High-frequency hearing in a hummingbird.

Some hummingbirds produce unique high-frequency vocalizations. It remains unknown whether these hummingbirds can hear these sounds, which are produced at frequencies beyond the range at which most birds can hear. Here, we show behavioral and neural evidence of high-frequency hearing in a hummingbird, the Ecuadorian Hillstar (Oreotrochilus chimborazo). In the field, hummingbirds responded to playback of high-frequency song with changes in body posture and approaching behavior. We assessed neural activation by inducing ZENK expression in the brain auditory areas in response to the high-frequency song. We found higher ZENK expression in the auditory regions of hummingbirds exposed to the high-frequency song compared to controls, while no difference was observed in the hippocampus between groups. The behavioral and neural responses show that this hummingbird can hear sounds at high frequencies. This is the first evidence of the use of high-frequency vocalizations and high-frequency hearing in conspecific communication in a bird.

Randomized clinical trial of postoperative hernia prophylaxis in open bariatric surgery.

BACKGROUND: Postoperative hernia following bariatric procedures is more common than in other groups of surgical patients, and remains a serious problem. Gastric bypass is the most often performed bariatric procedure and, despite the increasing popularity of a laparoscopic approach, many morbidly obese patients are still offered open procedures. The aim of this study was to assess the effects of prophylactic polypropylene mesh in morbidly obese patients undergoing gastric by-pass surgery. METHODS: The study randomized 74 patients undergoing open Roux-en-Y gastric bypass into two groups: wound closure with (n = 36) or without (n = 38) a polypropylene mesh. Mean(s.d.) body mass and body mass index in the mesh group were 137.3(24.5) kg and 46.2(7.1) kg/m(2) and in the non-mesh group were 139.0(24.9) kg and 46.8(7.6) kg/m(2) respectively. In the non-mesh group, the wound was closed with a polypropylene suture. Patients in the mesh group had in addition a polypropylene mesh inserted in a sublay manner. RESULTS: Patients were followed up for at least 6 (range 6-38) months. Hernia developed in eight patients in the non-mesh group but in none in the mesh group. The duration of hospital stay was similar in both groups: mean(s.d.) 8.4(3.2) and 10.3(5.9) days (P = 0.092). There were no serious complications in either group. CONCLUSION: The use of a mesh prevented hernia development and did not lengthen hospital stay.

Pharmacological characterization of the D1- and D2-like dopamine receptors from the brain of the leopard frog, Rana pipiens.

The pharmacological profiles of D1- and D2-like dopamine receptors were investigated for native brain receptors in the leopard frog, Rana pipiens, using direct binding assays, which characterize functional receptors rather than assess total receptor protein. We used homogenate assays of R. pipiens fore- and midbrains to determine, via saturation isotherms, that the dissociation constant, Kd, for (3)H-SCH-23390 binding to the D1-like receptors was 0.29 nM, and the maximal receptor density, Bmax, was 40 fmoles/mg protein. This compares with the more than 10-fold higher density of D1 sites in rat striatum. Specific binding for the D2-like receptors was measurable using these methods with (3)H-spiperone as the ligand. However, saturation of binding was not achieved. This contrasts with the > 400 fmoles/mg protein Bmax in rat striatum. Pharmacological profiles (rank order of potency of displacing drugs) for each receptor type were determined. We used non-radioactive SCH-23390, SKF-38393, sulpiride, and spiperone to displace (3)H-SCH-23390 and (3)H-spiperone at D1 and D2 receptors, respectively. Parallel displacement assays were performed with rat striatal controls. Results indicated that the relative rank order displacements in anuran dopamine receptors were characteristic of D1- and D2-like receptors. However, the rank orders were not identical to those in mammals. The rank order for affinity at D1-like receptors in both rats and frogs was SCH-23390 > SKF-38393 > spiperone > sulpiride. The rank order for affinity at D2-like receptors was spiperone > SCH-23390 > sulpiride > SKF-38393 in frogs, and spiperone > sulpiride > SCH-23390 > SKF-38393 in rats. SKF-38393 and spiperone had similar affinities for the 'D1' receptors in both species. SCH-23390 had a slightly lower affinity for the D1-like receptors in Rana, whereas sulpiride had a significantly lower affinity for Rana D1-like receptors compared to rat D1 receptors. In Rana D2-like receptors, spiperone and sulpiride were significantly less potent compared to rat. However, SCH-23390 and SKF-38393 were equally potent for the D2-like receptors in both species. The results indicate that amphibian brain dopamine receptors fall into two classes similar to the mammalian D1 and D2 subfamilies, but with binding characteristics slightly different from those typically described in mammals. This work represents the first pharmacological characterization of native brain dopaminergic receptors in an anuran amphibian. Because direct binding assays measure the initial aspect of the functional interaction between transmitter and receptor, these data provide an important complement to studies using cell expression systems.

