A central pacemaker that underlies the production of seasonal and sexually dimorphic social signals: functional aspects revealed by glutamate stimulation

The cyclic enrichment of behavioral repertoires is a common event in seasonal breeders. Breeding males Brachyhypopomus gauderio produce electric organ discharge (EOD) rate modulations called chirps while females respond with interruptions. The electromotor system is commanded by a pacemaker nucleus (PN) which sets the basal rate and produces the rate modulations. We focused on identifying functional, seasonal and sexual differences in this nucleus in correlation to these differences in behavior. The in vivo response to glutamate injection in the PN was seasonal, sexually dimorphic and site specific. Non-breeding adults and breeding females injected in dorsal and ventral sites generated EOD rate increases and interruptions, respectively. Reproductive males added a conspicuous communication signal to this repertoire. They chirped repetitively when we injected glutamate in a very restricted area of the ventral–rostral nucleus, surprisingly one with a low number of relay cell somata. This study shows that the PN is functionally organized in regions in a caudal–rostral axis, besides the previously documented dorsal–ventral division. Functional regions are revealed by seasonal changes that annually provide this nucleus with the cellular mechanisms that allow the bursting activity underlying chirp production, only in males.     

Sex differences in the electrocommunication signals of the electric fish Apteronotus bonapartii

The South American weakly-electric knifefish (Apteronotidae) produce highly diverse and readily quantifiable electrocommunication signals. The electric organ discharge frequency (EODf), and EOD modulations (chirps and gradual frequency rises (GFRs)), vary dramatically across sexes and species, presenting an ideal opportunity to examine the proximate and ultimate bases of sexually dimorphic behavior. We complemented previous studies on the sexual dimorphism of apteronotid communication signals by investigating electric signal features and their hormonal correlates in Apteronotus bonapartii, a species which exhibits strong sexual dimorphism in snout morphology. Electrocommunication signals were evoked and recorded using a playback paradigm, and were analyzed for signal features including EOD frequency and the structure of EOD modulations. To investigate the androgenic correlates of sexually dimorphic EOD signals, we measured plasma concentrations of testosterone and 11-ketotestosterone. A. bonapartii responded robustly to stimulus playbacks. EODf was sexually monomorphic, and males and females produced chirps with similar durations and amounts of frequency modulation. However, males were more likely than females to produce chirps with multiple frequency peaks. Sexual dimorphism in apteronotid electrocommunication signals appears to be highly evolutionarily labile. Extensive interspecific variation in the magnitude and direction of sex differences in EODf and in different aspects of chirp structure suggest that chirp signals may be an important locus of evolutionary change within the clade. The weakly-electric fish represent a rich source of data for understanding the selective pressures that shape, and the neuroendocrine mechanisms that underlie, diversity in the sexual dimorphism of behavior.     

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

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.     

Brain androgen receptor expression correlates with seasonal changes in the behavior of a weakly electric fish,Brachyhypopomus gauderio

Seasonal breeders are superb models for understanding natural relationships between reproductive behavior and its neural bases. We investigated the cellular bases of hormone effects in a weakly pulse-type electric fish with well-defined hormone-sensitive communication signals. Brachyhypopomus gauderio males emit social electric signals (SESs) consisting of rate modulations of the electric organ discharge during the breeding season. This discharge is commanded by a medullary pacemaker nucleus (PN), composed of pacemaker and relay neurons. We analyzed the contribution of androgen receptor (AR) expression to the seasonal generation of SESs, by examining the presence of ARs in the PN in different experimental groups: breeding, non-breeding, and testosterone (T)-implanted non-breeding males. AR presence and distribution in the CNS was assessed through western blotting and immunohistochemistry using the PG-21 antibody, which was raised against the human AR. We found AR immunoreactivity, for the first time in a pulse-type Gymnotiform, in several regions throughout the brain. In particular, this is the first report to reveal the presence of AR in both pacemaker and relay neurons within the Gymnotiform PN. The AR immunoreactivity was present in breeding males and could be induced in T-implanted non-breeding males. This seasonal and T-induced AR expression in the PN suggests that androgens may play an important role in the generation of SESs by modulating intrinsic electrophysiological properties of pacemaker and relay neurons.     

