Auditory perception exhibits sexual dimorphism and left telencephalic dominance in Xenopus laevis.

Sex differences in both vocalization and auditory processing have been commonly found in vocal animals, although the underlying neural mechanisms associated with sexual dimorphism of auditory processing are not well understood. In this study we investigated whether auditory perception exhibits sexual dimorphism in Xenopus laevis To do this we measured event-related potentials (ERPs) evoked by white noise (WN) and conspecific calls in the telencephalon, diencephalon and mesencephalon respectively. Results showed that (1) the N1 amplitudes evoked in the right telencephalon and right diencephalon of males by WN are significantly different from those evoked in females; (2) in males the N1 amplitudes evoked by conspecific calls are significantly different from those evoked by WN; (3) in females the N1 amplitude for the left mesencephalon was significantly lower than for other brain areas, while the P2 and P3 amplitudes for the right mesencephalon were the smallest; in contrast these amplitudes for the left mesencephalon were the smallest in males. These results suggest auditory perception is sexually dimorphic. Moreover, the amplitude of each ERP component (N1, P2 and P3) for the left telencephalon was the largest in females and/or males, suggesting that left telencephalic dominance exists for auditory perception in Xenopus.

Auditory sensitivity exhibits sexual dimorphism and seasonal plasticity in music frogs.

Seasonal changes in the structure and function of the vertebrate brain have been described in many species, particularly in seasonal breeders. However, it remains unclear whether sexual dimorphism varies between breeding seasons for specific brain regions. Auditory event-related potential (ERP) changes were evaluated in the Emei music frog (Babina daunchina) to assess sexual dimorphism and seasonal variations in auditory sensitivity. An acoustic playback experiment using an oddball paradigm design was conducted, in which two conspecific call types were used as deviant stimuli with synthesized white noise as standard stimulus. ERP components were analyzed for the telencephalon and mesencephalon of both sexes in the non-reproductive and reproductive states. Results show that auditory sensitivity is modulated by reproductive status, implying that seasonal plasticity is involved in auditory perception. Moreover, the amplitude of the N1 ERP component (mean amplitudes during the interval occurring 30-130 ms after stimulus onset) is higher in females for the telencephalon and higher in males for the mesencephalon, regardless of reproductive status and acoustic stimulus type. These results show that auditory ERP responses for specific brain regions exhibit sexual dimorphism in the absence of exogenous sexual stimulation during both the two reproductive states in the music frog.

The right thalamus may play an important role in anesthesia-awakening regulation in frogs.

BACKGROUND: Previous studies have shown that the mammalian thalamus is a key structure for anesthesia-induced unconsciousness and anesthesia-awakening regulation. However, both the dynamic characteristics and probable lateralization of thalamic functioning during anesthesia-awakening regulation are not fully understood, and little is known of the evolutionary basis of the role of the thalamus in anesthesia-awakening regulation. METHODS: An amphibian species, the South African clawed frog (Xenopus laevis) was used in the present study. The frogs were immersed in triciane methanesulfonate (MS-222) for general anesthesia. Electroencephalogram (EEG) signals were recorded continuously from both sides of the telencephalon, diencephalon (thalamus) and mesencephalon during the pre-anesthesia stage, administration stage, recovery stage and post-anesthesia stage. EEG data was analyzed including calculation of approximate entropy (ApEn) and permutation entropy (PE). RESULTS: Both ApEn and PE values differed significantly between anesthesia stages, with the highest values occurring during the awakening period and the lowest values during the anesthesia period. There was a significant correlation between the stage durations and ApEn or PE values during anesthesia-awakening cycle primarily for the right diencephalon (right thalamus). ApEn and PE values for females were significantly higher than those for males. DISCUSSION: ApEn and PE measurements are suitable for estimating depth of anesthesia and complexity of amphibian brain activity. The right thalamus appears physiologically positioned to play an important role in anesthesia-awakening regulation in frogs indicating an early evolutionary origin of the role of the thalamus in arousal and consciousness in land vertebrates. Sex differences exist in the neural regulation of general anesthesia in frogs.

