Harmonic calls and indifferent females: no preference for human consonance in an anuran

The human music faculty might have evolved from rudimentary components that occur in non-human animals. The evolutionary history of these rudimentary perceptual features is not well understood and rarely extends beyond a consideration of vertebrates that possess a cochlea. One such antecedent is a preferential response to what humans perceive as consonant harmonic sounds, which are common in many animal vocal repertoires. We tested the phonotactic response of female túngara frogs (Physalaemus pustulosus) to variations in the frequency ratios of their harmonically structured mating call to determine whether frequency ratio influences attraction to acoustic stimuli in this vertebrate that lacks a cochlea. We found that the ratio of frequencies present in acoustic stimuli did not influence female response. Instead, the amount of inner ear stimulation predicted female preference behaviour. We conclude that the harmonic relationships that characterize the vocalizations of these frogs did not evolve in response to a preference for frequency intervals with low-integer ratios. Instead, the presence of harmonics in their mating call, and perhaps in the vocalizations of many other animals, is more likely due to the biomechanics of sound production rather than any preference for ‘more musical’ sounds.     

Trade-off in short- and long-distance communication in tungara (Physalaemus pustulosus) and cricket (Acris crepitans) frogs

Female phonotaxis in túngara (Physalaemus pustulosus)and cricket (Acris crepitans) frogs is biased toward male advertisementcalls or call components of lower frequency. This behavioralbias might result in part from a mismatch between the spectralcharacteristics of the advertisement call and the most sensitive frequencyof the peripheral end organ implicated in reception of thesesounds. In both species, females are tuned to frequencies lowerthan average for the calls in their population. This mismatch,however, represents the situation during short-distance communication.Female frogs can also use the call to detect choruses at longdistances, and the spectral distribution of call energy canvary with transmission distance. We used computer simulationsto test the hypothesis that there is a better match betweentuning and call spectral energy at long distances from the callingmale than at short distances by comparing the performance (soundenergy received) of the natural tuning curve relative to anoptimal tuning curve (i.e., one centered at the call's dominantfrequency). The relative performance of the natural tuning curveincreased with distance in túngara frogs. For the twosubspecies of cricket frogs, however, the relative performancedecreased at longer distances. The performance did not equalthe optimal tuning curve at the distances tested. The resultsindicate that the relationship between calls and auditory tuningcannot be optimal for both long and short distance reception.The relationship between female tuning and call dominant frequencymay represent a compromise between short and long distance communication,and the bias toward short or long distances may vary among species.     

Seasonal changes in frequency tuning and temporal processing in single neurons in the frog auditory midbrain

Frogs rely on acoustic signaling to detect, discriminate, and localize mates. In the temperate zone, reproduction occurs in the spring, when frogs emerge from hibernation and engage in acoustically guided behaviors. In response to the species mating call, males typically show evoked vocal responses or other territorial behaviors, and females show phonotactic responses. Because of their strong seasonal behavior, it is possible that the frog auditory system also displays seasonal variation, as evidenced in their vocal control system. This hypothesis was tested in male Northern leopard frogs by evaluating the response characteristics of single neurons in the torus semicircularis (TS; a homolog of the inferior colliculus) to a synthetic mating call at different times of the year. We found that TS neurons displayed a seasonal change in frequency tuning and temporal properties. Frequency tuning shifted from a predominance of TS units sensitive to intermediate frequencies (700-1200 Hz) in the winter, to low frequencies (100-600 Hz) in the summer. In winter and early spring, most TS neurons showed poor, or weak, time locking to the envelope of the amplitude-modulated synthetic call, whereas in late spring and early summer the majority of TS neurons showed robust time-locked responses. These seasonal differences indicate that neural coding by auditory midbrain neurons in the Northern leopard frog is subject to seasonal fluctuation.     

Flexibility as a Strategy for Avoiding Call Overlap in Male Anhui Treefrogs

Male-male vocal competition is critical for mating success in anuran species; however, it remains unknown that how males regulate their competitive strategies dynamically during competition because calling is highly time-consuming, energetically demanding and likely to increase predation risks. Since different parts of calls will encode different information for vocal communication, we hypothesized that competitive strategies of male frogs may be modulated by the temporal and spectral features of different call notes. To test this hypothesis, the natural advertisement calls(OC), its modified versions with the first call note replaced by white noise(WN) or other notes and with the fifth call note replaced by WN, were played back to the Anhui tree frogs(Rhacophorus zhoukaiyae). Results showed that 1) males produced more competitive calls in response to acoustic stimuli compared to their baseline calling during silence; and 2) males emitted more non-overlapping calls compared to overlapping calls in response to the acoustic stimuli. These results are consistent with the idea that males are flexible to acoustic signals and their competition strategies are modulated dynamically by social contexts.     