Androgens enhance plasticity of an electric communication signal in female knifefish, Brachyhypopomus pinnicaudatus

Sex steroids were initially defined by their actions shaping sexually dimorphic behavioral patterns. More recently scientists have begun exploring the role of steroids in determining sex differences in behavioral plasticity. We investigated the role of androgens in potentiating circadian, pharmacological, and socially-induced plasticity in the amplitude and duration of electric organ discharges (EODs) of female gymnotiform fish. We first challenged female fish with injections of serotonin (5-HT) and adrenocorticotropic hormone (ACTH), and with social encounters with female and male conspecifics to characterize females' pre-implant responses to each treatment. Each individual was then implanted with a pellet containing dihydrotestosterone (DHT) concentrations of 0.0, 0.03, 0.1, 0.3, or 1.0 mg 10 g^− 1 body weight. We then repeated all challenges and compared each female's pre- and post-implant responses. The highest implant dose enhanced EOD duration modulations in response to all challenge types, responses to male challenge were also greater at the second highest dose, and responses to ACTH challenge were enhanced in females receiving all but the smallest dose (and blank) implants. Alternatively, amplitude modulations were enhanced only during female challenges and only when females received the highest DHT dose. Our results highlight the differential regulation of EOD duration and amplitude, and suggest that DHT enhanced the intrinsic plasticity of the electrogenic cells that produce the EOD rather than modifying behavioral phenotypes. The relative failure of DHT to enhance EOD amplitude plasticity also implies that factors other than androgens are involved in regulating/promoting male-typical EOD circadian rhythms and waveform modulations displayed in social contexts.     

Effects of sex, sensitivity and status on cue recognition in the weakly electric fish Apteronotus leptorhynchus

Maintaining a stable social organization necessitates that animals recognize their own dominance status relative to the status of other group members. The weakly electric brown ghost knifefish emits a sexually dimorphic sinusoidal electric organ discharge (EOD) for electrolocation. Dominant males discharge at the highest and females at the lowest EOD frequencies (EODFs). Each individual is most sensitive to its own EODF, which can be modulated for communication. To examine how sensitivity and social status influence an individual's response to different cues, we recorded the electrical signals emitted by 10 males and seven females in response to playbacks of sine waves mimicking a wide range of con- and heterospecific EODFs. While all individuals emit small chirps (LoCs) mostly to stimuli around their own EODF, they are more likely to emit rises (gradual nonchirp signals) to frequencies to which they are less sensitive; males similarly emit larger chirps (HiCs) to frequencies more distant from their own, especially to female mimics. Males with ‘dominant’ EODFs are less likely to emit rises, stimuli in the female range elicit more rises from both sexes, and females emit rises to male EOD mimics. Although low-ranking male EOD mimics elicit more LoCs from all males, males with lower-ranking EODFs chirp less at high EOD mimics than males with high-ranking EODFs chirp at low EOD mimics. We conclude that (1) although much of the variation in an individual's response is attributable to its sensitivity, individuals recognize sexual and status cues and have some internal representation of their own social status, and (2) whereas LoCs appear to function in intrasexual aggression, HiCs and rises could be used in both courtship and submissive signalling. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.      

Social electric signals in freely moving dyads of <Emphasis Type="Italic">Brachyhypopomus pinnicaudatus</Emphasis>

Brachyhypopomus pinnicaudatus (pulse-type weakly electric fish) is a gregarious species that displays reproductive behavior and agonistic encounters between males only during the breeding season. During social interactions, in addition to its basal electric organ discharge (EOD), fish emit social electric signals (SESs) in the contexts of reproduction and intrasexual aggression. We reproduced natural behavior in laboratory settings: SESs recorded in the field are indistinguishable from those observed in our experimental setup. SESs are nocturnal, change seasonally and exhibit sexual dimorphism. This study provides an exhaustive characterization and classification of SESs produced by males and females during the breeding season. In male–female dyads, males produce accelerations and chirps while females interrupt their EODs. The same SESs are observed in male–male dyads. We present a novel, thorough classification of male chirps into four independent types (A, B, C, and M) based on their duration and internal structure. The type M chirp is only observed in male–male dyads. Chirps and interruptions, both in male–female and male–male dyads, are emitted in bouts, which are also grouped throughout the night. Our data suggest the existence of a sophisticated electric dialog during reproductive and aggressive interaction whose precise timing and behavioral significance are being investigated.     

