Why the brown ghost chirps at night

Since the pioneering work by Moeller, Szabo, and Bullock, weakly electric fish have served as a valuable model for investigating spatial and social cognitive abilities in a vertebrate taxon usually less accessible than mammals or other terrestrial vertebrates. These fish, through their electric organ, generate low-intensity electric fields to navigate and interact with conspecifics, even in complete darkness. The brown ghost knifefish is appealing as a study subject due to a rich electric 'vocabulary', made by individually variable and sex-specific electric signals. These are mainly characterized by brief frequency modulations of the oscillating dipole moment continuously generated by their electric organ, and are known as chirps. Different types of chirps are believed to convey specific and behaviorally salient information, serving as behavioral readouts for different internal states during behavioral observations. Despite the success of this model in neuroethology over the past seven decades, the code to decipher their electric communication remains unknown. To this aim, in this study we re-evaluate the correlations between signals and behavior offering an alternative, and possibly complementary, explanation for why these freshwater bottom dwellers emit electric chirps. By uncovering correlations among chirping, electric field geometry, and detectability in enriched environments, we present evidence for a previously unexplored role of chirps as specialized self-directed signals, enhancing conspecific electrolocation during social encounters.

Supervised learning algorithm for analysis of communication signals in the weakly electric fish Apteronotus leptorhynchus

Signal analysis plays a preeminent role in neuroethological research. Traditionally, signal identification has been based on pre-defined signal (sub-)types, thus being subject to the investigator's bias. To address this deficiency, we have developed a supervised learning algorithm for the detection of subtypes of chirps-frequency/amplitude modulations of the electric organ discharge that are generated predominantly during electric interactions of individuals of the weakly electric fish Apteronotus leptorhynchus. This machine learning paradigm can learn, from a 'ground truth' data set, a function that assigns proper outputs (here: time instances of chirps and associated chirp types) to inputs (here: time-series frequency and amplitude data). By employing this artificial intelligence approach, we have validated previous classifications of chirps into different types and shown that further differentiation into subtypes is possible. This demonstration of its superiority compared to traditional methods might serve as proof-of-principle of the suitability of the supervised machine learning paradigm for a broad range of signals to be analyzed in neuroethology.

Understanding daily rhythms in weakly electric fish: the role of melatonin on the electric behavior of Brachyhypopomus gauderio

Living organisms display molecular, physiological and behavioral rhythms synchronized with natural environmental cycles. Understanding the interaction between environment, physiology and behavior requires taking into account the complexity of natural habitats and the diversity of behavioral and physiological adaptations. Brachyhypopomus gauderio is characterized by the emission of electric organ discharges (EOD), with a very stable rate modulated by social and environmental cues. The nocturnal arousal in B. gauderio coincides with a melatonin-dependent EOD rate increase. Here, we first show a daily cycle in both the EOD basal rate (EOD-BR) and EOD-BR variability of B. gauderio in nature. We approached the understanding of the role of melatonin in this natural behavior through both behavioral pharmacology and in vitro assays. We report, for the first time in gymnotiformes, a direct effect of melatonin on the pacemaker nucleus (PN) in in vitro preparation. Melatonin treatment lowered EOD-BR in freely moving fish and PN basal rate, while increasing the variability of both. These results show that melatonin plays a key role in modulating the electric behavior of B. gauderio through its effect on rate and variability, both of which must be under a tight temporal regulation to prepare the animal for the challenging nocturnal environment.

The effect of urethane and MS-222 anesthesia on the electric organ discharge of the weakly electric fish Apteronotus leptorhynchus

Urethane and MS-222 are agents widely employed for general anesthesia, yet, besides inducing a state of unconsciousness, little is known about their neurophysiological effects. To investigate these effects, we developed an in vivo assay using the electric organ discharge (EOD) of the weakly electric fish Apteronotus leptorhynchus as a proxy for the neural output of the pacemaker nucleus. The oscillatory neural activity of this brainstem nucleus drives the fish's EOD in a one-to-one fashion. Anesthesia induced by urethane or MS-222 resulted in pronounced decreases of the EOD frequency, which lasted for up to 3 h. In addition, each of the two agents caused a manifold increase in the generation of transient modulations of the EOD known as chirps. The reduction in EOD frequency can be explained by the modulatory effect of urethane on neurotransmission, and by the blocking of voltage-gated sodium channels by MS-222, both within the circuitry controlling the neural oscillations of the pacemaker nucleus. The present study demonstrates a marked effect of urethane and MS-222 on neural activity within the central nervous system and on the associated animal's behavior. This calls for caution when conducting neurophysiological experiments under general anesthesia and interpreting their results.

