Christopher J. Leary

Counting on Inhibition and Rate-Dependent Excitation in the Auditory System

The intervals between acoustic elements are important in audition. Although neurons have been recorded that show interval tuning, the underlying mechanisms are unclear. The anuran auditory system is well suited for addressing this problem. One class of midbrain neurons in anurans responds selectively over a narrow range of pulse-repetition rates (PRRs) and only after several sound pulses have occurred with the “correct” timing. This “interval-counting” process can be reset by a single incorrect interval. Here we show, from whole-cell patch recordings of midbrain neurons in vivo, that these computations result from interplay between inhibition and rate-dependent excitation. An individual pulse or slowly repeated pulses elicited inhibition and subthreshold excitation. Excitation was markedly enhanced, however, when PRR was increased over a neuron-specific range. Spikes were produced when the enhanced excitation overcame the inhibition. Interval-number thresholds were positively correlated with the strength of inhibition and number of intervals required to augment the excitation. Accordingly, interval-number thresholds decreased when inhibition was attenuated by loading cells with cesium fluoride. The selectivity of these neurons for the interpulse interval, and therefore PRR, was related to the time course of excitatory events and the rate dependence of enhancement; for cells that were tuned to longer intervals, EPSPs were broader, and enhancement occurred at slower PRRs. The frequency tuning of the inhibition generally spanned that of the excitation, consistent with its role in temporal computation. These findings provide the first mechanistic understanding of interval selectivity and counting in the nervous system.

Midbrain auditory neurons integrate excitation and inhibition to generate duration selectivity: an in vivo whole-cell patch study in anurans.

Sound duration can play a pivotal role in the reproductive behavior of anuran amphibians. Here, we report the first whole-cell recordings from duration-selective neurons in the anuran torus semicircularis, in vivo. We show that most short-pass duration-selective cells exhibited short-latency inhibition and delayed excitation. The duration of the inhibition increased with tone burst duration. Hence, for long-duration tone bursts, inhibition overlapped with excitation, reducing or eliminating spikes; no postinhibitory rebound was present. Other short-pass cells, however, showed inhibition only for long-duration tone bursts. Bandpass duration selectivity also involved interplay between inhibition and excitation; inhibition negated excitation with tone bursts that exceeded the optimum duration. Additionally, however, bandpass selectivity arose from stimulus-dependent excitation; tone bursts of sufficiently long duration were required to elicit excitation. Similarly, long-pass neurons showed inhibition and duration-dependent enhancement of excitation; long-pass selectivity resulted from enhanced excitation outlasting the transient inhibition or, in some cases, excitation overriding concurrent inhibition. Last, we evaluated the stimulus specificity of duration-selective neurons to variations in pulse repetition rate. We show that (1) most neurons that exhibited long-pass selectivity for tone-burst duration nonetheless responded to short-duration pulses when repeated at particular rates, and (2) some neurons that showed selectivity for tone burst duration also showed selectivity for pulse train duration. These novel response profiles appear to result from interplay between inhibition and time- and activity-dependent changes in excitation strength. These findings are discussed in the context of prevailing models of duration selectivity and acoustic communication in anurans.

Body size, age, growth and alternative mating tactics in toads: satellite males are smaller but not younger than calling males

Body size–age relationships may have important implications for the evolution and maintenance of alternative mating tactics, but these relationships remain largely unexplored in anuran amphibians (frogs and toads). In a single territorial anuran species, smaller noncalling ‘satellite’ males are younger than calling males. These results have led to the general assumption that such an age difference also holds for other anuran species with body size differences among behavioural phenotypes. However, individuals of many organisms show different developmental and/or growth trajectories that correspond to variation in adult morphology and behaviour. To test the generality of smaller satellite males being younger than larger calling males in anurans, we examined body size–age–behaviour relationships in Woodhouse’s toads, Bufo woodhousii, and Great Plains toads, B. cognatus. Age estimates, based on skeletochronologic analysis of lines of arrested growth (LAGs) from phalanges, indicated that males sampled while adopting a noncalling satellite tactic were smaller, but not younger, than males sampled while adopting a calling tactic. Furthermore, examination of inter-LAG areas of bone deposition indicated that smaller satellite male B. woodhousii showed lower growth rates relative to calling males. A similar trend occurred for B. cognatus .W e discuss our results in the context of developmental patterns associated with alternative mating tactics in other species and endocrine regulation of behavioural tactics in anurans.