Non‐sex‐biased Dominance in a Sexually Monomorphic Electric Fish: Fight Structure and Submissive Electric Signalling

Males and females commonly compete for limited resources. When interaction costs are similar for both sexes and there are no sexual differences in resource value estimation, a non‐sex‐biased dominance is expected. Moreover, only non‐sex‐biased assessment of contenders fighting ability (Resource Holding Potential, RHP) should influence contest decisions. To test these predictions, we evaluated non‐breeding agonistic intra‐ and intersexual dyadic interactions in the weakly electric fish, Gymnotus omarorum. During the non‐breeding season, resource value is not expected to depend on individuals’ reproductive status and should thus be equal for males and females. In addition, as G. omarorum presents no sexual differences in body size, interaction costs can be considered symmetric between sexes. We confirmed that body size differences, but not individuals’ gender, is the best predictor of dominance. We correlated RHP asymmetries with contest duration and evidenced that body size but not sex influences assessment in intrasexual and intersexual encounters. All dyads tested engaged in agonistic interactions (N = 33) in which a clear dominant emerged. The analysis of conflict phases evidenced the submissive role of electric displays. Electric organ discharge (EOD) interruptions appear early in the contest as an electric hiding attempt, whereas chirps are post‐resolution signals of subordinate status. Interestingly, the decision of interrupting the EOD was also influenced by RHP asymmetries, whereas chirping activity was influenced by the intensity of the attacks received. Our results confirm that body size is the best RHP proxy in non‐breeding intra‐ and intersexual contests of this monomorphic species and demonstrated a sequential pattern of submissive signalling by means of two different electric displays.     

Single electrocytes produce a sexually dimorphic signal in South American electric fish,Hypopomus occidentalis (Gymnotiformes,Hypopomidae)

Summary Hypopomus occidentalis is a weakly electric Gymnotiform fish with a pulse-type electric organ discharge (EOD).Hypopomus used in this study were taken from one of the northernmost boundaries of this species, the Atlantic drainage of Panama where the animals breed at the beginning of the dry season (December). In normal breeding populations,Hypopomus occidentalis exhibit a sexual dimorphism in EOD and morphology. Mature males are large with a broad tail and have an EOD characterized by a low peak power frequency. Females and immature males are smaller, having a slender tail and EODs with higher peak power frequencies (Fig. 1). This study describes differences in the EOD and electric organ morphology between breeding field populations of male and femaleHypopomus. Changes in physiology, morphology and EOD shape which may accompany this seasonal change were examined in steroid injected fish, using standard histological and physiological techniques.A group of females were injected with hormones (5-dihydrotestosterone (DHT), estrogen or saline) to assess changes in their morphology and EOD. Animals treated with DHT developed characteristics which mimicked the sexually dimorphic characteristics of a male, while the other groups did not (see Fig. 5). Tissue from the tails of breeding males and females, and females treated with DHT, were sampled to measure the size of the electrocytes in the tail. The broader tail of males and DHT-females is composed of large electrocytes, whereas the slender tail of normal females is composed of smaller electrocytes. Therefore, the increase in the tail width in the female DHT group is caused by an enlargement of the electrocytes in this area.Intracellular recordings from the electrocytes of saline and DHT injected females show a difference in the responses of the rostral faces of the electrocytes from the two groups, which reflect the differences in their EODs. Saline-treated animals had symmetrical EODs (the first and second phase of the EOD were equal in duration and amplitude), while the physiological responses from each face of the electrocytes yielded responses that were similarly equal in duration and amplitude. DHT-treated animals had asymmetrical EODs (the first phase of the EOD was similar to that of saline treated fish and larger in amplitude and shorter in duration than the second phase) and the physiological responses of the electrocytes reflected this asymmetry. The differential recordings across the caudal face were similar to those from saline treated fish, while the responses from the rostral face were longer in duration and smaller in amplitude.These data suggest that the effects of androgens underlie the changes in single electrocytes which produce the sexually dimorphic signals and morphology present in natural breeding populations ofHypopomus occidentalis.     

