Sex and species differences in neuromodulatory input to a premotor nucleus: A comparative study of substance P and communication behavior in weakly electric fish

Abstract

Many electric fish species modulate their electric organ discharges (EODs) to produce transient social signals that vary in number and structure. In Apteronotus leptorhynchus, males modulate their EOD more often than females, whereas in Apteronotus albifrons, males and females produce similar numbers of modulations. Sex differences in the number of EOD modulations in A. leptorhynchus are associated with sex differences in substance P in the diencephalic nucleus that controls transient EOD modulations, the CP/PPn. These sex differences in substance P have been hypothesized to regulate sex differences in the production of EOD modulations. To comparatively test this hypothesis, we examined substance P immunoreactivity in the CP/PPn of male and female A. leptorhynchus and A. albifrons. Because the number of EOD modulations is sexually monomorphic in A. albifrons, we predicted no sex difference in substance P in the CP/PPn of this species. Contrary to this prediction, male A. albifrons had significantly more substance P in the CP/PPn than females. This suggests that sex differences in substance P are not sufficient for controlling sex differences in the number of EOD modulations. Modulation structure (frequency excursion and/or duration), however, is also sexually dimorphic in A. leptorhynchus and is another possible behavioral correlate of the sexually dimorphic distribution of substance P. The present study found pronounced sex differences in the structure of EOD modulations in A. albifrons similar to those in A. leptorhynchus. Thus, sex differences in substance P may influence sex differences in the structure, rather than the number, of EOD modulations.

Evolution and hormonal regulation of sex differences in the electrocommunication behavior of ghost knifefishes (Apteronotidae)

“…The function of rises is unclear, in part because their variability has made them difficult to categorize and compare across studies. Unlike chirps, rises are often sexually monomorphic Kolodziejski et al, 2005;Zhou and Smith, 2006). Also unlike chirps, EOD playbacks to fish in chirp chambers inhibit rather than stimulate the production of rises in A. leptorhynchus, A. albifrons and A. bonapartii Kolodziejski et al, 2007).…”

“…High-frequency chirps have sometimes been subdivided into three types (types 1, 3 and 4) based on variation in duration and the frequency undershoot (Engler et al, 2000;Engler and Zupanc, 2001). Type 3 and 4 chirps are rare (<1% of chirps) (Engler and Zupanc, 2001;Kolodziejski et al, 2005), however, and it is unclear whether they have distinct functions or are one end of a continuum of high-frequency chirps. Two other chirp types (types 5 and 6) have been reported in interacting male A. leptorhynchus (Zupanc et al, 2006).…”

“…The chirps of A. albifrons, however, are much longer in duration than those of A. leptorhynchus, and A. albifrons high-frequency chirps lack frequency undershoots (Dunlap and Larkins-Ford, 2003a;Kolodziejski et al, 2005). Several other species (e.g.…”

“…The nature of sex differences in chirp structure varies across species. In A. leptorhynchus, males produce highfrequency chirps, which are produced very rarely by females, and the low-frequency chirps of females have less FM than male lowfrequency chirps (Fig.2) (Bastian et al, 2001;Kolodziejski et al, 2005). Males also produce chirps with greater FM in A. albifrons and S. nattereri (Formby et al, 2009;Kolodziejski et al, 2005).…”

“…In A. leptorhynchus, males produce highfrequency chirps, which are produced very rarely by females, and the low-frequency chirps of females have less FM than male lowfrequency chirps (Fig.2) (Bastian et al, 2001;Kolodziejski et al, 2005). Males also produce chirps with greater FM in A. albifrons and S. nattereri (Formby et al, 2009;Kolodziejski et al, 2005). In A. albifrons and P. hasemani, chirps are sexually dimorphic in duration; male chirps are longer than those of females (Dunlap and Larkins-Ford, 2003a;Kolodziejski et al, 2005;Petzold and Smith, 2012b).…”