It's easy to see how a deep voice or massive muscles can communicate fitness, but whether electric fish can send the same message with crackling electric discharges wasn't clear. Philip Stoddard, from Florida International University, knew that at night male Brachyhypopomus pinnicaudatus's electric buzzes were bigger and longer than the females. But why were the nocturnal males so much louder than the females when they also run the risk of attracting predators? Could the crackling communications convey information about a male's condition that could improve his chances of attracting a mate?Stoddard's student, Vicky Salazar, explains that this could be the case if the males were investing significant amounts of energy in their electric communications. But everyone assumed that the signal's metabolic cost was negligible, so the team decided to measure the metabolic cost of male and female electric discharges (p. 1012).

Salazar took on the challenging job of designing a respirometer that was watertight, so that she could accurately measure the animal's oxygen consumption, and transparent to the fish's electric discharge. Eventually she came up with a sealed unglazed ceramic tube that the nocturnal fish could rest in contentedly during the day as she recorded their metabolic rates.

Next Salazar had to devise a way of partitioning out the discharge's metabolic cost from the fish's routine costs. Fortunately the team already knew that metomidate, an anaesthetic, would only sedate the fish, and a curare analogue, flaxedil, would silence the fish's electric organ discharges. By sequentially administering the drugs, Salazar could systematically shut down different components of the fish's metabolic budget and dissect out the discharge's cost. Having recorded the fish's electric organ discharges after administering the metomidate, Salazar quickly removed the fish from the tube,injected it with curare, and returned it to the respirometer before measuring its standard metabolic rate. Fortunately the effects of both drugs wore off quickly after the experiment, and the fish soon recovered and began firing off again.

However, injecting the curare analogue when the fish became active at night was perilous. Salazar explains that the fish's electric discharge pattern became disturbed when she switched on her headlamp so that she could see to inject them. She tried to shield the fish with a red filter, but then she ran the risk of injecting herself. Realising that the nocturnal experiments were too hazardous, Salazar changed tack. She abandoned the respirometer tube,recording the metabolic rates of a pair of active fish in a tank, before calculating the metabolic cost of each individual's nocturnal electric discharges based on models of her daytime recordings.

Calculating the electric discharge's metabolic cost, Salazar realised that it cost the males dear. According to Salazar, the males invested anything from 11% up to a colossal 22% of their metabolic rate in their crackling serenades. However, the females hardly exerted themselves at all, expending a minimal 3%on electric discharges. And when Salazar compared the males' physical condition with the amount of effort they put into their electric hum, she realised that the males were probably broadcasting information about their condition too. The fattest and healthiest males could well use their big electric buzzes to entice females to mate with them while warning smaller males to steer clear.

Salazar, V. L. and Stoddard, P. K. (
). Sex differences in energetic costs explain sexual dimorphism in the circadian rhythm modulation of the electrocommunication signal of the gymnotiform fish Brachyhypopomus pinnicaudatus.
J. Exp. Biol.