Communication in a noisy environment: short-term acoustic adjustments and the underlying acoustic niche of a Neotropical stream-breeding frog

Acoustically active animals may show long- and short-term adaptations in acoustic traits for coping with ambient noise. Given the key role of calls in anurans’ life history, long- and short-term adaptations are expected in species inhabiting noisy habitats. However, to disentangle such adaptations is a difficult task, incipiently addressed for Neotropical frogs. We investigated if males of a stream-breeding frog (Crossodactylus schmidti) adjust call traits according to the background noise, and if the signal-to-noise ratio (SNR) varies between call harmonics and along call notes. We measured sound pressure levels of calls and noise in the field and used a fine-scale acoustic analysis to describe the signal and noise structure and test for noise-related call adjustments. The multi-note harmonic call of C. schmidti greatly varied in the spectral structure, including a trend for increasing note amplitude along the call, a wide frequency bandwidth of the 2nd harmonic, a minor call frequency modulation due to a trend for increasing note frequency within the same harmonic, and a major call frequency modulation due to the variable location of the dominant harmonic along the call. Calls had significantly higher frequencies than the noise at the range of the 1st and the 2nd call harmonics, and significantly louder sound pressure than the noise at the range of all harmonics. Males emitted the majority of call notes showing positive SNR, and though males also emitted some notes with negative SNR, when a given harmonic was negative the other harmonics in the same note did not tend to be SNR-negative. Our results indicate that male C. schmidti show short-term acoustic adjustments that make the advertisement call effective for coping with the interference of the stream-generated noise. We suggest that the call spectral plasticity serves for coping with temporary changes in the background noise, whilst we also discuss the possibility that the redundant, harmonic-structured call may have evolved to diminish masking interference on the acoustic signal by the background noise. This is the first study to uncouple noise-related acoustic adjustments and putative long-term acoustic adaptations for a Hylodidae, providing insights on behavioral plasticity and signal evolution of stream-breeding frogs.     

The vocal sac as a visual cue in anuran communication: an experimental analysis using video playback

Acoustic signals produced by most anurans are accompanied by inflation of a conspicuous vocal sac. We presented female túngara frogs, Physalaemus pustulosus, with acoustic playback of the male advertisement call, synchronized with a video playback of vocal sac inflation. Females significantly preferred a stimulus combination including vocal sac inflation over an identical set of stimuli with the vocal sac inflation removed. Neither a moving rectangle bearing the gross contrast and spatiotemporal properties of the vocal sac inflation sequence, nor the image of a noncalling male significantly enhanced the attractiveness of the acoustic stimulus. Both the form and spatiotemporal properties of the vocal sac thus appear to be salient to females. The results indicate that the vocal sac can serve as a visual cue, which may account for the conspicuous pigmentation found on the vocal sacs of males in many species. Gular inflation in synchrony with a call may function to facilitate female localization of individual males in an aggregation.     

Temporal call changes and prior experience affect graded signalling in the cricket frog

We investigated how male cricket frogs Acris crepitans, alter their advertisement calls in response to broadcasts of synthetic calls that were either 'attractive' or 'aggressive'. The stimulus calls differed in temporal but not spectral characteristics. Male cricket frogs produced a more aggressive call when presented with the aggressive stimulus, indicating that they perceived the temporal differences between the two call categories. The direction and degree of temporal and spectral changes depended on the relative dominant frequency of the resident and opponent. If the resident's dominant frequency was initially higher than the stimulus frequency, the pattern of change in dominant frequency mirrored that seen for the temporal call characters. In contrast, if the resident's initial dominant frequency was below that of the stimulus, then the temporal and spectral changes were in opposite directions. Furthermore, stimulus order influenced whether males responded differently to playbacks of aggressive and attractive calls; males that received the aggressive call first produced more aggressive calls during the aggressive stimulus, while males that received the attractive call first produced similar calls in response to the two stimuli. This suggests that experience with different types of signals influences the subsequent calling behaviour of male cricket frogs. Copyright 1999 The Association for the Study of Animal Behaviour.     

