Neural activity patterns in response to interspecific and intraspecific variation in mating calls in the túngara frog

Tuned in to communication sounds: Neuronal sensitivity in the túngara frog midbrain to frequency modulated signals

For complex communication signals, it is often difficult to identify the information-bearing elements and their parameters necessary to elicit functional behavior. Consequently, it may be difficult to design stimuli that test how neurons contribute to communicative processing. For túngara frogs (Physalaemus pustulosus), however, previous behavioral testing with numerous stimuli showed that a particular frequency modulated (FM) transition in the male call is required to elicit phonotaxis and vocal responses. Modeled on such behavioral experiments, we used awake in vivo recordings of single units in the midbrain to determine if their excitation was biased to behaviorally important FM parameters. Comparisons of stimulus driven action potentials revealed greatest excitation to the behaviorally important FM transition: a downward FM sweep or step that crosses ~600 Hz. Previous studies using long-duration acoustic exposure found immediate early gene expression in many midbrain neurons to be most sensitive to similar FM. However, those data could not determine if FM coding was accomplished by the population and/or individual neurons. Our data suggest both coding schemes could operate, as 1) individual neurons are more sensitive to the behaviorally significant FM transition and 2) when single unit recordings are analytically combined across cells, the combined code can produce high stimulus discrimination (FM vs. noise driven excitation), approaching that found in behavioral discrimination of call vs. noise.

Mechanisms, development, and comparative perspectives on experience-dependent plasticity in social behavior

Revealing the mechanisms underlying experience-dependent plasticity is a hallmark of behavioral neuroscience. While the study of social behavior has focused primarily on the neuroendocrine and neural control of social behaviors, the plasticity of these innate behaviors has received relatively less attention. Here, we review studies on mating-dependent changes to social behavior and neural circuitry across mammals, birds, and reptiles. We provide an overview of species similarities and differences in the effects of mating experiences on motivational and performative aspects of sexual behaviors, on sensory processing and preferences, and on the experience-dependent consolidation of sexual behavior. We also discuss recent insights into the neural mechanisms of and developmental influences on mating-dependent changes and outline promising approaches to investigate evolutionary parallels and divergences in experience-dependent plasticity.

Understanding the Role of Incentive Salience in Sexual Decision-Making

In the search for understanding female sexual decision-making, progress has been made in uncovering a variety of perceptual biases and most of these concern the animal's sensory biology and cognitive processes. We are now poised to dig deeper into the female's decision-making and ask if incentive salience, which plays a role in all types of appetitive behaviors, also influences a female's "taste for the beautiful." The incentive salience hypothesis suggests that dopamine assigns value or salience to objects or actions. After value is assigned to all potential actions, an action selection system then chooses among potential options to select the most valuable action. In this view, dopamine stimulates reward-seeking behavior by assigning incentive salience to specific behavioral actions, which in turn, increases pursuit and focus on objects or stimuli that represent the valuable action. Here, we apply this framework to understand why females are compelled to respond maximally to some male courtship signals over others and how this process may reveal a female's hidden mate preferences. We examine studies of dopamine and the mesolimbic reward system because these may play a role in expanding the female's perceptual landscape for novelty in male courtship signals and establishing novel hidden preferences. We review three avenues of research that may identify signatures of incentive salience in females during sexual decision-making. This review includes studies of dopamine agonist or antagonist administration in females during mate choice or partner preference tests, measures of neural activity in dopaminergic neural circuits during mate choice or partner preference tests, and social regulation of dopamine in females when entering reproductive contexts and/or exposure to mate signals. By applying the incentive salience hypothesis to female reproductive decision-making, it redefines how we see the female's role in sexual encounters. Females cannot be considered passive during reproductive encounters; rather they are seeking sexual encounters, particularly with males that tap into their perceptual biases and initiate a reward-seeking response. Incentive salience applied to reproductive behavior requires considering females as viewing sexual stimuli as rewarding and initiating action to seek out this reward, all of which indicates females are driving sexual encounters.