Auditory neural networks involved in attention modulation prefer biologically significant sounds and exhibit sexual dimorphism in anurans.

Allocating attention to biologically relevant stimuli in a complex environment is critically important for survival and reproductive success. In humans, attention modulation is regulated by the frontal cortex, and is often reflected by changes in specific components of the event-related potential (ERP). Although brain networks for attention modulation have been widely studied in primates and avian species, little is known about attention modulation in amphibians. The present study aimed to investigate the attention modulation networks in an anuran species, the Emei music frog (Babina daunchina). Male music frogs produce advertisement calls from within underground nest burrows that modify the acoustic features of the calls, and both males and females prefer calls produced from inside burrows. We broadcast call stimuli to male and female music frogs while simultaneously recording electroencephalographic (EEG) signals from the telencephalon and mesencephalon. Granger causal connectivity analysis was used to elucidate functional brain networks within the time window of ERP components. The results show that calls produced from inside nests which are highly sexually attractive result in the strongest brain connections; both ascending and descending connections involving the left telencephalon were stronger in males while those in females were stronger with the right telencephalon. Our findings indicate that the frog brain allocates neural attention resources to highly attractive sounds within the window of early components of ERP, and that such processing is sexually dimorphic, presumably reflecting the different reproductive strategies of males and females.

Males increase call frequency, not intensity, in response to noise, revealing no Lombard effect in the little torrent frog.

Noise is one of the main factors that can influence the processes of sound communication across a wide range of animal groups. Although the effects of ambient noise on animal communication, including anthropogenic noise, have received increasing attention, few studies have examined changes in the fine structure of acoustic signals produced by vocalizing species in constantly noisy environments. Here, we used natural recordings to determine the associations between stream noise and call parameters in the little torrent frog (Amolops torrentis). We also used playbacks of stream noise recorded in natural habitats and playbacks of white noise to examine how male vocal signals change with increasing noise levels. The results show that noise intensity has a significant effect on male call frequency, but not on call amplitude or other call characteristics. Based on this evidence, we suggest that in streamside species stream noise drives males to alter call frequency and call as loudly as possible in order to improve discriminability. These findings provide insights into the role played by ecological selection in the evolution of noise-dependent anuran vocal plasticity.

Correction to: Sometimes noise is beneficial: stream noise informs vocal communication in the little torrent frog Amolops torrentis.

[This corrects the article DOI: 10.1007/s10164-017-0515-y.].

Sometimes noise is beneficial: stream noise informs vocal communication in the little torrent frog Amolops torrentis.

Many kinds of environmental noise can interfere with acoustic communication and efficient decision making in terrestrial species. Here we identified an exception to this generalization in a streamside species, the little torrent frog (Amolops torrentis) which communicates in a stream noise environment. To determine whether stream noise can act as a cue regarding the microhabitat characteristics of senders, we performed phonotaxis experiments using stimulus pairs constructed with synthetic male calls (high or low dominant frequency) and stream noise with varied signal-to-noise ratios. We found that females prefer calls with high amplitude stream noise added compared to those with low amplitude stream noise added for both high and low dominant frequency stimulus pairs; however, stream noise itself was not attractive in the absence of calls. These results show that stream noise can function as a cue that may be used by females for enhancing the attractiveness of calls. Stream noise associates closely with rocks, topographies and vegetation and may thus provide useful microhabitat information for signal receivers, thereby acting on sexual selection. These data therefore contribute to our understanding of how the perception of mate attractiveness in heterogeneous ecological environments can evolve.

Male-male competition and female choice are differentially affected by male call acoustics in the serrate-legged small treefrog, Kurixalus odontotarsus.