Plasticity of Peripheral Auditory Frequency Sensitivity in Emei Music Frog

In anurans reproductive behavior is strongly seasonal. During the spring, frogs emerge from hibernation and males vocalize for mating or advertising territories. Female frogs have the ability to evaluate the quality of the males'' resources on the basis of these vocalizations. Although studies revealed that central single torus semicircularis neurons in frogs exhibit season plasticity, the plasticity of peripheral auditory sensitivity in frog is unknown. In this study the seasonally plasticity of peripheral auditory sensitivity was test in the Emei music frog Babina daunchina, by comparing thresholds and latencies of auditory brainstem responses (ABRs) evoked by tone pips and clicks in the reproductive and non-reproductive seasons. The results show that both ABR thresholds and latency differ significantly between the reproductive and non-reproductive seasons. The thresholds of tone pip evoked ABRs in the non-reproductive season increased significantly about 10 dB than those in the reproductive season for frequencies from 1 KHz to 6 KHz. ABR latencies to waveform valley values for tone pips for the same frequencies using appropriate threshold stimulus levels are longer than those in the reproductive season for frequencies from 1.5 to 6 KHz range, although from 0.2 to 1.5 KHz range it is shorter in the non-reproductive season. These results demonstrated that peripheral auditory frequency sensitivity exhibits seasonal plasticity changes which may be adaptive to seasonal reproductive behavior in frogs.     

Condition-dependent reproductive effort in frogs infected by a widespread pathogen

To minimize the negative effects of an infection on fitness, hosts can respond adaptively by altering their reproductive effort or by adjusting their timing of reproduction. We studied effects of the pathogenic fungus Batrachochytrium dendrobatidis on the probability of calling in a stream-breeding rainforest frog (Litoria rheocola). In uninfected frogs, calling probability was relatively constant across seasons and body conditions, but in infected frogs, calling probability differed among seasons (lowest in winter, highest in summer) and was strongly and positively related to body condition. Infected frogs in poor condition were up to 40% less likely to call than uninfected frogs, whereas infected frogs in good condition were up to 30% more likely to call than uninfected frogs. Our results suggest that frogs employed a pre-existing, plastic, life-history strategy in response to infection, which may have complex evolutionary implications. If infected males in good condition reproduce at rates equal to or greater than those of uninfected males, selection on factors affecting disease susceptibility may be minimal. However, because reproductive effort in infected males is positively related to body condition, there may be selection on mechanisms that limit the negative effects of infections on hosts.     

Calling at the highway: The spatiotemporal constraint of road noise on Pacific chorus frog communication

Loss of acoustic habitat due to anthropogenic noise is a key environmental stressor for vocal amphibian species, a taxonomic group that is experiencing global population declines. The Pacific chorus frog (Pseudacris regilla) is the most common vocal species of the Pacific Northwest and can occupy human‐dominated habitat types, including agricultural and urban wetlands. This species is exposed to anthropogenic noise, which can interfere with vocalizations during the breeding season. We hypothesized that Pacific chorus frogs would alter the spatial and temporal structure of their breeding vocalizations in response to road noise, a widespread anthropogenic stressor. We compared Pacific chorus frog call structure and ambient road noise levels along a gradient of road noise exposures in the Willamette Valley, Oregon, USA. We used both passive acoustic monitoring and directional recordings to determine source level (i.e., amplitude or volume), dominant frequency (i.e., pitch), call duration, and call rate of individual frogs and to quantify ambient road noise levels. Pacific chorus frogs were unable to change their vocalizations to compensate for road noise. A model of the active space and time (“spatiotemporal communication”) over which a Pacific chorus frog vocalization could be heard revealed that in high‐noise habitats, spatiotemporal communication was drastically reduced for an individual. This may have implications for the reproductive success of this species, which relies on specific call repertoires to portray relative fitness and attract mates. Using the acoustic call parameters defined by this study (frequency, source level, call rate, and call duration), we developed a simplified model of acoustic communication space–time for this species. This model can be used in combination with models that determine the insertion loss for various acoustic barriers to define the impact of anthropogenic noise on the radius of communication in threatened species. Additionally, this model can be applied to other vocal taxonomic groups provided the necessary acoustic parameters are determined, including the frequency parameters and perception thresholds. Reduction in acoustic habitat by anthropogenic noise may emerge as a compounding environmental stressor for an already sensitive taxonomic group.     