Androgen correlates of socially induced changes in the electric organ discharge waveform of a mormyrid fish

Weakly electric fish from the family Mormyridae produce pulsatile electric organ discharges (EODs) for use in communication. For many species, male EODs are seasonally longer in duration than those of females, and among males, there are also individual differences in EOD duration. While EOD elongation can be induced by the administration of exogenous androgens, androgen levels have never before been assessed under natural or seminatural conditions. By simulating the conditions occurring during the breeding season in the laboratory, we provide evidence of a sex difference in EOD duration as well as document levels of circulating androgens in males. In this study, we analyzed the nature of social influences on male EOD duration and plasma androgen levels in Brienomyrus brachyistius. Individual males, first housed with a single female and then placed into social groups consisting of three males and three females, showed status-dependent changes in EOD duration. Top-ranking males experienced a relatively large increase in EOD duration. Second-ranking males experienced a more modest increase, and low-ranking males experienced a decrease in EOD duration. These changes were paralleled by differences in circulating levels of plasma 11-ketotestosterone (11-KT), but not testosterone, suggesting that the changes in EOD duration may have been mediated by changes in plasma 11-KT levels. Thus, it appears that EOD duration is an accurate indicator of male status, which is under social and hormonal control.     

EOD modulations of brown ghost electric fish: JARs, chirps, rises, and dips.

Weakly electric "wave" fish make highly regular electric organ discharges (EODs) for precise electrolocation. Yet, they modulate the ongoing rhythmicity of their EOD during social interactions. These modulations may last from a few milliseconds to tens of minutes. In this paper we describe the different types of EOD modulations, what they may signal to recipient fish, and how they are generated on a neural level. Our main conclusions, based on a species called the brown ghost (Apteronotus leptorhynchus) are that fish: (1) show sexual dimorphism in the signals that they generate; (2) make different signals depending on Whether they are interacting with a fish of the opposite sex or, within their own sex, to a fish of that which is dominant or subordinate to it; (3) are able to assess relative dominance from electrical cues; (4) have a type of plasticity in the pacemaker nucleus, the control center for the EOD, that occurs after stimulation of NMDA receptors that causes a long-lasting (tens of minutes to hours) change in EOD frequency; (5) that this NMDA receptor-dependent change may occur in reflexive responses, like the jamming avoidance response (JAR), as well as after certain long-lasting social signals. We propose that NMDA-receptor dependent increases in EOD frequency during the JAR adaptively shift the EOD frequency to a new value to avoid jamming by another fish and that such increases in EOD frequency during social encounters may be advantageous since social dominance seems to be positively correlated with EOD frequency in both sexes.     

Sex differences in the electrocommunication signals of Sternarchogiton nattereri (Gymnotiformes: Apteronotidae)

In this study we examined electrocommunication behavior in Sternarchogiton nattereri (Apteronotidae), a weakly electric fish from South America. We focused on variation between females and males lacking external dentition and used playbacks of simulated conspecifics to elicit chirps (modulations of their electric organ discharge, EOD). Chirp responses were not affected by the frequency of the playback stimulus. EOD frequency, chirp rate, and chirp duration were not sexually dimorphic; however, the amount of chirp frequency modulation was significantly greater in toothless males than in females. These results reinforce that sex differences in chirp structure are highly diverse and widespread in the Apteronotidae.     

Serotonin in a diencephalic nucleus controlling communication in an electric fish: sexual dimorphism and relationship to indicators of dominance

Serotonin regulates aggressive behavior. The production or release of serotonin is sexually dimorphic and related to social rank in many species. We examined serotonin expression in the central posterior/prepacemaker nucleus (CP/PPn) of the electric fish Apteronotus leptorhynchus. The CP/PPn is a thalamic nucleus that controls agonistic and reproductive electrocommunication signals known as chirps and gradual frequency rises. In parts of the CP/PPn that control chirping, females had more than twice as many serotonergic fibers and terminals as did males. Serotonin immunoreactivity in chirp-controlling areas of the CP/PPn was also negatively correlated with two indicators of dominance: electric organ discharge (EOD) frequency and body mass. Within sexes, the negative correlation between EOD frequency and serotonergic innervation of the PPn was significant in females, but not in males. Females with higher EOD frequencies had less serotonin in the CP/PPn than did females with lower EOD frequencies. Thus, the CP/PPn contained more serotonin in females than in males, and in particular, more serotonin in females with EOD frequencies typical of social subordinates than in females with EOD frequencies typical of social dominants. These results, combined with previous findings that serotonin inhibits chirping and that females chirp much less than males, suggest that serotonin may link sex, social rank, and the production of agonistic communication signals. The relative simplicity of the neural circuits that control the EOD and chirping make the electromotor system well-suited for studying the cellular, physiological, and behavioral mechanisms by which serotonin modulates agonistic communication.     