Beyond cognitive templates: re-examining template metaphors used for animal recognition and navigation

The term 'cognitive template' originated from work in human-based cognitive science to describe a literal, stored, neural representation used in recognition tasks. As the study of cognition has expanded to non-human animals, the term has diffused to describe a wider range of animal cognitive tools and strategies that guide action through the recognition of and discrimination between external states. One potential reason for this non-standardized meaning and variable employment is that researchers interested in the broad range of animal recognition tasks enjoy the simplicity of the cognitive template concept and have allowed it to become shorthand for many dissimilar or unknown neural processes without deep scrutiny of how this metaphor might comport with underlying neurophysiology. We review the functional evidence for cognitive templates in fields such as perception, navigation, communication, and learning, highlighting any neural correlates identified by these studies. We find that the concept of cognitive templates has facilitated valuable exploration at the interface between animal behavior and cognition, but the quest for a literal template has failed to attain mechanistic support at the level of neurophysiology. This may be the result of a misled search for a single physical locus for the 'template' itself. We argue that recognition and discrimination processes are best treated as emergent and, as such, may not be physically localized within single structures of the brain. Rather, current evidence suggests that such tasks are accomplished through synergies between multiple distributed processes in animal nervous systems. We thus advocate for researchers to move towards a more ecological, process-oriented conception, especially when discussing the neural underpinnings of recognition-based cognitive tasks.

Neurobiology of Female Mate Choice in Frogs: Auditory Filtering and Valuation

Mate choice is a decision making process with profound implication for the reproductive success of both the sender and the chooser. Preferences for conspecific over heterospecific males and for some conspecifics over others are typically mediated by a female's response to signals produced by males. And although one can experimentally describe a female's preference function, there is relatively little understood about the neural mechanisms mediating these preferences. In anurans, mating preferences have often been explained in terms of sensory biases. Indeed, in the túngara frog (Physalaemus pustulosus), the auditory system appears to act as a filter for conspecific calls. However, auditory responses are not good predictors of intraspecific mating preferences in túngara frogs. Rather, neural activity in the preoptic area, which can be gated by estradiol, is a better predictor of mating preferences. A similar pattern holds in spadefoot toads (Spea bombifrons): the preoptic area, but not the auditory midbrain, integrates physiological cues in its response to mating calls in a pattern that predicts preferences. Neuroanatomically, the anuran preoptic area is poised to mediate forebrain influences on auditory response of the midbrain and it has descending projections to the medulla and spinal cord that could directly influence motor responses. Indeed, lesions of the preoptic area abolish phonotaxis. A role for the preoptic area in mating preferences is supported by studies in mammals that show the preoptic area is required for the expression of preferences. Further, activity of the preoptic area correlates with mating preference in fish. This leads to a model for the neurobiological mechanisms of mate choice, in which sensory systems filter relevant signals from irrelevant ones, but the preoptic area assigns value to the range of relevant signals.

Sex differences in behavioural and neural responsiveness to mate calls in a parrot

Vocalisation in songbirds and parrots has become a prominent model system for speech and language in humans. We investigated possible sex differences in behavioural and neural responsiveness to mate calls in the budgerigar, a vocally-learning parrot. Males and females were paired for 5 weeks and then separated, after which we measured vocal responsiveness to playback calls (a call of their mate versus a call of an unfamiliar conspecific). Both sexes learned to recognise mate calls during the pairing period. In males, but not females, mate calls evoked significantly fewer vocal responses than unfamiliar calls at one month after separation. Furthermore, in females, there was significantly greater molecular neuronal activation in response to mate calls compared to silence in the caudomedial mesopallium (CMM), a higher-order auditory region, in both brain hemispheres. In males, we found right-sided dominance of molecular neuronal activation in response to mate calls in the CMM. This is the first evidence suggesting sex differences in functional asymmetry of brain regions related to recognition of learned vocalisation in birds. Thus, sex differences related to recognition of learned vocalisations may be found at the behavioural and neural levels in avian vocal learners as it is in humans.