Detection probabilities of two introduced frogs in Hawaii: implications for assessing non-native species distributions

Two nonnative Caribbean frogs, the Puerto Rican coqui and the Cuban greenhouse frog, recently invaded Hawaii. Because of its louder breeding call, management efforts have focused on the coqui, while little has been done to address the more cryptic greenhouse frog, even though it may be as widespread and have similar ecological impacts. The goal of this research was to determine the distribution and detection probability of both species on the island of Hawaii. We conducted a breeding call presence/absence survey at 446 sites every 2 km along major road networks. We re-surveyed 125 sites twice to determine detection and occupancy probabilities. Greenhouse frog detection probabilities (0.24, 0.29, 0.48, for each of the three visits, respectively) were lower than coqui detection probabilities (0.58, 0.73, 0.50, respectively) and increased with visits while those of the coqui did not. Greenhouse frog detection probabilities were lower in the presence of coquis for the first two surveys (0.12, 0.14) than in sites with greenhouse frogs alone (0.41), while greenhouse frogs had no effect on the detection of coquis. Site occupancy estimates for the greenhouse and coqui frog were 0.35 and 0.31, respectively, suggesting the species are similarly widespread. Results suggest multiple visits to sites are required to detect the greenhouse frog. Furthermore, results suggest that accounting for detectability is essential when determining the extent of invasion of cryptic species.     

The effect of airplane noise on frogs: a case study on the Critically Endangered Pickersgill’s reed frog (<Emphasis Type="Italic">Hyperolius pickersgilli</Emphasis>)

Species communicating acoustically may develop behavioral responses that aid them to transmit information and overcome signal masking in habitats disturbed by anthropogenic noise. Although many studies have concentrated on road traffic noise, very few studies mentioned effects of low flying airplane flyby noise on the vocal behavior of frogs. We studied the Critically Endangered Pickersgill’s Reed frog (Hyperolius pickersgilli) native to the eastern coastal regions of South Africa as a case study. In order to evaluate the call of H. pickersgilli, we compared a site with high levels of airplane flyby noise to a reference site without any airplane activity. Our results show that H. pickersgilli males made changes in both temporal and spectral properties of their call. Males call significantly more during and after an airplane flyby in relation to the call rate before the noise stimulus, but resumed normal call rhythms when measurements were taken 15 min after overflight. We found that males call at higher mean dominant frequencies (df difference = 161.4 Hz, P < 0.05) when exposed to high-intensity airplane flyby noise. In comparison with call rate 5 min before the airplane flyby, males called 12 % more during and 18 % more after the airplane flyby. Although changes in the spectral and temporal properties of the call of H. pickersgilli were observed, this species was actively calling for much longer than any other local species. This is the first study from Africa to report effects of anthropogenic noise on anuran communication.     

Information transfer during cricket frog contests

We investigated information transfer during vocal interactions between cricket frogs, Acris crepitans, with a specific focus on information about size and intention. In response to opponents, cricket frogs alter both temporal and spectral (frequency) aspects of their calls. Previous work suggests that males use dominant frequency, which is correlated with size, to provide information about fighting ability, and use temporal call characters, which are independent of size, to provide information about aggressive intent. We tested this hypothesis by investigating the relationship between call characters and contest behaviour. We presented a focal male with a simulated opponent, and categorized his behavioural response as attack, abandon or tolerate. We found that information about opponent size does not appear to influence a male's decision to fight, flee or tolerate an intruder. Whether or not males use the size information that is encoded in call frequency remains unclear. In contrast, changes in call frequency, which are not correlated with size, predicted the outcome of simulated contests, suggesting that male cricket frogs signal information about agonistic intent. Temporal call characters indicated whether or not a resident tolerated an opponent, but they did not predict contest outcome (attack versus abandon). Furthermore, the difference between the temporal call characters of a focal male and the simulated opponent predicted whether the resident tolerated the opponent. We suggest, therefore, that temporal call changes may be a cooperative signal designed to facilitate assessment of size through physical contact in wrestling bouts. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Temporal peculiarities of responses of auditory system neurons of the frog <Emphasis Type="Italic">Rana temporaria</Emphasis> to tone bursts with different modulation frequencies