Social context influences androgenic effects on calling in the green treefrog (Hyla cinerea).

Courtship behavior in frogs is an ideal model for investigating the relationships among social experience, gonadal steroids, and behavior. Reception of mating calls causes an increase in androgen levels in listening males, and calling, in turn, depends on the presence of androgens. However, previous studies found that androgen replacement does not always restore calling to intact levels, and the relationship between androgens and calling may be context dependent. We examined the influence of androgens on calling behavior in the presence and the absence of social signals in male green treefrogs (Hyla cinerea). We categorized calling during an acoustic stimulus (mating chorus or tones) as evoked and calling in the absence of a stimulus as spontaneous. Intact males received a cholesterol implant, castrated males were castrated and received a cholesterol implant, and T-implanted males were castrated and received a testosterone implant. The androgen levels (mean +/- SE ng/ml of plasma) achieved by the implants were as follows: castrated males, 1.2 +/- 0.2; intact males 21.9 +/- 7.0; T-implanted males, 254.6 +/- 39.5. As in other frogs, calling depends on the presence of androgens, as castration abolished and T replacement maintained calling. However, among intact and T-implanted males, the influence of androgens on calling differed between spontaneous and evoked calling. There was a positive effect of androgen treatment on spontaneous call rate and a positive correlation between spontaneous call rate and androgen levels. The influence of androgen levels on evoked call rate was more complex and interacted with acoustic treatment. Surprisingly, T implants suppressed the chorus-specific increase in calling that is evident in intact males. In addition, in response to the chorus, T-implanted males called less than did intact males, in spite of higher androgen levels. Furthermore, variation in androgens did not explain variation in evoked call rate. These data indicate that androgens influence the motivation to call, but that, when socially stimulated, androgens are necessary but insufficient for calling.

Behavioral and hormonal effects of exogenous vasotocin and corticosterone in the green treefrog.

Vasotocin (AVT) promotes courtship in a wide range of vertebrates. However, this effect is not independent of steroid hormones. For example, androgens may work in concert with AVT and corticosterone (CORT) may work to oppose AVT action. In frogs, AVT promotes calling, and in some species, CORT inhibits calling. In addition, androgens are known to modulate AVT in the brain, and CORT may depress androgen secretion. Previous work in amphibians has suggested that AVT promotes courtship by overcoming a CORT-mediated stress response. Possible behavioral and hormonal interactions among AVT, CORT, and androgens were investigated in wild, free-living green treefrogs (Hyla cinerea). Saline, AVT, CORT, or a combination of AVT and CORT were administered to calling males, and several measures of spontaneous calling were evaluated for 1.5 h following injection. Plasma testosterone, dihydrotestosterone, and CORT were also measured. Saline-injected males had low CORT levels, and AVT and CORT injection elevated plasma CORT levels. AVT increased the likelihood of calling, but, in males who did call, AVT did not influence latency to call or how often they were observed calling. Very few saline-injected males resumed calling after injection, and therefore a CORT effect was only detectable in AVT-injected males. CORT inhibited calling in AVT-injected males only at the highest dose of CORT (40 microg); lower levels of CORT were unsuccessful at inhibiting AVT-induced calling. AVT appeared to have a specific effect on calling motivation. Further, the data suggest that disinhibition of a CORT response is not the primary mechanism by which AVT increases calling. In addition, CORT injection reduced endogenous androgen levels. Finally, endogenous androgens were negatively correlated with latency to begin calling, suggesting that they may have a positive effect on calling. These data indicate that AVT has positive effects on calling but provide only weak evidence that CORT inhibits courtship in this species.

Effects of medial and dorsal cortex lesions on spatial memory in lizards.