BACKGROUND: The evolution of exaggerated vocal signals in anuran species is an important topic. Males and females have both evolved the ability to discriminate communication sounds. However, the nature of sexual dimorphism in cognition and sensory discrimination and in the evolution and limitation of sexual signal exaggeration remain relatively unexplored. METHODS: In the present study, we used male calls of varied complexity in the serrate-legged small treefrog, Kurixalus odontotarsus, as probes to investigate how both sexes respond to variations in call complexity and how sex differences in signal discrimination play a role in the evolution of sexual signal exaggeration. The compound calls of male K. odontotarsus consist of a series of one or more harmonic notes (A notes) which may be followed by one or more short broadband notes (B notes). RESULTS: Male playback experiments and female phonotaxis tests showed that increasing the number of A notes in stimulus calls elicits increased numbers of response calls by males and increases the attractiveness of the stimulus calls to females. The addition of B notes, however, reduces male calling responses. Moreover, call stimuli which contain only B notes suppress spontaneous male calling responses. Phonotaxis experiments show that females prefer calls with greater numbers of A notes and calls containing both A notes and B notes, but do not prefer calls with only B notes. DISCUSSION: Male-male competition and female choice appear to have played different roles in the evolution and limitation of signal complexity in K. odontotarsus. These results provide new insights into how exaggerated compound signals evolve and how signal complexity may be limited in anurans.

Competitive pressures affect sexual signal complexity in Kurixalus odontotarsus: insights into the evolution of compound calls.

Male-male vocal competition in anuran species is critical for mating success; however, it is also energetically demanding and highly time-consuming. Thus, we hypothesized that males may change signal elaboration in response to competition in real time. Male serrate-legged small treefrogs (Kurixalus odontotarsus) produce compound calls that contain two kinds of notes, harmonic sounds called 'A notes' and short broadband sounds called 'B notes'. Using male evoked vocal response experiments, we found that competition influences the temporal structure and complexity of vocal signals produced by males. Males produce calls with a higher ratio of notes:call, and more compound calls including more A notes but fewer B notes with contest escalation. In doing so, males minimize the energy costs and maximize the benefits of competition when the level of competition is high. This means that the evolution of sexual signal complexity in frogs may be susceptible to selection for plasticity related to adjusting performance to the pressures of competition, and supports the idea that more complex social contexts can lead to greater vocal complexity.

The thermal background determines how the infrared and visual systems interact in pit vipers.

The thermal infrared (IR) sensing system of pit vipers is believed to complement vision and provide a substitute imaging system in dark environments. Theoretically, the IR system would best image a scene consisting of a homothermal target in cold surroundings as a bright spot on a dark background. To test this hypothesis, we evaluated how the pit viper (Gloydius brevicaudus) discriminates and strikes prey when the background temperature is either higher or lower than that of the prey (approximately 32-33°C) in different parts of the scene. Snakes were tested in a modified predation cage in which background temperatures were set to 26°C on one side and either 33 or 40°C on the opposite side when the eyes, the pit organs or neither sensory system was occluded. When the eyes were blocked, snakes preferred to strike prey on the 26°C side rather than on the 33°C side but showed no bias in the other conditions. Snakes showed no preference for 26 versus 40°C background temperature, although more missed strikes occurred when the eyes were occluded. The results thus revealed that the pit viper IR system can accomplish a 'brightness constancy' computation reflecting the difference between the target and background temperatures, much as the visual system compares the luminance of a figure and the background. Furthermore, the results show that the IR system performs less well for locating prey when the background is warmer than the target.

The First Call Note Plays a Crucial Role in Frog Vocal Communication.

Vocal Communication plays a crucial role in survival and reproductive success in most amphibian species. Although amphibian communication sounds are often complex consisting of many temporal features, we know little about the biological significance of each temporal component. The present study examined the biological significance of notes of the male advertisement calls of the Emei music frog (Babina daunchina) using the optimized electroencephalogram (EEG) paradigm of mismatch negativity (MMN). Music frog calls generally contain four to six notes separated approximately by 150 millisecond intervals. A standard stimulus (white noise) and five deviant stimuli (five notes from one advertisement call) were played back to each subject while simultaneously recording multi-channel EEG signals. The results showed that the MMN amplitude for the first call note was significantly larger than for that of the others. Moreover, the MMN amplitudes evoked from the left forebrain and midbrain were typically larger than those from the right counterpart. These results are consistent with the ideas that the first call note conveys more information than the others for auditory recognition and that there is left-hemisphere dominance for processing information derived from conspecific calls in frogs.

