Electric Fish Trade-Off Energy between Competing Metabolic Demands

Physiological trade-offs are driven by internal competition of two or more functional demands for the same limited resource. This study is the first to quantify a direct energetic trade-off between reproductive behavior and other metabolic functions. Males of the weakly electric fish  Brachyhypopomus gauderio produce reproductive signals that are among the most energetically costly of any animal measured, consuming up to a third of the animal’s daily energy budget. In this study, we determined that high energetic demand of male electric signals is matched by a metabolic trade-off with other cellular functions. We used thyroxine implants to modulate the signal metabolism, partitioned the energy budget pharmacologically, and measured energy consumption using oxygen respirometry. In males, total energy consumption was unchanged pre- and post-thyroxine treatment, while signal metabolism rose and the standard metabolic rate fell in an even trade-off. In contrast to males, total metabolism in females rose under thyroxine and females traded off metabolic functions in the opposite direction from males, boosting standard metabolic rate at the expense of signal metabolism. These results reveal metabolic trade-offs between signaling and cellular metabolism in electric fish and suggest that thyroid hormones regulate the allocation of energy between electric signals and somatic maintenance in favor of reproduction, likely at the expense of survival.