Effects of estradiol on neural responses to social signals in female túngara frogs

Estradiol plays an important role in mediating changes in female sexual behavior across reproductive cycles. In the túngara frog [Physalaemus (=Engystomops) pustulosus], the relationship between gonadal activity and female sexual behavior, as expressed by phonotaxis, is mediated primarily by estradiol. Estradiol receptors are expressed in auditory and motivational brain areas and the hormone could serve as an important modulator of neural responses to conspecific calls. To better understand how estradiol modifies neural responses to conspecific social signals, we manipulated estradiol levels and measured expression of the immediate early gene egr-1 in the auditory midbrain, thalamus and limbic forebrain in response to conspecific or heterospecific calls. We found that estradiol and conspecific calls increased egr-1 expression in the auditory midbrain and limbic forebrain, but in the thalamus, only conspecific calls were effective. In the preoptic area, estradiol enhanced the effect of the conspecific call on egr-1 expression, suggesting that the preoptic area could act as a hormonal gatekeeper to phonotaxis. Overall, the results suggest that estradiol has broad influences on the neural circuit involved in female reproduction, particularly those implicated in phonotaxis.

Geographic variation in acoustic communication in anurans and its neuroethological implications

Geographic variation of traits may represent the first step for evolutionary divergence potentially leading to speciation. Signals are behavioral traits of particular interest for the study of variation at a geographic scale. The anuran acoustic communication system represents an excellent model for studies of this kind, because their vocalizations play a main role in reproduction and the extant variation in this system may determine the evolution of this group. This review is committed to studies on geographic variation of acoustic communication systems in anurans, focusing on temporal and spectral characteristics of signals, environmental constraints affecting them and sound producing and receiving organs. In addition to the review of the literature on these topics, I highlight the deficit of investigation in some areas and propose alternative directions to overcome these drawbacks. Further, I propose the four-eyed frog, Pleurodema thaul, as an excellent model system to study geographic variation using a wide spectrum of approaches.

Male mice song syntax depends on social contexts and influences female preferences

In 2005, Holy and Guo advanced the idea that male mice produce ultrasonic vocalizations (USV) with some features similar to courtship songs of songbirds. Since then, studies showed that male mice emit USV songs in different contexts (sexual and other) and possess a multisyllabic repertoire. Debate still exists for and against plasticity in their vocalizations. But the use of a multisyllabic repertoire can increase potential flexibility and information, in how elements are organized and recombined, namely syntax. In many bird species, modulating song syntax has ethological relevance for sexual behavior and mate preferences. In this study we exposed adult male mice to different social contexts and developed a new approach of analyzing their USVs based on songbird syntax analysis. We found that male mice modify their syntax, including specific sequences, length of sequence, repertoire composition, and spectral features, according to stimulus and social context. Males emit longer and simpler syllables and sequences when singing to females, but more complex syllables and sequences in response to fresh female urine. Playback experiments show that the females prefer the complex songs over the simpler ones. We propose the complex songs are to lure females in, whereas the directed simpler sequences are used for direct courtship. These results suggest that although mice have a much more limited ability of song modification, they could still be used as animal models for understanding some vocal communication features that songbirds are used for.

From uni- to multimodality: towards an integrative view on anuran communication

Undeniably, acoustic signals are the predominant mode of communication in frogs and toads. Acoustically active species are found throughout the vast diversity of anuran families. However, additional or alternative signal modalities have gained increasing attention. In several anurans, seismic, visual and chemical communications have convergently evolved due to ecological constraints such as noisy environments. The production of a visual cue, like the inevitably moving vocal sac of acoustically advertising males, is emphasized by conspicuously coloured throats. Limb movements accompanied by dynamic displays of bright colours are additional examples of striking visual signals independent of vocalizations. In some multimodal anuran communication systems, the acoustic component acts as an alert signal, which alters the receiver attention to the following visual display. Recent findings of colourful glands on vocal sacs, producing volatile species-specific scent bouquets suggest the possibility of integration of acoustic, visual and chemical cues in species recognition and mate choice. The combination of signal components facilitates a broadened display repertoire in challenging environmental conditions. Thus, the complexity of the communication systems of frogs and toads may have been underestimated.