Effect of the Level of Anesthesia on the Auditory Brainstem Response in the Emei Music Frog (Babina daunchina).

Anesthesia is known to affect the auditory brainstem response (ABR) in mice, rats, birds and lizards. The present study investigated how the level of anesthesia affects ABR recordings in an amphibian species, Babina daunchina. To do this, we compared ABRs evoked by tone pip stimuli recorded from 35 frogs when Tricaine methane sulphonate (MS-222) anesthetic immersion times varied from 0, 5 and 10 minutes after anesthesia induction at sound frequencies between 0.5 and 6 kHz. ABR thresholds increased significantly with immersion time across the 0.5 kHz to 2.5 kHz frequency range, which is the most sensitive frequency range for hearing and the main frequency range of male calls. There were no significant differences for anesthetic levels across the 3 kHz to 6 kHz range. ABR latency was significantly longer in the 10 min group than in the 0 and 5 min groups at frequencies of 0.5, 1.0, 1.5, 2.5 kHz, while ABR latency did not differ across the 3 kHz to 4 kHz range and at 2.0 kHz. Taken together, these results show that the level of anesthesia affects the amplitude, threshold and latency of ABRs in frogs.

An exception to the matched filter hypothesis: A mismatch of male call frequency and female best hearing frequency in a torrent frog.

The matched filter hypothesis proposes that the tuning of auditory sensitivity and the spectral character of calls will match in order to maximize auditory processing efficiency during courtship. In this study, we analyzed the acoustic structure of male calls and both male and female hearing sensitivities in the little torrent frog (Amolops torrentis), an anuran species who transmits acoustic signals across streams. The results were in striking contradiction to the matched filter hypothesis. Auditory brainstem response results showed that the best hearing range was 1.6-2 kHz consistent with the best sensitive frequency of most terrestrial lentic taxa, yet completely mismatched with the dominant frequency of conspecific calls (4.3 kHz). Moreover, phonotaxis tests show that females strongly prefer high-frequency (4.3 kHz) over low-frequency calls (1.6 kHz) regardless of ambient noise levels, although peripheral auditory sensitivity is highest in the 1.6-2 kHz range. These results are consistent with the idea that A. torrentis evolved from nonstreamside species and that high-frequency calls evolved under the pressure of stream noise. Our results also suggest that female preferences based on central auditory system characteristics may evolve independently of peripheral auditory system sensitivity in order to maximize communication effectiveness in noisy environments.

A lateralized functional auditory network is involved in anuran sexual selection.

Right ear advantage (REA) exists in many land vertebrates in which the right ear and left hemisphere preferentially process conspecific acoustic stimuli such as those related to sexual selection. Although ecological and neural mechanisms for sexual selection have been widely studied, the brain networks involved are still poorly understood. In this study we used multi-channel electroencephalographic data in combination with Granger causal connectivity analysis to demonstrate, for the first time, that auditory neural network interconnecting the left and right midbrain and forebrain function asymmetrically in the Emei music frog (Babina daunchina), an anuran species which exhibits REA. The results showed the network was lateralized. Ascending connections between the mesencephalon and telencephalon were stronger in the left side while descending ones were stronger in the right, which matched with the REA in this species and implied that inhibition from the forebrainmay induce REA partly. Connections from the telencephalon to ipsilateral mesencephalon in response to white noise were the highest in the non-reproductive stage while those to advertisement calls were the highest in reproductive stage, implying the attention resources and living strategy shift when entered the reproductive season. Finally, these connection changes were sexually dimorphic, revealing sex differences in reproductive roles.

Resting-state brain networks revealed by granger causal connectivity in frogs.