Activity of medullar and midbrain auditory neurons at action of amplitude-modulated tone burst was recorded in immobilized common frogs Rana temporaria. Depth of modulation amounted to 10% or 80%, frequency of modulation varied from 5 to 150 Hz, and carrier intensity was in the range of 20–30 dB. Phasic neurons in medulla clearly reproduced the modulation frequency, but only at the 80% modulation depth. However, during presentation of signal with the 10% modulation depth, these neurons practically did not respond. Tonic neurons were able to reproduce the modulation frequency up to 10–150 Hz, but at the 10% modulation depth, the degree of reproduction of envelope was rather low, although for several first modulation periods it rose statistically significantly. In midbrain, the highest frequency of the reproduced modulation sharply fell. At greater modulation frequencies, the response of these neurons qualitatively reminds that of medullar neurons. At the low modulation frequencies, there is identified a group of midbrain neurons with a prominent increase of the signal modulation. This occurs in the frequency diapason up to 60 Hz; at an increase of the modulation frequency the time of achievement of maximal synchronization decreases. The optimal modulation frequency in many neurons of semicircular torus corresponds to parameters of the male nuptial call.     

The mixed-species chorus as public information: tungara frogs eavesdrop on a heterospecific

Multispecies choruses represent a promising but uninvestigatedforum for public information. Although frogs exposed to a potentialpredator call more readily in the presence of conspecific callsthan in their absence, none are known to make comparable useof heterospecific calls. To test for heterospecific eavesdropping,we isolated calling male túngara frogs (Physalaemus pustulosus),presented them with a potential predator, and recorded theirresponses to playbacks of 1 of 4 stimuli: calls of a conspecific,a sympatric heterospecific (Leptodactylus labialis), an allopatriccongener (Physalaemus enesefae), or silence. We found that malescalled more in response to the L. labialis call than to eitherthe silent stimulus or the P. enesefae call. In contrast, theP. enesefae call did not result in significantly more callingthan the silent stimulus. The conspecific call was the mosteffective at promoting calling. The data indicate that túngarafrogs selectively attend to the call of a heterospecific. Wehypothesize that such heterospecific eavesdropping contributesto the emergent behavior of mixed-species choruses.     

Modelling the prey detection performance of Rhinonicteris aurantia (Chiroptera: Hipposideridae) in different atmospheric conditions discounts the notional role of relative humidity in adaptive evolution

We examined a recent notion that differences in echolocation call frequency amongst geographic groups of constant frequency (CF)-emitting bats is the result of a trade-off between maximising prey detection range at lower frequencies and enhancing small-prey resolution at higher frequencies in different atmospheric (relative humidity; RH) environments. Isolated populations of the endemic Australian orange leaf-nosed bat Rhinonicteris aurantia were used as an example since geographic isolation in different environments has been a precursor to differences in their characteristic echolocation call frequencies (mean difference c. 6 kHz; means of 114.64 and 120.99 kHz). The influence of both atmospheric temperature and RH on maximum prey detection range was explored through mathematical modelling. This revealed that temperature was of similar importance to relative humidity and that under certain circumstances, each could reduce the effect of the other on ultrasound attenuation rates. The newly developed models contain significant conceptual improvements in method compared to other recent approaches, and can be applied to the situation of any other species of bat. For a given set of atmospheric conditions, the prey detection range of R. aurantia was reduced slightly when call frequency increased by 6 kHz, but an increase in RH, temperature or both reduced detection range significantly. A similar trend was also evident in prey detection volume ratios calculated for the same conditions. Spatial volume ratios were applied to assess the impact of changed atmospheric conditions and prey size on foraging ecology. Reductions in detection range associated with increases in RH and/or temperature also varied in relation to the size (cross sectional area) of insect prey. Modelling demonstrated that small (6 kHz) movements in call frequency could not compensate for the changes in prey detection range and spatial detection volumes that result from significant changes in atmospheric temperature or RH. The notion that differences in RH are the primary cause leading to adaptive evolution and speciation in CF-emitting bats by precipitating intraspecific differences in the mean call frequency of geographically isolated bat populations was not supported by the results of this case study.     

Female call preferences in tree-hole frogs: why are there so many unattractive males?