In mammals and birds, the hippocampus is a major learning and memory center that plays a prominent role in spatial memory, the use of distal cues to guide navigation. The role of reptilian hippocampal homologues, the medial and dorsal cortex, in spatial memory has not been thoroughly investigated. The medial and dorsal cortex of reptiles is known to play a role in learning both tasks that are hippocampally dependent and tasks that are not hippocampally dependent in mammals and birds. In order to examine the specific role of the medial and dorsal cortex in spatial memory, we trained medial cortex, dorsal cortex, and sham lesioned Cnemidophorus inornatus lizards to locate the one heated rock of four identical rocks spaced evenly around the perimeter of a circular, sand filled, arena in a cool room. We used probe trials to examine the strategies used by lizards to locate the goal. Medial cortex lesions and dorsal cortex lesions slowed acquisition and altered the strategies used to locate the goal. However, none of the lizards adopted a spatial strategy to locate the goal suggesting that the dorsal cortex and medial cortex are involved in using non-spatial strategies for navigation.

Social influences on androgen levels in the southern leopard frog, Rana sphenocephala.

Social cues play an important role in modulating neuroendocrine and hormonal states in many vertebrates. Social cues in the form of conspecific advertisement calls have been demonstrated to modulate plasma androgen levels in male Rana sphenocephala, the Southern leopard frog. Male R. sphenocephala were captured and brought to the laboratory for testing. Animals were exposed to one of two acoustical environments: a recording of an active conspecific breeding chorus or a recording of the same chorus in which all the spectral frequencies had been digitally shifted 500 Hz higher. Following 12 nights of exposure to either stimulus, plasma androgen levels were assessed by radioimmunoassay and gonadal morphology was examined to compare group differences. Plasma levels of dihydrotestosterone were significantly higher in males exposed to the conspecific chorus, compared with males exposed to the altered version of the chorus. Gonadal morphology did not differ between the two groups. These results suggest that in addition to environmental cues such as rainfall and other seasonal changes, social cues may also serve as environmental synchronizers among anurans to coordinate reproductive effort.

Evolution of calls and auditory tuning in the Physalaemus pustulosus species group.

In species within the Physalaemus pustulosus species group, male frogs produce a whine-like advertisement call consisting of a frequency sweep typically descending from 1,000 to 400 Hz (depending on the species). One species, Physalaemus pustulosus, the túngara frog, has evolved a second call syllable, the chuck, which males place after their whine. Most energy in the chuck is above 1,500 Hz and peaks at 2,400 Hz. We investigated whether the evolution of this new call component in P. pustulosus coincided with evolution of auditory tuning. We used multiunit electrophysiological recordings of auditory-evoked activity in the midbrain to characterize auditory tuning in Physalaemus pustulosus, four other Physalaemus species within the P. pustulosus clade, and three additional, closely related Physalaemus species as outgroups. All eight species had similar sensitivity profiles, with a broad area of enhanced sensitivity from 100 to 1,100 Hz, which we presume represents amphibian papilla (AP) tuning, and a second, narrower area of enhanced sensitivity centered above 2,100 Hz, which we presume represents basilar papilla (BP) tuning. For all species, the whine stimulates the AP. The P. pustulosus chuck stimulates the BP. The frequency with greatest AP sensitivity differed significantly among species. Although in all cases the AP peak lay within the frequency sweep of the whine, phylogenetically corrected correlations revealed no significant relationships between AP tuning and any spectral feature of the whine. BP tuning was similar among all species, with mean BP best excitatory frequencies (BEFs) around 2,100-2,200 Hz, with the exception of P. pustulatus, with a mean BP BEF of 2,549 Hz. Physalaemus pustulosus, the only investigated species that produces a call component stimulating the BP, had a BP BEF that was not significantly different from any of the species within its clade except P. pustulatus, or from any of the outgroup species. A phylogenetic reconstruction of ancestral BP tuning confirms that the only point of evolutionary change in BP tuning is in the line of descent leading to P. pustulatus, not in the line leading to P. pustulosus despite this being the species using the BP for communication. The results indicate that BP tuning around 2,200 Hz is a conserved trait in the Physalaemus pustulosus species group and that no evolution of BP tuning accompanied the subsequent evolution of the call component (the chuck) that stimulates it. This supports the sensory exploitation idea, which posits that signals evolve to match preexisting features of receiver systems.