Resting-state networks (RSNs) refer to the spontaneous brain activity generated under resting conditions, which maintain the dynamic connectivity of functional brain networks for automatic perception or higher order cognitive functions. Here, Granger causal connectivity analysis (GCCA) was used to explore brain RSNs in the music frog (Babina daunchina) during different behavioral activity phases. The results reveal that a causal network in the frog brain can be identified during the resting state which reflects both brain lateralization and sexual dimorphism. Specifically (1) ascending causal connections from the left mesencephalon to both sides of the telencephalon are significantly higher than those from the right mesencephalon, while the right telencephalon gives rise to the strongest efferent projections among all brain regions; (2) causal connections from the left mesencephalon in females are significantly higher than those in males and (3) these connections are similar during both the high and low behavioral activity phases in this species although almost all electroencephalograph (EEG) spectral bands showed higher power in the high activity phase for all nodes. The functional features of this network match important characteristics of auditory perception in this species. Thus we propose that this causal network maintains auditory perception during the resting state for unexpected auditory inputs as resting-state networks do in other species. These results are also consistent with the idea that females are more sensitive to auditory stimuli than males during the reproductive season. In addition, these results imply that even when not behaviorally active, the frogs remain vigilant for detecting external stimuli.

Bigger Is Not Always Better: Females Prefer Males of Mean Body Size in Philautus odontotarsus.

Most species are believed to evolve larger body sizes over evolutionary time. Previous studies have suggested that sexual selection, through male-male competition and female choice, favors larger males. However, there is little evidence of selection against large size. The female serrate-legged small treefrogs (Philautus odontotarsus) must carry passive males from leks to breeding grounds over relatively long distances after amplexus to find a suitable place to lay eggs. The costs of large male size may therefore decrease mating success due to reduced agility and/or higher energy requirements. Thus, we hypothesized that selection would not favor larger males in P. odontotarsus. Females can assess male body size on the basis of the dominant frequency of male calls in frogs. To assess female P. odontotarsus preferences for a potential mate's body size, male calls of high, average and low dominant frequency were played back to the females in phonotaxis experiments. Results showed that most females prefer the advertisement call with average dominant frequency. In addition, we compared the body mass distribution of amplectant males with that of single males in nature. The body masses of amplectant males are more narrowly distributed in the intermediate range than that of single males. The phonotaxis results and the data of actual female preferences in the field show that females strongly prefer potential mates of mean body sizes, consistent with the view that, in this species at least, larger males are not always perceived as better by females. In the present study, P. odontotarsus provides an example of an amphibian species in which large size does not have an advantage in mating success for males. Instead, our results provide evidences that stabilizing selection favors the optimal intermediate size of males.

Sound Classification and Call Discrimination Are Decoded in Order as Revealed by Event-Related Potential Components in Frogs.

Species that use communication sounds to coordinate social and reproductive behavior must be able to distinguish vocalizations from nonvocal sounds as well as to identify individual vocalization types. In this study we sought to identify the neural localization of the processes involved and the temporal order in which they occur in an anuran species, the music frog Babina daunchina. To do this we measured telencephalic and mesencephalic event-related potentials (ERPs) elicited by synthesized white noise (WN), highly sexually attractive (HSA) calls produced by males from inside nests and male calls of low sexual attractiveness (LSA) produced outside of nests. Each stimulus possessed similar temporal structures. The results showed the following: (1) the amplitudes of the first negative ERP component (N1) at ∼ 100 ms differed significantly between WN and conspecific calls but not between HSA and LSA calls, indicating that discrimination between conspecific calls and nonvocal sounds occurs in ∼ 100 ms, (2) the amplitudes of the second positive ERP component (P2) at ∼ 200 ms in the difference waves between HSA calls and WN were significantly higher than between LSA calls and WN in the right telencephalon, implying that call characteristic identification occurs in ∼ 200 ms and (3) WN evoked a larger third positive ERP component (P3) at ∼ 300 ms than conspecific calls, suggesting the frogs had classified the conspecific calls into one category and perceived WN as novel. Thus, both the detection of sounds and the identification of call characteristics are accomplished quickly in a specific temporal order, as reflected by ERP components. In addition, the most dynamic ERP patterns appeared in the left mesencephalon and the right telencephalon, indicating the two brain regions might play key roles in anuran vocal communication.