Electrocommunication signals and aggressive behavior vary among male morphs in an apteronotid fish, Compsaraia samueli

Within-species variation in male morphology is common among vertebrates and is often characterized by dramatic differences in behavior and hormonal profiles. Males with divergent morphs also often use communication signals in a status-dependent way. Weakly electric knifefish are an excellent system for studying variation in male morphology and communication and its hormonal control. Knifefish transiently modulate the frequency of their electric organ discharge (EOD) during social encounters to produce chirps and rises. In the knifefish Compsaraia samueli, males vary extensively in jaw length. EODs and their modulations (chirps and rises) have never been investigated in this species, so it is unclear whether jaw length is related to the function of these signals. We used three behavioral assays to analyze EOD modulations in male C. samueli: (1) artificial playbacks, (2) relatively brief, live agonistic dyadic encounters, and (3) long-term overnight recordings. We also measured circulating levels of two androgens, 11-ketotestosterone and testosterone. Chirp structure varied within and across individuals in response to artificial playback, but was unrelated to jaw length. Males with longer jaws were more often dominant in dyadic interactions. Chirps and rises were correlated with and preceded attacks regardless of status, suggesting these signals function in aggression. In longer-term interactions, chirp rate declined after one week of pairing, but was unrelated to male morphology. Levels of circulating androgens were low and not predictive of jaw length or EOD signal parameters. These results suggest that communication signals and variation in male morphology are linked to outcomes of non-breeding agonistic contests.

Vocal and Electric Fish: Revisiting a Comparison of Two Teleost Models in the Neuroethology of Social Behavior

The communication behaviors of vocal fish and electric fish are among the vertebrate social behaviors best understood at the level of neural circuits. Both forms of signaling rely on midbrain inputs to hindbrain pattern generators that activate peripheral effectors (sonic muscles and electrocytes) to produce pulsatile signals that are modulated by frequency/repetition rate, amplitude and call duration. To generate signals that vary by sex, male phenotype, and social context, these circuits are responsive to a wide range of hormones and neuromodulators acting on different timescales at multiple loci. Bass and Zakon (2005) reviewed the behavioral neuroendocrinology of these two teleost groups, comparing how the regulation of their communication systems have both converged and diverged during their parallel evolution. Here, we revisit this comparison and review the complementary developments over the past 16 years. We (a) summarize recent work that expands our knowledge of the neural circuits underlying these two communication systems, (b) review parallel studies on the action of neuromodulators (e.g., serotonin, AVT, melatonin), brain steroidogenesis (via aromatase), and social stimuli on the output of these circuits, (c) highlight recent transcriptomic studies that illustrate how contemporary molecular methods have elucidated the genetic regulation of social behavior in these fish, and (d) describe recent studies of mochokid catfish, which use both vocal and electric communication, and that use both vocal and electric communication and consider how these two systems are spliced together in the same species. Finally, we offer avenues for future research to further probe how similarities and differences between these two communication systems emerge over ontogeny and evolution.