Prior experience with conspecific signals enhances auditory midbrain responsiveness to conspecific vocalizations

There is a long history in neuroethology of investigating how communication signals influence the brain and behavior. It has become increasingly clear that brain areas associated with sensory processing are plastic in adults and that this plasticity is related to reproductive condition. However, the role of communication signal reception in adult auditory plasticity has received relatively little attention. Here, we investigated whether the reception of communication signals (a frog chorus) could enhance the responsiveness of the auditory system to future reception of communication signals (a single male call). We found that animals that had been exposed to 10 days of a male chorus had stronger auditory midbrain immediate early gene expression than animals that had been exposed to 10 days of random tones when tested with 30 min of male calls or 30 min of tones. Our results suggest that exposure to dynamic social stimuli, like frog choruses, may play an important role in shaping the neural and behavioral responses to communication signals.

Identification and characterization of an Egr ortholog as a neural immediate early gene in the European honeybee (Apis mellifera L.)

To date, there are only few reports of immediate early genes (IEGs) available in insects. Aiming at identifying a conserved IEG in insects, we characterized an Egr homolog of the honeybee (AmEgr: Apis mellifera Egr). AmEgr was transiently induced in whole worker brains after seizure induction. In situ hybridization for AmEgr indicated that neural activity of a certain mushroom body (a higher brain center) neuron subtype, which is the same as that we previously identified using another non-coding IEG, termed kakusei, is more enhanced in forager brains. These findings suggest that Egr can be utilized as an IEG in insects.

Exposure to advertisement calls of reproductive competitors activates vocal-acoustic and catecholaminergic neurons in the plainfin midshipman fish, Porichthys notatus

While the neural circuitry and physiology of the auditory system is well studied among vertebrates, far less is known about how the auditory system interacts with other neural substrates to mediate behavioral responses to social acoustic signals. One species that has been the subject of intensive neuroethological investigation with regard to the production and perception of social acoustic signals is the plainfin midshipman fish, Porichthys notatus, in part because acoustic communication is essential to their reproductive behavior. Nesting male midshipman vocally court females by producing a long duration advertisement call. Females localize males by their advertisement call, spawn and deposit all their eggs in their mate’s nest. As multiple courting males establish nests in close proximity to one another, the perception of another male’s call may modulate individual calling behavior in competition for females. We tested the hypothesis that nesting males exposed to advertisement calls of other males would show elevated neural activity in auditory and vocal-acoustic brain centers as well as differential activation of catecholaminergic neurons compared to males exposed only to ambient noise. Experimental brains were then double labeled by immunofluorescence (-ir) for tyrosine hydroxylase (TH), an enzyme necessary for catecholamine synthesis, and cFos, an immediate-early gene product used as a marker for neural activation. Males exposed to other advertisement calls showed a significantly greater percentage of TH-ir cells colocalized with cFos-ir in the noradrenergic locus coeruleus and the dopaminergic periventricular posterior tuberculum, as well as increased numbers of cFos-ir neurons in several levels of the auditory and vocal-acoustic pathway. Increased activation of catecholaminergic neurons may serve to coordinate appropriate behavioral responses to male competitors. Additionally, these results implicate a role for specific catecholaminergic neuronal groups in auditory-driven social behavior in fishes, consistent with a conserved function in social acoustic behavior across vertebrates.

Social signals increase monoamine levels in the tegmentum of juvenile Mexican spadefoot toads (Spea multiplicata)

Monoamines are important neuromodulators that respond to social cues and that can, in turn, modify social responses. Yet we know very little about the ontogeny of monoaminergic systems and whether they contribute to the development of social behavior. Anurans are an excellent model for studying the development of social behavior because one of its primary components, phonotaxis, is expressed early in life. To examine the effect of social signals on monoamines early in ontogeny, we presented juvenile Mexican spadefoot toads (Spea multiplicata) with a male mating call or no sound and measured norepinephrine, epinephrine, dopamine, serotonin, and a serotonin metabolite, across the brain using high-pressure liquid chromatography. Our results demonstrate that adult-like monoaminergic systems are in place shortly after metamorphosis. Perhaps more interestingly, we found that mating calls increased the level of monoamines in the juvenile tegmentum, a midbrain region involved in sensory-motor integration and that contributes to brain arousal and attention. We saw no such increase in the auditory midbrain or in forebrain regions. We suggest that changes in monoamine levels in the juvenile tegmentum may reflect the effects of social signals on arousal state and could contribute to context-dependent modulation of social behavior.

