If I stridulate, you won’t eat me, will you?
Spiders use acoustic signals for several purposes, both in communication with other species (e.g. to threaten potential enemies) or within the species (e.g. during courtship or competition among males). They have several mechanisms to produce sounds, such as percussion and stridulation; the latter one consists of rubbing two rigid body parts against each other. The stridulatory aparatus is located on different parts of the body, for instance members of the genus Palpimanus from the family Palpimanidae have a chelicera-pedipalp stridulatory apparatus, which produces very quiet, but audible sound. These spiders are specialist hunters of other spiders of all ontogenetic stages and are morphologically and behaviorally adapted for active nocturnal hunting, mainly on the ground.
When two Palpimanus spiders meet, which is quite probable due to their aggregated distribution, they are theoretically in danger of their lives, as they can fall prey to the other one. That is why they have evolved an ability to recognize kin in order to avoid cannibalism. Acoustic signals are an effective means, especially in low-light conditions and at short distances. Despite cannibalism being quite common in spiders, it is not always an optimal life strategy, as it is quite risky (danger of injury or that a hunter turns out to be the prey himself in the end). The research team thus hypothesised that the stridulation function is a defensive signal against cannibalism, or more probably against any predator.
For the experiments, they chose morphologically very similar species, P. gibbulus and P. orientalis, collected in Portugal and Greece, respectively. After having performed various behavioral experiments, the scientists concluded that the spiders started to stridulate after being touched and stridulation was of two intensities - lower and higher, depending on the intensity of the contact. Cannibalism was observed in only 19% of the cases and it was more frequent when the size difference between the two individuals placed together in a Petri dish was bigger. The lower-intensity stridulation was usually used during the very first contact with the other spider and regardless of the size of both participants. The higher intensity was used when the touching continued or when a smaller spider was touched or grasped by a bigger one. In the majority of cases, the spiders separated after stridulation, which confirms the hypothesis that stridulation is a defensive signal. When the stridulatory apparatus was artificially damaged, the spiders were more often cannibalized (almost 60%).
The researchers suppose that stridulation is probably a tool preventing cannibalism, particularly among conspecifics differing by body size or during mating behavior, as the males are smaller than females and they indeed often stridulate during courtship and mating.