Social signals influence hormones independently of calling behavior in the treefrog (Hyla cinerea).

Social signals play an important role in regulating hormone-behavior relationships. In anurans (frogs and toads), acoustic signals are an essential aspect of reproductive behavior; however, the physiological consequences of receiving social signals has remained largely undescribed. Each night for 5, 10, or 20 days, we presented acoustically isolated male treefrogs with a conspecific mating chorus, an array of tones, or no sound. We recorded calling rate of individuals throughout the experiment and collected blood before and after treatment. Days of stimulus exposure had no effect on any dependent measure. Acoustic treatment influenced steroid levels; testosterone, dihydrotestosterone, and corticosterone increased only in the group exposed to the chorus. Chorus-exposed males also showed an increase in stimulus-evoked calling. We found no correlation between androgens and calling within each treatment group. In addition, noncallers in the chorus group had higher levels of androgens than males in the tone or no sound groups. Further, chorus-exposed males with zero, low, or high rate of calling had similar levels of androgens. These data indicate that social signals increase circulating androgens independently of calling behavior. Elevated corticosterone associated with chorus reception did not inhibit calling behavior, and corticosterone showed no correlation with androgen levels.

Anticholinergic effects in frogs in a Morris water maze analog.

We determined the effect of two doses of the centrally acting anticholinergic drug, atropine sulfate (AS), on the performance of female Northern Leopard frogs (Rana pipiens) in a visual cue analog of the Morris water task. Untreated frogs learned the visually cued task, while frogs treated with 150 mg/kg AS were significantly slower than controls in learning to escape warm water by finding a visible platform, and there was a dose-dependent response, with frogs treated with 50 mg/kg AS performing midway between the higher dose and control frogs. These results suggest that the general role of the cholinergic system in learning is important in amphibians, and that this role is evolutionarily conserved across vertebrate species.

Female preferences for temporal order of call components in the túngara frog: a Bayesian analysis.

We employed a Bayesian statistical approach to examine female preferences in the Neotropical frog Physalaemus pustulosus for the temporal relationship of the two parts of the conspecific advertisement call. The male advertisement call consists of a 'whine', which is necessary for species recognition, followed immediately by one or more 'chucks', which make the whine more attractive to females. We conducted 42 two-choice experiments with a total of 840 individual tests to compare the attractiveness of a whine with a chuck in a variety of positions relative to the start of the whine against a normal whine or against a normal whine/chuck. Females have a bimodal preference function for chuck position. Chucks placed in a variety of positions after the whine (including the position of a chuck in a normal call) were generally as attractive as a normal whine/chuck. Chucks placed before the whine were generally more attractive than a whine alone, and in some positions as attractive as the normal whine/chuck. Chucks overlapping the end of the whine make the call more attractive than a whine alone, but not as attractive as a normal whine/chuck, while chucks overlapping an initial portion of the whine beginning 50 ms into it are ignored; previous work (Wilczynski et al. 1995, Animal Behaviour, 49, 911-929) indicates this region to be critical for species recognition. These results suggest that female P. pustulosus have preferences for the temporal relationship of the two parts of the conspecific male advertisement call that far exceed the vocal signals produced by males, and that male calls have evolved calls to exploit one peak of this function. Copyright 1999 The Association for the Study of Animal Behaviour.

Forebrain arginine vasotocin correlates of alternative mating strategies in cricket frogs.

In cricket frogs, Acris crepitans, sexually active males can switch between calling and noncalling (satellite) mating strategies and injections of the neuropeptide arginine vasotocin (AVT) stimulate calling behavior. We report here that this behavioral variation of animals under field conditions is associated with variations in AVT-immunoreactive (AVT-ir) staining in distinct brain nuclei. In both calling and satellite males, one AVT-ir brain region was found in a continuous string of cells between the medial amygdala and the nucleus accumbens (ACC). Satellite males possessed significantly more AVT-ir staining in the brain (cells and fibers) than calling males at the level of the ACC, although not in the medial amygdala. This difference in AVT-ir staining in the ACC can, in part, be explained by differences in the density of staining within the cells and in cell size. In addition, satellite males had significantly higher AVT-ir staining in the fibers medial to the ACC than calling males. Because other studies have demonstrated that AVT stimulates calling behavior, a plausible hypothesis is that calling males are releasing more AVT from neurons in the ACC, depleting reserves within the cells, and that the released AVT elicits calling behavior. AVT immunoreactivity levels are also higher in the ACC in both calling and satellite males than in female cricket frogs, which do not call. Satellite males may therefore have AVT reserves that might allow them to call depending on the social conditions.