Encoding and Perception of Electro-communication Signals in Apteronotus leptorhynchus

Animal communication plays an essential role in triggering diverse behaviors. It is believed in this regard that signal production by a sender and its perception by a receiver is co-evolving in order to have beneficial effects such as to ensure that conspecifics remain sensitive to these signals. However, in order to give appropriate responses to a communication signal, the receiver has to first detect and interpret it in a meaningful way. The detection of communication signals can be limited under some circumstances, for example when the signal is masked by the background noise in which it occurs (e.g., the cocktail-party problem). Moreover, some signals are very alike despite having different meanings making it hard to discriminate between them. How the central nervous system copes with these tasks and problems is a central question in systems neuroscience. Gymnotiform weakly electric fish pose an interesting system to answer these questions for various reasons: (1) they use a variety of communication signals called “chirps” during different behavioral encounters; (2) the central physiology of the electrosensory system is well known; and (3) most importantly, these fish give reliable behavioral responses to artificial stimuli that resemble natural communication signals, making it possible to uncover the neural mechanisms that lead to the observed behaviors.

Dominance in Habitat Preference and Diurnal Explorative Behavior of the Weakly Electric Fish Apteronotus leptorhynchus

Electrocommunication and -localization behaviors of weakly electric fish have been studied extensively in the lab, mostly by means of short-term observations on constrained fish. Far less is known about their behaviors in more natural-like settings, where fish are less constrained in space and time. We tracked individual fish in a population of fourteen brown ghost knifefish (Apteronotus leptorhynchus) housed in a large 2 m³ indoor tank based on their electric organ discharges (EOD). The tank contained four different natural-like microhabitats (gravel, plants, isolated stones, stacked stones). In particular during the day individual fish showed preferences for specific habitats which provided appropriate shelter. Male fish with higher EOD frequencies spent more time in their preferred habitat during the day, moved more often between habitats during the night, and less often during the day in comparison to low-frequency males. Our data thus revealed a link between dominance indicated by higher EOD frequency, territoriality, and a more explorative personality in male A. leptorhynchus. In females, movement activity during both day and night correlated positively with EOD frequency. In the night, fish of either sex moved to another habitat after about 6 s on average. During the day, the average transition time was also very short at about 20 s. However, these activity phases were interrupted by phases of inactivity that lasted on average about 20 min during the day, but only 3 min in the night. The individual preference for daytime retreat sites did not reflect the frequent explorative movements at night.

Chirping and asymmetric jamming avoidance responses in the electric fish Distocyclus conirostris

Electrosensory systems of weakly electric fish must accommodate competing demands of sensing the environment (electrolocation) and receiving social information (electrocommunication). The jamming avoidance response (JAR) is a behavioral strategy thought to reduce electrosensory interference from conspecific signals close in frequency. We used playback experiments to characterize electric organ discharge frequency (EODf), chirping behavior and the JAR of Distocyclus conirostris, a gregarious electric fish species. EODs of D. conirostris had low frequencies (∼80-200 Hz) that shifted in response to playback stimuli. Fish consistently lowered EODf in response to higher-frequency stimuli but inconsistently raised or lowered EODf in response to lower-frequency stimuli. This led to jamming avoidance or anti-jamming avoidance, respectively. We compare these behaviors with those of closely related electric fish (Eigenmannia and Sternopygus) and suggest that the JAR may have additional social functions and may not solely minimize the deleterious effects of jamming, as its name suggests.

Influence of long-term social interaction on chirping behavior, steroid levels and neurogenesis in weakly electric fish

Social interactions dramatically affect the brain and behavior of animals. Studies in birds and mammals indicate that socially induced changes in adult neurogenesis participate in the regulation of social behavior, but little is known about this relationship in fish. Here, we review studies in electric fish (Apteronotus leptorhychus) that link social stimulation, changes in electrocommunication behavior and adult neurogenesis in brain regions associated with electrocommunication. Compared with isolated fish, fish living in pairs have greater production of chirps, an electrocommunication signal, during dyadic interactions and in response to standardized artificial social stimuli. Social interaction also promotes neurogenesis in the periventricular zone, which contributes born cells to the prepacemaker nucleus, the brain region that regulates chirping. Both long-term chirp rate and periventricular cell addition depend on the signal dynamics (amplitude and waveform variation), modulations (chirps) and novelty of the stimuli from the partner fish. Socially elevated cortisol levels and cortisol binding to glucocorticoid receptors mediate, at least in part, the effect of social interaction on chirping behavior and brain cell addition. In a closely related electric fish (Brachyhypopomus gauderio), social interaction enhances cell proliferation specifically in brain regions for electrocommunication and only during the breeding season, when social signaling is most elaborate. Together, these studies demonstrate a consistent correlation between brain cell addition and environmentally regulated chirping behavior across many social and steroidal treatments and suggest a causal relationship.

