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First published online February 29, 2008
Journal of Experimental Biology 211, 1012-1020 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.014795

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Sex differences in energetic costs explain sexual dimorphism in the circadian rhythm modulation of the electrocommunication signal of the gymnotiform fish Brachyhypopomus pinnicaudatus

Vielka L. Salazar^* and Philip K. Stoddard

Department of Biological Sciences, Florida International University, University Park, Miami, FL 33199, USA

View larger version (5K): [in this window] [in a new window]   Fig. 1. The gymnotiform fish Brachyhypopomus pinnicaudatus shows strong sexual dimorphism in tail morphology and amplitude and duration of the electric organ discharge (EOD) waveform (Hopkins et al., 1990). Males show pronounced day–night differences in the EOD (Franchina and Stoddard, 1998). Shaded areas on each fish's body show the location of the bilateral electric organs.

View larger version (17K): [in this window] [in a new window]   Fig. 2. (A) Schematic diagram of the respirometry tank set-up used to record EOD values and oxygen consumption rates simultaneously. EODs were digitized from carbon electrodes at opposite sides of the tank while the fish rested inside an unglazed ceramic tube. This tube was connected to a peristaltic pump and to the oxygen electrodes, which measured oxygen levels in the water as it was pumped at a constant flow rate. ADC, analog–digital converter. (B) Sample data from one male showing reduction of oxygen consumption with the two drug treatments. Metomidate abolishes activity, but not electrogenesis. Flaxedil, a curare analog, blocks the EOD as well. The difference between oxygen consumed at baseline with no drugs versus oxygen consumed following metomidate treatment is proportional to the energetic cost of ventilation and muscle tone plus any locomotion the fish attempts in the respirometry tube (O2,other). The difference between oxygen consumed following metomidate versus flaxedil and metomidate treatment (O2,EOD) is proportional to the energetic cost of electrogenesis at rest. The remaining oxygen consumption (O2,SMR) indicates the relative standard metabolic rate, the energetic cost of cellular metabolism at ambient temperature (27°C).

View larger version (27K): [in this window] [in a new window]   Fig. 3. (A) Measured and modeled means and 95% confidence intervals of sex differences in the day and night energetic costs of bioelectrogenesis (O2,EOD), standard metabolic rate (O2,SMR), and activity (O2,other). The energetic cost of electrogenesis is a small component of the total energy budget in females but a significant component in males. In both sexes, aerobic activity represented a big portion of the total budget and thus, as expected, O2,other (which includes locomotion) increased dramatically at night. Energy budgets at night are modeled two ways: model 1 assumes electrogenesis costs increase linearly with EOD power; model 2 assumes individuals of each sex follow the same relationship between O2,EOD and EOD power as that between individuals of the same sex (see B). (B) In both sexes the oxygen consumed per EOD, and thus its energetic cost, increases proportionally with the EOD power (males: y=3.32e–05x1.80–1.03e–05, R2=0.99; females: logy=1.06 logx–2.64, R2=0.79). Comparing males and females with similar oxygen consumption per EOD indicates that males emit more powerful EODs. However, male costs increase exponentially as EOD power increases. Error lines represent the 95% confidence intervals. One female outlier (not shown) fell more than 4 s.d. beyond the regression line and was removed from all analyses. (C) In males, EOD rates are linearly related to residual O2,EOD corrected for EOD power (R2=0.56). The same relationship probably occurs in females as well, though the measured relationship does not significantly exceed measurement noise (R2=0.15).

View larger version (5K): [in this window] [in a new window]   Fig. 4. Stepwise regression leverage plot shows residual male body mass and O2,EOD after partialing out body length. Body mass adjusted for body length (i.e. how fat a male is for a given length) is a good indicator of body condition. How much energy a male is spending on electrogenesis can be inferred from the relationship between waveform and discharge rate (see Fig. 3B,C and Fig. 5B). Thus the amount of energy spent on electrogenesis, even during the day, can reveal a male's body condition. Dashed lines represent the 95% confidence interval around the regression line.

View larger version (9K): [in this window] [in a new window]   Fig. 5. (A) B. pinnicaudatus of both sexes decrease their discharge rates by day, and males in particular decrease the amplitude and duration of the individual waveforms. (B) Shown for each sex are the relative costs in energetic expense attributable to diurnal reductions of EOD discharge rate (abscissa), EOD waveform (ordinate), and the combination of the two (area).

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