Padrão estridulatório e sua função no escorpião Jaguajir rochae (Borelli, 1910) (Arachnida: Scorpiones)

Abstract

Animals can communicate in many ways and stridulation, which is the production of sound from friction of two specialised rigid body parts, represents one of the least investigated aspects in some taxa. Admittedly present in scorpions, as in the Caatinga sp ecies Jaguajir rochae (Borelli, 1910), whose stridulatory pattern was described in 1995. However, this communication’s ecological role or the morphofunctional effects of the stridulatory apparatus in sound production have yet to be explored. Thus, this paper sought to redescribe and analyse the stridulatory patterns produced by J. rochae scorpions of different sexes, due to possible morphometric variations in the stridulatory apparatus, as well as to investigate if there are changes in the stridulatory reaction to different levels of stress and presence of chemical traces of a vertebrate predator. Thereunto adult specimen from the cities of Sertânia - PE and São José dos Cordeiros - PB were utilised, and their stridulatory reactions were registered under different situations: (1) minimal mechanical stimulus; (2) lower stress mechanical stimulus; (3) higher stress mechanical stimulus; (4) mechanical stimulus added to chemical stimulus from a vertebrate predator. Furthermore, morphometric characters of the stridulatory apparatus were measured to evaluate possible variations which could cause sound variation between the sexes of this species. The mean of measured structures was bigger in females, although the allometric rate of the measured lengths was bigger in males, whose pecten are proportionally bigger in relation to total body size. On the other hand, the pecten’s number of teeth presented similar values to both sexes. The sound spectrogram showed frequencies between 13 and 27 kHz, with peak frequency little over 20 kHz and average length of de 0,62 ± 0,05 s for males e 0,75 ± 0,08 s for females. The sound pattern of males and females was differentiated mainly through parameters of delta time, peak power frequency and minimum frequency, which presented higher values in females. Regarding the animals’ stridulatory reaction to different stimuli (mechanical and chemical), there weren’t differences amongst the investigated treatments. The results corroborate the morphometric variation influencing sound pattern hypothesis, such that bigger structures produce longer sounds (delta time), as seen in other arthropods. The similarity in reaction to different types of stimuli showed that stridulation in J. rochae can be triggered merely by mechanical stimulus. As far as we know, this is the first study to investigate the ecological function of stridulation in scorpions. The results here obtained are unprecedented for the taxon and strengthen the hypothesis of stridulation in scorpions having an exclusively defensive function.