Stress Hormone Is Implicated in Satellite‐Caller Associations and Sexual Selection in the Great Plains Toad

The effects of androgens on male‐typical traits suggest that variation among males in circulating levels can play a major role in sexual selection. We examined whether variation in vocal attractiveness is attributable to differences in androgen levels among Great Plains toads (Bufo cognatus). We found that noncalling “satellite” males practicing an alternative mating tactic were more likely to associate with males producing long calls. However, callers with satellites did not have higher androgen levels than callers without satellites. Rather, callers with satellites had significantly lower corticosterone (CORT) levels than callers without satellites. A CORT manipulation experiment suggested that differences in calls for males with and without satellites were related to differences in CORT levels. Furthermore, there was a negative correlation between CORT level and call duration within most nights of chorus activity. However, the correlation was weak for the pooled data (across all nights), suggesting that local environmental and/or social factors also affect call duration. Last, we show that females preferred broadcast calls of longer duration, characteristic of males with satellites and low CORT. These results imply that satellites optimize their reproductive success by associating with males producing long calls. However, this association should negatively affect the fitness of attractive callers.

Evidence of convergent character displacement in release vocalizations of Bufo fowleri and Bufo terrestris (Anura; Bufonidae)

Anuran release vocalizations prevent prolonged amplexus of males by conspecific and heterospecific males, whereas advertisement vocalizations potentially act as a premating isolation mechanism. Selection favouring prompt release during heterospecific amplexus should result in convergent character displacement in release vocalizations. Conversely, selection should produce divergent character displacement in advertisement vocalizations. I examined call properties important to mate recognition from release and advertisement vocalizations of allopatric and sympatric populations of Bufo fowleri and B. terrestris to determine whether the two calls exhibit opposing patterns of character displacement. Results indicated convergent character displacement in the periodicity of release vocalizations (a measure equal to the inverse of pulse rate and an estimate of release vibration rate). The periodicity of advertisement vocalizations was significantly displaced from the allopatric character state resulting in greater divergence in sympatry. However, this parameter was already largely divergent in allopatry and, therefore, results were not consistent with the pattern expected for divergent character displacement. The dominant frequency of sympatric advertisement vocalizations was not significantly displaced from allopatry. Results for advertisement vocalizations provided evidence that convergence of release vocalizations was not the result of hybridization. There was no evidence that clinal variation accounted for displacement of call characteristics. Convergent character displacement in the periodicity of release vocalizations may facilitate interspecific communication during heterospecific amplexus, whereas divergent advertisement vocalizations promote species isolation.  2001 The Association for the Study of Animal Behaviour

Relationships among steroid hormone levels, vocal effort and body condition in an explosive-breeding toad

Mutually reinforcing hormone–behaviour relationships may drive temporal changes in steroid hormone levels and vocal effort in anuran amphibians. Recent models propose that chorus activity stimulates the production of androgens in signallers and receivers, thereby mediating an increase in vocal effort. The energetic demands associated with high vocal effort should, in turn, elevate circulating corticosterone levels to promote the mobilization of energy reserves. High threshold corticosterone levels, however, are expected to negatively affect androgen levels and, hence, vocal effort. Steroid hormone levels and vocal effort are thus expected to be low at the onset of chorus activity, increase over sequential nights of calling until peak levels are reached, and subsequently decline; energy reserves should show inverse temporal trends. To test these predictions, we examined temporal changes in androgen (dihydrotestosterone and testosterone) levels, corticosterone levels, vocal effort and body condition in Woodhouses toads, Bufo woodhousii . Contrary to model predictions, androgen levels, vocal effort and body condition remained relatively unchanged and circulating corticosterone levels tended to decrease within and across nights of chorus activity. Furthermore, although vocal effort was not correlated with circulating androgen levels, it was positively correlated with circulating corticosterone levels, supporting the prediction that high vocal effort promotes the production of corticosterone. Yet despite this correlation, poor body condition was not associated with high corticosterone levels, suggesting that factors other than energetic state influence corticosterone levels in B. woodhousii . We discuss alternative hypotheses regarding relationships among the measured parameters in this explosive-breeding species.

Interval-counting neurons in the anuran auditory midbrain: factors underlying diversity of interval tuning

In anurans, the temporal patterning of sound pulses is the primary information used for differentiating between spectrally similar calls. One class of midbrain neurons, referred to as ‘interval-counting’ cells, appears to be particularly important for discriminating among calls that differ in pulse repetition rate (PRR). These cells only respond after several pulses are presented with appropriate interpulse intervals. Here we show that the range of selectivity and sharpness of interval tuning vary considerably across neurons. Whole-cell recordings revealed that neurons showing temporally summating excitatory postsynaptic potentials (EPSPs) with little or no inhibition or activity-dependent enhancement of excitation exhibited low-pass or band-pass tuning to slow PRRs. Neurons that showed inhibition and rate-dependent enhancement of excitation, however, were band-pass or high-pass to intermediate or fast PRRs. Surprisingly, across cells, interval tuning based on membrane depolarization and spike rate measures were not significantly correlated. Neurons that lacked inhibition showed the greatest disparities between these two measures of interval tuning. Cells that showed broad membrane potential-based tuning, for example, varied considerably in their spike rate-based tuning; narrow spike rate-based tuning resulted from ‘thresholding’ processes, whereby only the largest depolarizations triggered spikes. The potential constraints associated with generating interval tuning in this manner are discussed.

