The Ash Twins Blog

Humans have exceptionally large brains relative to their body size (about 2% of body mass). Perhaps surprisingly, some animals have brains which are even larger relative to body size. Take the mormyrid electric fish (better known as the elephant nose), whose brain takes up 3% of its body mass. The majority of this size increase is represented by a pornographically enlarged valvula, the medial lobe of the cerebellum (Fig. 1). In this species, the valvula takes on complex electrosensory and electromodulatory functions.

Fig.1 Sagittal section through the brain of a Mormyrid electric fish. Note the exceptionally large valvula dorsally. TH - thalamus, hyp - hypothalamus, Dm - dorsalis pars medialis, LC - caudal lobe, LP - posterior lateral line lobe, lfb - lateral forebrain bundle. Courtesy Prechtl et al. (1998).

In animals, muscles are electrically controlled: influxes and effluxes of electrically charged ions into myofiber cells drive the contraction and relaxation of muscles. Subsequently, animal motion generates electric fields. Due to the high conductance of water, these electrical fields can be sensed by marine life. Electrosensation allows fish to localize other animals in three dimensions, either for help in interception (in the case of sharks) or evasion (in the case of mormyrids). This sixth sense provides a high-speed complement to the slower-timescale chemosensation (i.e. smell).

In the case of mormyrids, not only can these animals sense electric fields in the nearby water, they can also induce electric fields around them. This ability aids navigation in turbid waters and can serve as a jamming signal to misdirect electrosensing carnivores. The valvula is believed to compute the fearsome biophysical differential equations necessary to perform this stealth operation.

Source: Striedter 2008 Principles of Brain Evolution