The neuroethology of electrocommunication: how signal background influences sensory encoding and behaviour in Apteronotus leptorhynchus

Weakly-electric fish are a well-established model system for neuroethological studies on communication and aggression. Sensory encoding of their electric communication signals, as well as behavioural responses to these signals, have been investigated in great detail under laboratory conditions. In the wave-type brown ghost knifefish, Apteronotus leptorhynchus, transient increases in the frequency of the generated electric field, called chirps, are particularly well-studied, since they can be readily evoked by stimulating a fish with artificial signals mimicking conspecifics. When two fish interact, both their quasi-sinusoidal electric fields (called electric organ discharge, EOD) superimpose, resulting in a beat, an amplitude modulation at the frequency difference between the two EODs. Although chirps themselves are highly stereotyped signals, the shape of the amplitude modulation resulting from a chirp superimposed on a beat background depends on a number of parameters, such as the beat frequency, modulation depth, and beat phase at which the chirp is emitted. Here we review the influence of these beat parameters on chirp encoding in the three primary stages of the electrosensory pathway: electroreceptor afferents, the hindbrain electrosensory lateral line lobe, and midbrain torus semicircularis. We then examine the role of these parameters, which represent specific features of various social contexts, on the behavioural responses of A. leptorhynchus. Some aspects of the behaviour may be explained by the coding properties of early sensory neurons to chirp stimuli. However, the complexity and diversity of behavioural responses to chirps in the context of different background parameters cannot be explained solely on the basis of the sensory responses and thus suggest that critical roles are played by higher processing stages.

Glucocorticoid receptor blockade inhibits brain cell addition and aggressive signaling in electric fish, Apteronotus leptorhynchus

When animals are under stress, glucocorticoids commonly inhibit adult neurogenesis by acting through glucocorticoid receptors (GRs). However, in some cases, conditions that elevate glucocorticoids promote adult neurogenesis, and the role of glucocorticoid receptors in these circumstances is not well understood. We examined the involvement of GRs in social enhancement of brain cell addition and aggressive signaling in electric fish, Apteronotus leptorhynchus. In this species, long-term social interaction simultaneously elevates plasma cortisol, enhances brain cell addition and increases production of aggressive electrocommunication signals ("chirps"). We implanted isolated and paired fish with capsules containing nothing (controls) or the GR antagonist, RU486, recorded chirp production and locomotion for 7d, and measured the density of newborn cells in the periventricular zone. Compared to isolated controls, paired controls showed elevated chirping in two phases: much higher chirp rates in the first 5h and moderately higher nocturnal rates thereafter. Treating paired fish with RU486 reduced chirp rates in both phases to those of isolated fish, demonstrating that GR activation is crucial for socially induced chirping. Neither RU486 nor social interaction affected locomotion. RU486 treatment to paired fish had a partial effect on cell addition: paired RU486 fish had less cell addition than paired control fish but more than isolated fish. This suggests that cortisol activation of GRs contributes to social enhancement of cell addition but works in parallel with another GR-independent mechanism. RU486 also reduced cell addition in isolated fish, indicating that GRs participate in the regulation of cell addition even when cortisol levels are low.