Interruption of pacemaker signals by a diencephalic nucleus in the African electric fish, Gymnarchus niloticus

The African electric fish Gymnarchus niloticus rhythmically emits electric organ discharges (EODs) for communication and navigation. The EODs are generated by the electric organ in the tail in response to the command signals from the medullary pacemaker complex, which consists of a pacemaker nucleus (PN), two lateral relay nuclei (LRN) and a medial relay nucleus (MRN). The premotor structure and its modulatory influences on the pacemaker complex have been investigated in this paper. A bilateral prepacemaker nucleus (PPn) was found in the area of the dorsal posterior nucleus (DP) of the thalamus by retrograde labeling from the PN. No retrogradely labeled neurons outside the pacemaker complex were found after tracer injection into the LRN or MRN. Accordingly, anterogradely labeled terminal fibers from PPn neurons were found only in the PN. Iontophoresis of l-glutamate into the region of the PPn induced EOD interruptions. Despite the exclusive projection of the PPn neurons to the PN, extracellular and intracellular recordings showed that PN neurons continue their firing while MRN neurons ceased their firing during EOD interruption. This mode of EOD interruption differs from those found in any other weakly electric fishes in which EOD cessation mechanisms have been known.     

Mate preference in female electric fish, Brachyhypopomus pinnicaudatus

Weakly electric fish communicate with brief electrostatic field pulses called electric organ discharges (EODs). EOD waveforms are sexually dimorphic in most genera, a condition thought to result from mate choice acting to shape the electric signal's constituent action potentials. We have no direct behavioural evidence that sexual selection by either mate choice or intrasexual competition is responsible for sex differences in the EOD waveforms of electric fish. We explored sexual selection in electric fish by conducting two-choice unforced preference tests with live, unaltered gymnotiform electric fish,Brachyhypopomus pinnicaudatus , which are sexually dimorphic. In the initial test, gravid females selected males over females only when the males were larger than average. Gravid females in later tests preferred larger males to smaller males in a significant majority of those trials in which they showed a preference. In about one-third of those trials, females spawned with their preferred male, confirming their preference. We concluded that passage through the choice apparatus was related to mate choice. The signals of chosen males had larger EOD amplitudes and longer EOD durations. These findings show that femaleB. pinnicaudatus do have a preference for a certain male phenotype. The system requires additional study to dissociate correlated male phenotypic characters to identify which male traits the female prefers. Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.      

Human chorionic gonadotropin-induced shifts in the electrosensory system of the weakly electric fish, Sternopygus

Sternopygus macrurus of both sexes were injected with human chorionic gonadotropin (hCG) or saline. Electric organ discharge (EOD) frequency rose after hCG injections in females and gradually declined to baseline levels over the next few weeks. EOD changes in males were more complex and variable; most males showed an initial minor rise in EOD frequency followed by a larger decrease, or simply a decrease. hCG treatment also resulted in a rise in electroreceptor best frequency and shortened electric organ pulse duration in females, and had the opposite effect on these parameters in males. The saline-injected controls showed no changes in any of these parameters. Levels of testosterone (T) and 11-ketotestosterone, but not estrogen (E), were elevated in males preceding the fall in EOD frequency, whereas neither T nor E changed significantly in females before EOD frequency increases. Saline injections caused a drop in T in the male control group and had no effect in the female control group. We presume that the effect of hCG on the electrosensory system of males is mediated via androgens. Whether the effects of hCG on females are mediated by slight increases in circulating levels of gonadal steroids, the release of hormones other than T or E, or are due to direct effects on the nervous system is not known.     

The evolutionary origins of electric signal complexity.

