Electrically serenading female electric knifefishes is energetically expensive for the males, according to a study published in a recent issue of the Journal of Experimental Biology.
Vielka Salazar and Philip Stoddard measured the energetic cost of electrogenesis relative to the total energy budget for the knifefish species Brachyhypopomus pinnicaudatus, and found that males spend a significant amount of energy (11-22% of their total energy budget) in electric signal generation compared to females (3%).
To calculate this energy budget, the authors measured the oxygen consumption of fishes when they were generating EODs (electric organ discharges) and when they were at rest (motor activity in the fishes was inhibited by drugs).
Not only did the authors find males to be using more of their energy budget for EODs, they also found the most powerful signals to be generated by the healthiest males (i.e. the EODs of the males are also condition-dependent signals).
By broadcasting more powerful signals, the males are also exposing themselves to greater predation risks (since they become more conspicuous to electroreceptive predators this way); the authors consider this to be another method by which male electric knifefishes signal their condition to females.
For more information, see the paper, Salazar, VL and PK Stoddard (2008) Sex differences in energetic costs explain sexual dimorphism in the circadian rhythm modulation of the electrocommunication signal of the gymnotiform fish Brachyhypopomus pinnicaudatus. Journal of Experimental Biology 211, pp. 1012-1020.