Echo response to chirping in the weakly electric brown ghost knifefish (Apteronotus leptorhynchus): role of frequency and amplitude modulations

Teleost fish of the order Gymnotiformes are distinguished by their ability to produce electric discharges by means of specialized organs. These electric organ discharges serve various behavioral functions, including communication. During such electric interactions, male brown ghost knifefish ( Apteronotus leptorhynchus (Ellis in Eigenmann, 1912)) generate several types of transient frequency and amplitude modulations (“chirps”) of the otherwise nearly constant discharges. Previous studies have shown that the chirps generated by one individual follow those of the other with a preferred latency of approximately 500–1000 ms. As demonstrated in the present study, signals consisting of either frequency modulations or amplitude modulations are able to trigger this echo response. Signals composed of just amplitude modulations are effective in triggering an echo response only if the reduction in amplitude is large (approximately 40%, relative to baseline of the electric organ discharge of the emitting fish). By contrast, in frequency-modulated signals, a maximum frequency increase as small as 1.2% relative to baseline frequency is sufficient to trigger an echo response. This remarkable sensitivity might be an adaptation for the detection of so-called type-2 chirps, as chirps of this type are composed of rather small frequency increases and negligible amplitude modulations. In line with this hypothesis is the observation that during electric interactions of two fish, the generation of these chirps dominates the production of any of the other five chirp types known.

A central pacemaker that underlies the production of seasonal and sexually dimorphic social signals: anatomical and electrophysiological aspects

Our long-term goal is to approach the understanding of the anatomical and physiological bases for communication signal diversity in gymnotiform fishes as a model for vertebrate motor pattern generation. Brachyhypopomus gauderio emits, in addition to its electric organ discharge (EOD) at basal rate, a rich repertoire of rate modulations. We examined the structure of the pacemaker nucleus, responsible for the EOD rate, to explore whether its high output signal diversity was correlated to complexity in its neural components or regional organization. We confirm the existence of only two neuron types and show that the previously reported dorsal-caudal segregation of these neurons is accompanied by rostral-caudal regionalization. Pacemaker cells are grouped dorsally in the rostral half of the nucleus, and relay cells are mainly ventral and more abundant in the caudal half. Relay cells are loosely distributed from the center to the periphery of the nucleus in correlation to somata size. Our findings support the hypothesis that regional organization enables a higher diversity of rate modulations, possibly offering distinct target areas to modulatory inputs. Since no anatomical or electrophysiological seasonal or sexual differences were found, we explored these aspects from a functional point of view in a companion article.

Phylogenetic comparative analysis of electric communication signals in ghost knifefishes (Gymnotiformes: Apteronotidae)

Electrocommunication signals in electric fish are diverse, easily recorded and have well-characterized neural control. Two signal features, the frequency and waveform of the electric organ discharge (EOD), vary widely across species. Modulations of the EOD (i.e. chirps and gradual frequency rises) also function as active communication signals during social interactions, but they have been studied in relatively few species. We compared the electrocommunication signals of 13 species in the largest gymnotiform family,Apteronotidae. Playback stimuli were used to elicit chirps and rises. We analyzed EOD frequency and waveform and the production and structure of chirps and rises. Species diversity in these signals was characterized with discriminant function analyses, and correlations between signal parameters were tested with phylogenetic comparative methods. Signals varied markedly across species and even between congeners and populations of the same species. Chirps and EODs were particularly evolutionarily labile, whereas rises differed little across species. Although all chirp parameters contributed to species differences in these signals, chirp amplitude modulation, frequency modulation (FM) and duration were particularly diverse. Within this diversity,however, interspecific correlations between chirp parameters suggest that mechanistic trade-offs may shape some aspects of signal evolution. In particular, a consistent trade-off between FM and EOD amplitude during chirps is likely to have influenced the evolution of chirp structure. These patterns suggest that functional or mechanistic linkages between signal parameters(e.g. the inability of electromotor neurons increase their firing rates without a loss of synchrony or amplitude of action potentials) constrain the evolution of signal structure.