Spatial and reversal learning in congeneric lizards with different foraging strategies.

Environmental demands that require intensive search for mates, food and nest sites are correlated with efficient spatial memory in many mammalian and avian species. This convergence of evidence has led to the view that spatial memory, and the neurological structures associated with it, have been selected in niches that require memory for the location of goal objects. Whether such evolutionary demands are also correlated with nonspatial abilities that require flexible use of associations similar to those required for spatial memory has not been well studied. In addition, correlations between niche types and the use of spatial or nonspatial memory have not been investigated in nonmammalian, nonavian taxa. In this study, we investigated the relationship between foraging strategies and performance on two tasks, one spatial and the other nonspatial, in congeneric lizard species: Acanthodactylus boskianus, an active forager that collects clumped sedentary prey, Acanthodactylus scutellatus, a sit-and-wait predator that collects distributed mobile prey. The two species did not differ in their performance of a spatial memory task, but A. boskianus, the active forager, performed better on the reversal of a visual discrimination, a nonspatial task. These findings question the generality of the spatial adaptation model for vertebrates. We present the pliancy hypothesis, which we developed to account for these results. Copyright 1999 The Association for the Study of Animal Behaviour.

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.

Relative medial and dorsal cortex volume in relation to foraging ecology in congeneric lizards.

The need to locate distributed resources such as mates, food, and nests is correlated with an enlarged hippocampus in many mammalian and avian species. This correlation is believed to be a consequence of selection for spatial ability. Little is known about how such ecological needs affect non-mammalian, non-avian species. In lizards, the putative hippocampal homologues are the dorsal cortex (DC) and medial cortex (MC). We examined the relationship between foraging ecology and the size of the DC and MC in congeneric male lizards. We predicted based on the mammalian and avian literature that Acanthodactylus boskianus, an active forager that captures clumped, immobile prey would have a larger MC and DC than A. scutellatus, a sit-and-wait predator, that captures mobile prey. Our previous behavioral studies showed that A. boskianus did not differ from A. scutellatus on a spatial task but that A. boskianus was significantly better at the reversal of a visual discrimination, another task that is hippocampally dependent in mammals. In the current study, we found that, relative to telencephalon volume, the MC and DC were larger in the active forager whereas a control region, the lateral, olfactory, cortex, was similar in size between species. The current anatomical results suggest that MC and DC size is related to active foraging in lizards and, along with our previous behavioral studies, show that it is possible for this relationship to occur in the absence of evidence for species differences in spatial memory.

Intraspecific variation in laryngeal and ear morphology in male cricket frogs (Acris crepitans)

In a previous report, the authors found significant population variation in the calls of cricket frogs (Acris crepitans) that could not be explained by geographic variation in body size alone. Here we extend that work by investigating intraspecific population variation in the morphological characteristics underlying acoustic communication in male cricket frogs from several sites in Texas. We measured the volumes of laryngeal and auditory components responsible for the generation or reception of species-specific vocalizations in male frogs from eight populations. We found significant differences among populations in body size, as well as all the laryngeal and ear components we measured. With the exception of vocal cord and extracolumella volumes, the volumes of these anatomical structures differ among populations independently of body size as determined by a covariate analysis with snout-vent length as the covariate. Call dominant frequency differs among populations in a clinal pattern and head width, arytenoid cartilage, vocal cord and dilator muscle volume show a similar pattern when the residuals of the regression of morphological component on SVL are assessed for this trend. The results show that both larynx and ear structures can change in size independently of body size, yielding significant geographic variation in the behavioral and physiological expressions of the acoustic communication system underlying mate choice.Copyright 1998 The Linnean Society of London

The effects of arginine vasotocin on the calling behavior of male cricket frogs in changing social contexts.