The stress of elaborate male traits: integrating glucocorticoids with androgen-based models of sexual selection

Stress hormones are emerging as major factors regulating the expression of elaborate male traits. Surprisingly, however, the effects of glucocorticoids on such traits have not been formally integrated with androgen-based models of sexual selection. Here we point out that consideration of glucocorticoid-mediated effects on the phenotype provides new insight into long-standing hypotheses and controversies associated with such models. In particular, androgen-based ‘handicap’ models of sexual selection characteristically hinge on graded effects of androgens on male traits, but few studies have found support for such a relationship, suggesting that androgens may not be a primary target of selection. We propose, however, that in many instances, androgens may not appear to have a graded effect on the phenotype because elevated glucocorticoids mask the effects of androgens. Glucocorticoids may be inextricably linked to elaborate traits because the energetic demands associated with such traits promote glucocorticoid production. We thus propose that glucocorticoid-mediated effects on male traits warrant re-evaluation of androgen-based models of sexual selection. In particular, we argue that androgen-based handicap models cannot be dismissed based on the lack of evidence for a graded relationship between androgen level and the extent or magnitude of the trait. Our review of the literature indicates that most studies have examined either the effects of androgens or the effects of glucocorticoids, but not both. A more integrated approach involving the effects of both steroids is necessary to fully understand the role of androgens and the endocrine targets of selection associated with the expression of elaborate male traits.

Elevated stress hormone diminishes the strength of female preferences for acoustic signals in the green treefrog

Mate selection can be stressful; time spent searching for mates can increase predation risk and/or decrease food consumption, resulting in elevated stress hormone levels. Both high predation risk and low food availability are often associated with increased variation in mate choice by females, but it is not clear whether stress hormone levels contribute to such variation in female behavior. We examined how the stress hormone corticosterone (CORT) affects female preferences for acoustic signals in the green treefrog, Hyla cinerea. Specifically, we assessed whether CORT administration affects female preferences for call rate - an acoustic feature that is typically under directional selection via mate choice by females in most anurans and other species that communicate using acoustic signals. Using a dual speaker playback paradigm, we show that females that were administered higher doses of CORT were less likely to choose male advertisement calls broadcast at high rates. Neither CORT dose nor level was related to the latency of female phonotactic responses, suggesting that elevated CORT does not influence the motivation to mate. Results were also not related to circulating sex steroids (i.e., progesterone, androgens or estradiol) that have traditionally been the focus of studies examining the hormonal basis for variation in female mate choice. Our results thus indicate that elevated CORT levels decrease the strength of female preferences for acoustic signals.

A test of the Energetics-Hormone Vocalization model in the green treefrog

Male courtship displays may be regulated by, and affect the production of, circulating hormones. The Energetics-Hormone Vocalization (EHV) model, for example, posits that interactions among chorusing male anuran amphibians stimulate androgen production that then mediates an increase in vocal effort. Increased vocal effort is expected to deplete energy reserves and increase glucocorticoid levels that, in turn, negatively affect androgen levels and vocalization. Androgen levels, glucocorticoid levels, and vocal effort are thus expected to increase across and within nights of chorus activity and should be positively correlated in calling males; energy reserves should decline temporally and be inversely related to glucocorticoid levels. We tested predictions of the EHV model in the green treefrog, Hyla cinerea. Consistent with the model, both testosterone and dihydrotestosterone levels increased across the breeding season in calling males. However, testosterone levels decreased and dihydrotestosterone levels did not change within nights of chorus activity, suggesting that chorusing behavior did not drive the seasonal elevation in androgens. Corticosterone (CORT) level remained relatively stable across the breeding season and decreased within nights of chorus activity, contrary to model predictions. Body condition, the proxy for energetic state, was inversely correlated with CORT level but discrepancies between model predictions and temporal patterns of CORT production arose because there was no evidence of a temporal decrease in body condition or increase in vocal effort. Moreover, androgen and CORT levels were not positively correlated with vocal effort. Additional ecological and physiological measures may be needed to support predictions of the EHV model.