Stimulus frequency differentially affects chirping in two species of weakly electric fish: implications for the evolution of signal structure and function

During social interactions, apteronotid electric fish modulate their electric organ discharges (EODs) to produce transient communication signals known as chirps. Chirps vary widely across species and sex in both number and structure. In Apteronotus leptorhynchus, males chirp far more than females and their chirps have greater frequency modulation than those of females. High-frequency chirps are produced by males most often in response to female-like electric signals. As such, they have been hypothesized to function in courtship. The more common low-frequency chirps, produced by both males and females in response to same-sex signals, are hypothesized to function as aggressive signals. To determine whether the two chirp types in the closely related Apteronotus albifrons have similar functions, we stimulated chirping in male and female A. leptorhynchus and A. albifrons with playbacks simulating the EODs of same-sex versusopposite-sex conspecifics. As in A. leptorhynchus, male and female A. albifrons produced low-frequency chirps most often to same-sex signals. Unlike A. leptorhynchus, however, A. albifrons also produced more high-frequency chirps to same-sex stimuli than to opposite-sex stimuli. This suggests that high-frequency chirps in A. albifrons,unlike those in A. leptorhynchus, may not function as courtship signals and that the function of similar chirp types has diversified in Apteronotus. Examples such as this, in which the function of a communication signal has changed in closely related species, are rare. The electrocommunication signals of apteronotids may thus provide a remarkable opportunity to investigate the evolutionary interactions of signal structure and function.

Environmental, seasonal, and social modulations of basal activity in a weakly electric fish

The electric organ discharge (EOD) of weakly electric fish encodes information about species, sex, behavioral, and physiological states throughout the lifetime. Its central command is crucial for sensory-motor coordination, and is therefore the target of plastic mechanisms that adapt fish to environmental and social challenges. The EOD waveform of Brachyhypopomus pinnicaudatus is modulated by environmental factors and the neuroendocrine system. In this study we investigate the effects of water temperature and day-night cycle upon EOD rate in this species during the breeding and non-breeding seasons. During the non-breeding season, EOD rate is a linear function of water temperature and exhibits counterclockwise hysteresis. During breeding, a thermal resistance strategy prevents the decrease of EOD rate to cooling. A nocturnal increase of EOD basal rate independent of water temperature and locomotor activity was demonstrated in isolated non-breeding adults and in male-female dyads all year round. An additional increase of nocturnal EOD rate, probably acting as a social courtship signal, was found in breeding dyads. This additional increase of nocturnal EOD rate could not be fully explained by gonadal maturation and was modulated by social stimuli. This study provides novel data on the complex interactions between environment, reproductive cycle, social behavior, and electromotor output in an advantageous model of the vertebrate central nervous system.

Electric interactions through chirping behavior in the weakly electric fish, Apteronotus leptorhynchus

The weakly electric fish Apteronotus leptorhynchus produces wave-like electric organ discharges distinguished by a high degree of regularity. Transient amplitude and frequency modulations ("chirps") can be evoked in males by stimulation with the electric field of a conspecific. During these interactions, the males examined in this study produced six types of chirps, including two novel ones. Stimulation of a test fish with a conspecific at various distances showed that two electrically interacting fish must be within 10 cm of each other to evoke chirping behavior in the neighboring fish. The chirp rate of all but one chirp type elicited by the neighboring fish was found to be negatively correlated with the absolute value of the frequency difference between the two interacting fish, but independent of the sign of this difference. Correlation analysis of the instantaneous rates of chirp occurrence revealed two modes of interactions characterized by reciprocal stimulation and reciprocal inhibition. Further analysis of the temporal relationship between the chirps generated by the two fish during electric interactions showed that the chirps generated by one individual follow the chirps of the other with a short latency of approximately 500-1,000 ms. We hypothesize that this "echo response" serves a communicatory function.

Reciprocal neural connections between the central posterior/prepacemaker nucleus and nucleus G in the gymnotiform fish, Apteronotus leptorhynchus

The central posterior nucleus of teleost fish is a cluster of neurons in the dorsal thalamus that plays an important role in controlling social behaviors. In the weakly electric gymnotiform fish, Apteronotus leptorhynchus, this nucleus forms a larger complex together with the prepacemaker nucleus, hence called central posterior/prepacemaker nucleus (CP/PPn). This complex is crucially involved in neural control of transient modulations of the electric organ discharge, which occur both spontaneously and in the context of social interactions. This control function is intimately linked to its pattern of connectivity with other brain regions. By employing an in vitro neuronal tract-tracing technique, we have, in the present study, identified a novel reciprocal connection between the CP/PPn and a cell group situated in the region between the ventral thalamus and the inferior lobe. Despite the previous interpretation by other authors of this cell group as the glomerular nucleus, the lack of a projection of this nucleus to the hypothalamus, as also demonstrated in the present investigation, makes such a homology unlikely. We, therefore, interpret this nucleus as a brain structure of unknown homology in other teleosts and suggest 'nucleus G' to identify it.