This study explores the evolutionary origins of waveform complexity in electric organ discharges (EODs) of weakly electric fish. I attempt to answer the basic question of what selective forces led to the transition from the simplest signal to the second simplest signal in the gymnotiform electric fishes. The simplest electric signal is a monophasic pulse and the second simplest is a biphasic pulse. I consider five adaptive hypotheses for the evolutionary transition from a monophasic to a biphasic EOD: (i) electrolocation, (ii) sexual selection, (iii) species isolation, (iv) territory defense, (v) crypsis from electroreceptive predators. Evaluating these hypotheses with data drawn largely from the literature, I find best support for predation. Predation is typically viewed as a restraining force on evolution of communication signals, but among the electric fishes, predation appears to have served as a creative catalyst. In suppressing spectral energy in the sensitivity range of predators (a spectral simplification), the EOD waveforms have become more complex in their time domain structure. Complexity in the time domain is readily discernable by the high frequency electroreceptor systems of gymnotiform and mormyrid electric fish. The addition of phases to the EOD can cloak the EOD from predators, but also provides a substrate for subsequent modification by sexual selection. But, while juveniles and females remain protected from predators, breeding males modify their EODs in ways that enhance their conspicuousness to predators.     

Electrocommunication signals in female brown ghost electric knifefish, Apteronotus leptorhynchus

Female communication behaviors are often overlooked by researchers in favor of male behaviors, which are usually more overt and easier to elicit. Very little is known about female electrocommunication behaviors in brown ghost knifefish, a weakly electric wavetype Gymnotiform fish. Most behavioral studies have focused on males, and fish are usually restrained and played a stimulus near their own electric organ discharge frequency to evoke chirps (abrupt short-term frequency rises) or the jamming avoidance response. Our study focuses on categorizing and describing spontaneous and evoked electric organ discharge modulations in free-swimming female fish that were either electrically coupled to tanks containing a conspecific (male or female), or left isolated. Cluster analysis of signals produced under isolated and social conditions revealed three categories of rises: short rise, medium rise and long rise; and one category of frequency decrease (dip). Females produce significantly more short rises when electrically coupled to tanks containing lower-frequency females, and produce more long rises when electrically coupled to tanks containing males. Short rises may have an intrasexual aggressive function, while long rises may serve as an advertisement of status or reproductive condition in intersexual interactions.     

Seasonal activation and inactivation of song motor memories in wild canaries is not reflected in neuroanatomical changes of forebrain song areas

Seasonal, testosterone-dependent changes in sexual behaviors are common in male vertebrates. In songbirds such seasonal changes occur in a learned behavior--singing. Domesticated male canaries (Serinus canaria) appear to lose song units (syllables) after the breeding season and learn new ones until the next breeding season. Here we demonstrate in a longitudinal field study of individual, free-living nondomesticated (wild) canaries (S. canaria) a different mode of seasonal behavioral plasticity, seasonal activation, and inactivation of auditory-motor memories. The song repertoire composition of wild canaries changes seasonally: about 25% of the syllables are sung seasonally; the remainder occur year-round, despite seasonal changes in the temporal patterns of song. In the breeding season, males sing an increased number of fast frequency-modulated syllables, which are sexually attractive for females, in correlation with seasonally increased testosterone levels. About 50% of the syllables that were lost after one breeding season reappear in the following breeding season. Furthermore, some identical syllable sequences are reactivated on an annual basis. The seasonal plasticity in vocal behavior occurred despite the gross anatomical and ultrastructural stability of the forebrain song control areas HVc and RA that are involved in syllable motor control.     

Hormone-induced and maturational changes in electric organ discharges and electroreceptor tuning in the weakly electric fishApteronotus

Summary Plasticity in the frequency of the electric organ discharge (EOD) and electroreceptor tuning of weakly electric fish was studied in the genusApteronotus. Both hormone-induced and maturational changes in EOD frequency and electroreceptor tuning were examined.Apteronotus is different from all other steroid-responsive weakly electric fish in that estradiol-17, rather than androgens, induces discharge frequency decreases. This result can account for the reversed discharge frequency dimorphism found inApteronotus in which, counter to all other known sexually dimorphic electric fish, females have lower discharge frequencies than males. Studies of electroreceptor tuning inApteronotus indicate that electroreceptors are closely tuned to the frequency of the EOD. This finding was noted not only in adult animals, but also in juvenile animals shortly after the onset of their EODs. Tuning plasticity inApteronotus, as in other species studied, is associated with altered EOD frequencies and was noted in both maturational EOD changes and in estrogen-induced changes. Thus, tuning plasticity appears to be a general phenomenon which occurs concurrent with a variety of EOD changes.