In tree-hole frogs, Metaphrynella sundana, the fundamental call frequency varies widely between males. In field playback experiments, females strongly preferred calls from the lower range of frequencies found in the population. There was no correlation, however, between male size and call frequency, as is normally the case for anurans, so large males were not necessarily more attractive to females. Presence or absence of upper harmonics in the call had no effect on female choice. Tree holes with shallow air columns were more often used by calling frogs, and were presumably more common, than deep holes. Since male M. sundana actively exploit the resonant properties of tree holes for mate attraction, and high frequencies match comparatively shallow holes, the benefits of attaining acoustic matching probably select for high-frequency calls. In addition, males with high-frequency calls may be heard from a greater distance in the vicinity of torrent streams. Since the level of such noise in the forest varies in time and space, different frequencies may prove optimal in different contexts, thereby preserving the observed variation within the population. Having an ‘unattractive’ high-frequency call should be potentially beneficial only when calling males do not congregate, a condition that our data suggest is fulfilled in this system.     

Stream Noise,Hybridization, and Uncoupled Evolution of Call Traits in Two Lineages of Poison Frogs: Oophaga histrionica and Oophaga lehmanni

According to the acoustic adaptation hypothesis, communication signals are evolutionary shaped in a way that minimizes its degradation and maximizes its contrast against the background noise. To compare the importance for call divergence of acoustic adaptation and hybridization, an evolutionary force allegedly promoting phenotypic variation, we compared the mate recognition signal of two species of poison frogs (Oophaga histrionica and O. lehmanni) at five localities: two (one per species) alongside noisy streams, two away from streams, and one interspecific hybrid. We recorded the calls of 47 males and characterized the microgeographic variation in their spectral and temporal features, measuring ambient noise level, body size, and body temperature as covariates. As predicted, frogs living in noisy habitats uttered high frequency calls and, in one species, were much smaller in size. These results support a previously unconsidered role of noise on streams as a selective force promoting an increase in call frequency and pleiotropic effects in body size. Regarding hybrid frogs, their calls overlapped in the signal space with the calls of one of the parental lineages. Our data support acoustic adaptation following two evolutionary routes but do not support the presumed role of hybridization in promoting phenotypic diversity.     

Processing of simple and complex acoustic signals in a tonotopically organized ear

Processing of complex signals in the hearing organ remains poorly understood. This paper aims to contribute to this topic by presenting investigations on the mechanical and neuronal response of the hearing organ of the tropical bushcricket species Mecopoda elongata to simple pure tone signals as well as to the conspecific song as a complex acoustic signal. The high-frequency hearing organ of bushcrickets, the crista acustica (CA), is tonotopically tuned to frequencies between about 4 and 70 kHz. Laser Doppler vibrometer measurements revealed a strong and dominant low-frequency-induced motion of the CA when stimulated with either pure tone or complex stimuli. Consequently, the high-frequency distal area of the CA is more strongly deflected by low-frequency-induced waves than by high-frequency-induced waves. This low-frequency dominance will have strong effects on the processing of complex signals. Therefore, we additionally studied the neuronal response of the CA to native and frequency-manipulated chirps. Again, we found a dominant influence of low-frequency components within the conspecific song, indicating that the mechanical vibration pattern highly determines the neuronal response of the sensory cells. Thus, we conclude that the encoding of communication signals is modulated by ear mechanics.     

Sight of a Predator Induces a Corticosterone Stress Response and Generates Fear in an Amphibian

Amphibians, like other animals, generate corticosterone or cortisol glucocorticoid responses to stimuli perceived to be threatening. It is generally assumed that the corticosterone response of animals to capture and handling reflects the corticosterone response to stimuli such as the sight of a predator that are thought to be natural stressors. Fijian ground frogs ( Platymantisvitiana ) are preyed upon by the introduced cane toads ( Rhinellamarina ), and we used ground frogs to test the hypothesis that the sight of a predator will induce a corticosterone stress response in an amphibian. Urinary corticosterone metabolite concentrations increased in male ground frogs exposed to the sight of a toad for 1, 3 or 6 h, whereas corticosterone did not change in frogs exposed to another male ground frog, a ball, or when no stimulus was present in the test compartment. The frogs exposed to a toad initially moved towards the stimulus then moved away, whereas frogs exposed to another frog moved towards the test frog and remained closer to the frog than at the start of the test. Tonic immobility (TI) was measured as an index of fearfulness immediately after the test exposure of the frogs to a stimulus. The duration of TI was longer in frogs exposed to a toad than to another frog or to a ball. The results provide novel evidence that the sight of a predator can induce a corticosterone response and lead to increased fearfulness in amphibians. In addition, they show that endemic frogs can recognise an introduced predator as a threat.