Localizing brain regions associated with female mate preference behavior in a swordtail

Female mate choice behavior is a critical component of sexual selection, yet identifying the neural basis of this behavior is largely unresolved. Previous studies have implicated sensory processing and hypothalamic brain regions during female mate choice and there is a conserved network of brain regions (Social Behavior Network, SBN) that underlies sexual behaviors. However, we are only beginning to understand the role this network has in pre-copulatory female mate choice. Using in situ hybridization, we identify brain regions associated with mate preference in female Xiphophorus nigrensis, a swordtail species with a female choice mating system. We measure gene expression in 10 brain regions (linked to sexual behavior, reward, sensory integration or other processes) and find significant correlations between female preference behavior and gene expression in two telencephalic areas associated with reward, learning and multi-sensory processing (medial and lateral zones of the dorsal telencephalon) as well as an SBN region traditionally associated with sexual response (preoptic area). Network analysis shows that these brain regions may also be important in mate preference and that correlated patterns of neuroserpin expression between regions co-vary with differential compositions of the mate choice environment. Our results expand the emerging network for female preference from one that focused on sensory processing and midbrain sexual response centers to a more complex coordination involving forebrain areas that integrate primary sensory processing and reward.

Birdsong: is it music to their ears?

Since the time of Darwin, biologists have wondered whether birdsong and music may serve similar purposes or have the same evolutionary precursors. Most attempts to compare song with music have focused on the qualities of the sounds themselves, such as melody and rhythm. Song is a signal, however, and as such its meaning is tied inextricably to the response of the receiver. Imaging studies in humans have revealed that hearing music induces neural responses in the mesolimbic reward pathway. In this study, we tested whether the homologous pathway responds in songbirds exposed to conspecific song. We played male song to laboratory-housed white-throated sparrows, and immunolabeled the immediate early gene product Egr-1 in each region of the reward pathway that has a clear or putative homologue in humans. We found that the responses, and how well they mirrored those of humans listening to music, depended on sex and endocrine state. In females with breeding-typical plasma levels of estradiol, all of the regions of the mesolimbic reward pathway that respond to music in humans responded to song. In males, we saw responses in the amygdala but not the nucleus accumbens – similar to the pattern reported in humans listening to unpleasant music. The shared responses in the evolutionarily ancient mesolimbic reward system suggest that birdsong and music engage the same neuroaffective mechanisms in the intended listeners.

Modulation of sensory-motor integration as a general mechanism for context dependence of behavior

Social communication is context-dependent, with both the production of signals and the responses of receivers tailored to each animal's internal needs and external environmental conditions. We propose that this context dependence arises because of neural modulation of the sensory-motor transformation that underlies the social behavior. Neural systems that are restricted to individual behaviors may be modulated at early stages of the sensory or motor pathways for optimal energy expenditure. However, when neural systems contribute to multiple important behaviors, we argue that the sensory-motor relay is the likely site of modulation. Plasticity in the sensory-motor relay enables subtle context dependence of the social behavior while preserving other functions of the sensory and motor systems. We review evidence that the robust responses of anurans to conspecific signals are dependent on reproductive state, sex, prior experience, and current context. A well-characterized midbrain sensory-motor relay establishes signal selectivity and gates locomotive responses to sound. The social decision-making network may modulate this auditory-motor transformation to confer context dependence of anuran reproductive responses to sound. We argue that similar modulation may be a general mechanism by which vertebrates prioritize their behaviors.

