibrating the leg hairs over the same frequency range that the spiders could hear triggered nerve signals in the spiders’ brain, which were the same as the signals that they had recorded when playing sound to the arachnids. This suggests that the sensory hairs are likely involved in perceiving sound, although the authors caution that proving a direct link between sound perception, the leg hairs and nerve signals in the brain is a nontrivial task that is yet to be completed.
In what started as a fortuitous creaky chair incident, Shamble and colleagues have found that jumping spiders can detect and respond to long-range airborne sound – both at the neural and behavioural levels – which may be important for predator detection and finding a mate. So next time you reach for a can of Raid, just remember, spiders may be able to hear you plotting their demise.
