Bathed in their own gently oscillating weak electric field, glass knifefish (Eigenmannia virescens) interpret distortions of the field to learn about their surroundings, although the fish rarely enjoy the opportunity to analyse perturbations in their isolated fields. According to Sarah Stamper and Manu Madhav from Johns Hopkins University, USA, these fish spend most of their time in groups and in order to avoid jamming each other's electric fields with their own oscillations, the fish adjust the frequencies of their fields to prevent a clash. In addition, the team explains that the complex oscillating field that results when the fish congregate is enfolded by an envelope – known as the social envelope – which also has a characteristic low frequency ripple. Curious to find out whether glass knifefish are able to identify the social envelope produced by a small crowd and modulate their own electrical fields in response, Stamper, Madhav, Noah Cowan and Eric Fortune simulated the electric fields produced by two glass knifefish and measured the response of a third fish as they systematically varied the field's social envelope (p. 4196).
Presenting a fish with a simulated composite field with a low frequency (2 Hz) social envelope, the team found that the fish altered the frequency of its own oscillating electric field significantly, although fish located in a field with a higher frequency social envelope (4–8 Hz) responded more weakly. Thus, the fish were able to detect low frequency social envelopes. And when the team analysed the impact of the third fish on the structure of the social envelope, they realised that the additional fish raised the frequency of the social envelope to 5–15 Hz. However, the fish were not simply increasing the frequency of the oscillating field to avoid jamming the signals of the other nearby fish. The team suspects instead that the fish increase the social envelope frequency to improve their electrical perception, as very low frequency electric field envelopes (~2 Hz) may impair their ability to perceive their surroundings and objects moving in the vicinity.