Sexual signals of the Amazonian frog Allobates paleovarzensis: geographic variation and stereotypy of acoustic traits

Abstract

Because of its close relationship with the process of evolutionary differentiation, it is expected that geographic variability in acoustic sexual traits should be greater among than within populations. This is particularly expected in organisms with typically high population genetic structure and low dispersal abilities, such as anuran amphibians. We studied the acoustic traits of the advertisement call in the small-sized dendrobatoid frog Allobates paleovarzensisthrough its range in Central Amazonia. We accessed the variability of call traits from the within-male to the among-population levels, and evaluated the degree of stereotypy of the call characteristics. Call variability had comparable magnitudes within and among populations, and was independent of the degree of stereotypy of call measurements. Therefore, none of the call traits stood out as a potential cue for discrimination between populations. Spectral call measurements were static and strongly related with body size, which explained between 30 and 35% of the variation of these acoustic traits. Temporal characters of the notes were dynamic and influenced by environmental temperature (e.g., 27% of note rate variation), whilst temporal measurements of the entire calls were not related to the co-factors analysed. Both spectral and temporal call traits varied among populations and between sides of the Amazon River. Our results also indicate that body size and sampling site jointly affected the variability of the call traits. However, geographic distances among populations and the river barrier had no significant effect on the overall acoustic variation, indicating that local stabilising selective forces may be important in the process of call differentiation.

Auditory selectivity for acoustic features that confer species recognition in the túngara frog

In anurans, recognition of species-specific acoustic signals is essential to finding a mate. In many species, behavioral tests have elucidated which acoustic features contribute to species recognition, but the mechanisms by which the brain encodes these species-specific signal components are less well understood. The túngara frog produces a `whine' mating call that is characterized by a descending frequency sweep. However, much of the signal is unnecessary for recognition, as recognition behavior can be triggered by a descending two-tone step that mimics the frequency change in a portion of the whine. To identify the brain regions that contribute to species recognition in the túngara frog, we exposed females to a full-spectrum whine, a descending two-tone step that elicits recognition, the reversed two-tone step that does not elicit recognition, or no sound, and we measured expression of the neural activity-dependent gene egr-1 in the auditory brainstem and thalamus. We found that the behavioral relevance of the stimuli was the best predictor of egr-1 expression in the laminar nucleus of the torus semicircularis but not elsewhere. That is, the laminar nucleus responded more to the whine and the two-tone step that elicits recognition than to the reversed two-tone step. In contrast, in other brainstem and thalamic nuclei, whines induced egr-1 expression but tones did not. These data demonstrate that neural responses in the laminar nucleus correspond to behavioral responses of females and they suggest that the laminar nucleus may act as a feature detector for the descending frequencies characteristic of conspecific calls.

Ultrasound-evoked immediate early gene expression in the brainstem of the Chinese torrent frog, Odorrana tormota

The concave-eared torrent frog, Odorrana tormota, has evolved the extraordinary ability to communicate ultrasonically (i.e., using frequencies > 20 kHz), and electrophysiological experiments have demonstrated that neurons in the frog’s midbrain (torus semicircularis) respond to frequencies up to 34 kHz. However, at this time, it is unclear which region(s) of the torus and what other brainstem nuclei are involved in the detection of ultrasound. To gain insight into the anatomical substrate of ultrasound detection, we mapped expression of the activity-dependent gene, egr-1, in the brain in response to a full-spectrum mating call, a filtered, ultrasound-only call, and no sound. We found that the ultrasound-only call elicited egr-1 expression in the superior olivary and principal nucleus of the torus semicircularis. In sampled areas of the principal nucleus, the ultrasound-only call tended to evoke higher egr-1 expression than the full-spectrum call and, in the center of the nucleus, induced significantly higher egr-1 levels than the no-sound control. In the superior olivary nucleus, the full-spectrum and ultrasound-only calls evoked similar levels of expression that were significantly greater than the control, and egr-1 induction in the laminar nucleus showed no evidence of acoustic modulation. These data suggest that the sampled areas of the principal nucleus are among the regions sensitive to ultrasound in this species.