From oscillators to modulators: behavioral and neural control of modulations of the electric organ discharge in the gymnotiform fish, Apteronotus leptorhynchus

The brown ghost (Apteronotus leptorhynchus) is a weakly electric gymnotiform fish that produces wave-like electric organ discharges distinguished by their enormous degree of regularity. Transient modulations of these discharges occur both spontaneously and when stimulating the fish with external electric signals that mimic encounters with a neighboring fish. Two prominent forms of modulations are chirps and gradual frequency rises. Chirps are complex frequency and amplitude modulations lasting between 20 ms and more than 200 ms. Based on their biophysical characteristics, they can be divided into four distinct categories. Gradual frequency rises consist of a rise in discharge frequency, followed by a slow return to baseline frequency. Although the modulatory phase may vary considerably between a few 100 ms and almost 100 s, there is no evidence for the existence of distinct categories of this type of modulation signal. Stimulation of the fish with external electric signals results almost exclusively in the generation of type-2 chirps. This effect is independent of the chirp type generated by the respective individual under non-evoked conditions. By contrast, no proper stimulation condition is known to evoke the other three types of chirps or gradual frequency rises in non-breeding fish. In contrast to the type-2 chirps evoked when subjecting the fish to external electric stimulation, the rate of spontaneously produced chirps is quite low. However, their rate appears to be optimized according to the probability of encountering a conspecific. As a result, the rate of non-evoked chirping is increased during the night when the fish exhibit high locomotor activity and in the time period following external electric stimulation. These, as well as other, observations demonstrate that both the type and rate of modulatory behavior are affected by a variety of behavioral conditions. This diversity at the behavioral level correlates with, and is likely to be causally linked to, the diversity of inputs received by the neurons that control chirps and gradual frequency rises, respectively. These neurons form two distinct sub-nuclei within the central posterior/prepacemaker nucleus in the dorsal thalamus. In vitro tract-tracing experiments have elucidated some of the connections of this complex with other brain regions. Direct input is received from the optic tectum. Indirect input arising from telencephalic and hypothalamic regions, as well as from the preoptic area, is relayed to the central posterior/prepacemaker nucleus via the preglomerular nucleus. Feedback loops may be provided by projections of the central posterior/prepacemaker nucleus to the preglomerular nucleus and the nucleus preopticus periventricularis.

Diversity in the structure of electrocommunication signals within a genus of electric fish, Apteronotus

Some gymnotiform electric fish modulate their electric organ discharge for intraspecific communication. In Apteronotus leptorhynchus, chirps are usually rapid (10-30 ms) modulations that are activated through non- N-methyl- d-aspartate (non-NMDA) glutamate receptors in the hindbrain pacemaker nucleus. Males produce longer chirp types than females and chirp at higher rates. In Apteronotus albifrons, chirp rate is sexually monomorphic, but chirp structure (change in frequency and amplitude during a chirp) was unknown. To better understand the neural regulation and evolution of chirping behavior, we compared chirp structure in these two species under identical stimulus regimes. A. albifrons, like A. leptorhynchus, produced distinct types of chirps that varied, in part, by frequency excursion. However, unlike in A. leptorhynchus, chirp types in A. albifrons varied little in duration, and chirps were all longer (70-200 ms) than those of A. leptorhynchus. Chirp type production was not sexually dimorphic in A. albifrons, but within two chirp types males produced longer chirps than females. We suggest that species differences in chirp duration might be attributable to differences in the relative proportions of fast-acting (non-NMDA) and slow-acting (NMDA) glutamate receptors in the pacemaker. Additionally, we map species difference onto a phylogeny and hypothesize an evolutionary sequence for the diversification of chirp structure.