We investigated the effects of the neurohypophysial peptide, arginine vasotocin (AVT), on the calling behavior of male Acris crepitans during and immediately following a simulated acoustic agonistic encounter. AVT did not block the aggressive response to agonistic calls, as the changes in temporal call characteristics in response to the encounter were similar to those of saline-treated males. However, AVT caused males to begin calling sooner during the agonistic encounter and to call significantly more than saline males during and after the agonistic encounter. In addition, AVT-treated males maintained a higher dominant frequency compared to saline animals during and following the agonistic encounter. Changes in temporal characteristics in the period following the agonistic encounter indicated that control males were more likely to exhibit a rebound effect which resulted in larger changes in calling parameters compared to AVT-treated animals. The results indicate that AVT causes changes in calling behavior in male A. crepitans during and following an agonistic encounter that are consistent with animals highly motivated to maintain vigorous active calling throughout changing social conditions.

Sexual dimorphism and species differences in the neurophysiology and morphology of the acoustic communication system of two neotropical hylids.

We examined auditory tuning and the morphology of the anatomical structures underlying acoustic communication in female Hyla microcephala and H. ebraccata and compared our findings to data from a previous study (Wilczynski et al. 1993) in which we showed species differences in the traits that in males relate to differences in the species-typical calls. Female species differences in the best excitatory frequency (BEF) of the basilar papilla (BP) were similar to the differences seen in males, and females had a significantly lower BEF in H. ebraccata, but not H. microcephala. In both species, females had lower BP thresholds. Snout-vent length, head width, and tympanic membrane diameters were sexually dimorphic in both species and larger in females, whereas laryngeal components were sexually dimorphic and larger in males. Middle and inner ear volumes were not sexually dimorphic. Despite the significant species differences in laryngeal morphology seen in males, female larynges are not significantly different. Furthermore, the interaction of species and sex differences resulted in significantly different degrees of sex dimorphism in the species, particularly for the larynx, which is more sexually dimorphic in H. microcephala, and measures of body size, which are more dimorphic in H. ebraccata.

Correlations between call characteristics and morphology in male cricket frogs (Acris crepitans).

We investigated the relationships among spectral and temporal advertisement-call characteristics and the sizes of the laryngeal and ear components thought to underlie the generation and reception of species-specific vocalizations in male cricket frogs (Acris crepitans). We tested the predictions that the volumes of the structural elements necessary for acoustic communication would be correlated with various parameters of the vocalizations. The anatomy of laryngeal and ear structures was reconstructed from serial sections of the heads of male cricket frogs of two subspecies collected from several sites across the range of this species in Texas, USA. The relationships among the anatomy and call parameters were assessed using several univariate and multivariate analyses. Highly significant univariate correlations among the laryngeal components suggest that the temporal and spectral characteristics of the calls are not independently produced. Dominant frequency correlates strongly with most of the other call and morphological characteristics. Removing body size effects, however, removes the relationship between dominant frequency and the volume of the whole larynx and ear. This is also the case for call pulse rate, indicating that for this species both spectral and temporal call parameters are biomechanically related to laryngeal size which is, in turn, largely mediated by body size. General body size effects might also explain the existence of significant relationships between ear size and temporal characteristics of the call that probably do not have a functional basis.

Arginine vasotocin injection increases probability of calling in cricket frogs, but causes call changes characteristic of less aggressive males.

Male cricket frogs, Acris crepitans communicate to males and females using advertisement calls, which are arranged into call groups. Calls at the middle and end, but not beginning of the call group, are modified in response to male-male aggressive interactions. We found in this field study of male cricket frogs in natural breeding choruses that the peptide hormone arginine vasotocin (AVT) not only increased the probability that males called after injections, but also caused modifications in middle and end calls to produce calls characteristic of less aggressive males. Moreover, AVT-injected males showed significantly greater increases in call dominant frequency than saline-injected males, again, a characteristic of less aggressive males. Cricket frog calls are used to both repel males and attract females, thus call changes may relate to male-male and/or male-female interactions. Saline-injected males also demonstrated significant changes in several call traits, including changes that occurred in the beginning and middle calls of the call groups, but not the end calls. AVT appeared to block some call changes produced through handling. These data suggest that AVT can influence acoustic communication in frogs in several ways, including effects on call characteristics and dominant frequency, as well as potentially blocking some handling effects.