Core and Shell Song Systems Unique to the Parrot Brain.

The ability to imitate complex sounds is rare, and among birds has been found only in parrots, songbirds, and hummingbirds. Parrots exhibit the most advanced vocal mimicry among non-human animals. A few studies have noted differences in connectivity, brain position and shape in the vocal learning systems of parrots relative to songbirds and hummingbirds. However, only one parrot species, the budgerigar, has been examined and no differences in the presence of song system structures were found with other avian vocal learners. Motivated by questions of whether there are important differences in the vocal systems of parrots relative to other vocal learners, we used specialized constitutive gene expression, singing-driven gene expression, and neural connectivity tracing experiments to further characterize the song system of budgerigars and/or other parrots. We found that the parrot brain uniquely contains a song system within a song system. The parrot "core" song system is similar to the song systems of songbirds and hummingbirds, whereas the "shell" song system is unique to parrots. The core with only rudimentary shell regions were found in the New Zealand kea, representing one of the only living species at a basal divergence with all other parrots, implying that parrots evolved vocal learning systems at least 29 million years ago. Relative size differences in the core and shell regions occur among species, which we suggest could be related to species differences in vocal and cognitive abilities.

The biological significance of acoustic stimuli determines ear preference in the music frog.

Behavioral and neurophysiological studies support the idea that right ear advantage (REA) exists for perception of conspecific vocal signals in birds and mammals. Nevertheless, few studies have focused on anuran species that typically communicate through vocalization. The present study examined the direction and latencies of orientation behaviors in Emei music frogs (Babina daunchina) produced in response to six auditory stimuli emitted by a speaker placed directly behind the subjects. The stimuli included male advertisement calls produced from within burrow nests, which have been shown to be highly sexually attractive (HSA), calls produced from outside burrows, which are of low sexual attractiveness (LSA), screech calls produced when frogs are attacked by snakes, white noise, thunder and silence. For all sound stimuli except the screech, the frogs preferentially turned to the right. Right ear preference was strongest for HSA calls. For the screech and thunder stimuli, there was an increased tendency for subjects to move further from the speaker rather than turning. These results support the idea that in anurans, right ear preference is associated with perception of positive or neutral signals such as the conspecific advertisement call and white noise, while a left ear preference is associated with perception of negative signals such as predatory attack.

Right ear advantage for vocal communication in frogs results from both structural asymmetry and attention modulation.

Right-ear/left-hemisphere advantage (REA) in processing species-specific vocalizations has been demonstrated in mammals including humans. Two models for REA are typically proposed, a structural model and an attentional model. These hypotheses were tested in an anuran species, the Emei music frog (Babina daunchina) in which females strongly prefer male calls produced from inside mud-retuse burrows (high sexual attractiveness or HSA calls) to those produced in open fields (low sexual attractiveness or LSA calls). Isochronic playbacks were used to control for attention to stimuli presented to either the left or right sides of female subjects while electroencephalogram (EEG) signals were recorded from the left and right midbrain and telencephalon. The results show that relative EEG power in the delta band declined while those of the alpha and beta bands increased with time in the left but not the right midbrain. Since the anuran midbrain receives auditory information derived primarily from the contralateral auditory nerve, these results support the idea that REA occurs in frogs because communication sounds are processed preferentially in the left midbrain. Furthermore, though differences in the dynamic changes of the delta, alpha and beta bands in the left midbrain between acoustic stimuli were not statistically significant, these changes were stronger during the playback of HSA calls toward which females tend to allocate greater attentional resources. These results imply that REA in frogs results from the combined effects of structural asymmetry and attention modulation.