Michael S. Reichert

Trade-Offs and Upper Limits to Signal Performance during Close-Range Vocal Competition in Gray Tree Frogs Hyla versicolor

Performance limitations on signal production constrain signal evolution. Variation in signaling performance may be related to signaler quality and therefore is likely to be a salient aspect of communication systems. When multiple signal components are involved in communication, there may be trade-offs between components, and performance can be measured as the degree to which signalers approach the upper limits of the trade-off function. We examined vocal performance in the gray tree frog Hyla versicolor, in which females prefer values of call duration and rate exceeding the usual range of variation within and among males. We recorded interactions between pairs of males calling on mobile platforms that allowed us to manipulate intermale distance and place males in highly competitive environments. We found that, although there was a clear upper boundary on the ability of males to maximize call duration and call rate simultaneously, call effort did not remain constant in this highly competitive situation. Our estimates of an upper limit to vocal performance were corroborated by analyses of calling behavior in the context of close-range mate attraction. We discuss potential constraints on signaling performance and the relevance of this measure of performance for both intrasexual and intersexual communication.

Gray tree frogs, Hyla versicolor, give lower-frequency aggressive calls in more escalated contests

As animal contests escalate, variation in the performance of aggressive signaling behaviors can give important insights into contest dynamics. In anuran amphibians, males of numerous species utilize distinctive aggressive vocalizations during disputes over calling spaces. Little is known, however, about the causes and consequences of variation in aggressive-call characteristics. We analyzed recordings of calls made during staged aggressive interactions between male gray tree frogs, Hyla versicolor, to determine how variation in a key aggressive-call characteristic, dominant frequency, was affected by increasing contest escalation. We found that dominant frequencies of aggressive calls were lower than those of advertisement calls used to attract females. Furthermore, we found that males lowered their aggressive-call frequencies with increases in escalation. Winners tended to have lower-frequency aggressive calls than losers. We conclude that aggressive calls in H. versicolor are similar to the graded aggressive calls that have been described in several other species. This gradation may allow males to balance the energetic costs of producing lower-frequency calls with the benefits of efficiently repelling rival males. Other processes related to motivation and the physiological effects of participating in contests may also be responsible for the observed variation in aggressive-call frequency with contest escalation. Our results demonstrate that detailed experimental studies of aggressive calling behavior in anurans, which to this point have rarely been performed, are feasible and generate important insights relating to general problems in animal contest behavior and animal communication.

The role of body size on the outcome, escalation and duration of contests in the grey treefrog, Hyla versicolor

Aggressive interactions in animals are often resolved in favour of the individual with superior fighting ability, or resource-holding potential (RHP). A recent revival of studies of aggressive behaviour has focused on the assessment strategies used in animal contests. Strategies of dispute resolution through mutual or self-assessment of RHP differ in the predicted relationship between interaction duration and each competitor’s relative and absolute RHP. We studied potential components of RHP (mass, length, body condition) and their relationship to contest duration and the level of escalation in the grey treefrog, Hyla versicolor, using a novel method to stage aggressive interactions in the laboratory. Overall, large males were more likely to win than small males, but they only had an advantage in less escalated interactions and were not more successful in physical fights. There was limited evidence for an effect of body size on interaction duration or the level of escalation. Specifically, the body condition of both the smaller and larger contestant was weakly negatively related to the duration and level of escalation of contests. This relationship is the opposite of what would be expected under any assessment strategy. Given these data, coupled with the lack of relationships between other size measures and interaction duration, we conclude that assessment of body size does not occur in contests in H. versicolor. Other unmeasured components of RHP may play a role in determining interaction duration, and the relatively weak and ineffective fighting abilities of this species may limit the dominance of larger individuals. Relatively little is known about aggressive behaviour in frogs. Our method for staging aggressive interactions allows us to address predictions of game theory models in an important group for studies of animal communication.

Modality interactions alter the shape of acoustic mate preference functions in gray treefrogs

Sexual selection takes place in complex environments where females evaluating male mating signals are confronted with stimuli from multiple sources and modalities. The pattern of expression of female preferences may be influenced by interactions between modalities, changing the shape of female preference functions, and thus ultimately altering the selective landscape acting on male signal evolution. We tested the hypothesis that the responses of female gray treefrogs, Hyla versicolor, to acoustic male advertisement calls are affected by interactions with visual stimuli. We measured preference functions for several call traits under two experimental conditions: unimodal (only acoustic signals presented), and multimodal (acoustic signals presented along with a video‐animated calling male). We found that females were more responsive to multimodal stimulus presentations and, compared to unimodal playbacks, had weaker preferences for temporal call characteristics. We compared the preference functions obtained in these two treatments to the distribution of male call characteristics to make inferences on the strength and direction of selection expected to act on male calls. Modality interactions have the potential to influence the course of signal evolution and thus are an important consideration in sexual selection studies.

Noise affects the shape of female preference functions for acoustic signals

The shape of female mate preference functions influences the speed and direction of sexual signal evolution. However, the expression of female preferences is modulated by interactions between environmental conditions and the females sensory processing system. Noise is an especially relevant environmental condition because it interferes directly with the neural processing of signals. Although noise is therefore likely a significant force in the evolution of communication systems, little is known about its effects on preference function shape. In the grasshopper Chorthippus biguttulus, female preferences for male calling song characteristics are likely to be affected by noise because its auditory system is sensitive to fine temporal details of songs. We measured female preference functions for variation in male song characteristics in several levels of masking noise and found strong effects of noise on preference function shape. The overall responsiveness to signals in noise generally decreased. Preference strength increased for some signal characteristics and decreased for others, largely corresponding to expectations based on neurophysiological studies of acoustic signal processing. These results suggest that different signal characteristics will be favored under different noise conditions, and thus that signal evolution may proceed differently depending on the extent and temporal patterning of environmental noise.

Aggressive thresholds in Dendropsophus ebraccatus: habituation and sensitization to different call types

Males in many chorusing anuran species use aggressive calls during defense of calling spaces from other males. The minimal intensity of another male’s vocalizations that elicits an aggressive call response has been termed the aggressive threshold. Previous studies of aggressive thresholds have shown that they are plastic: males habituated (increased their aggressive thresholds) in response to repeated presentation of stimuli above initial threshold levels. Habituation likely contributes to the stable chorus structure of these species, in which aggressive calling is rare compared to advertisement calls. I have observed high levels of aggressive calling in the treefrog Dendropsophus ebraccatus, suggesting that males of this species do not habituate. In this study, I investigated the plasticity of aggressive thresholds in D. ebraccatus. I measured the aggressive thresholds of males before and after suprathreshold stimulation by both advertisement and aggressive calls. I found that the different call types had different effects: males habituated to advertisement calls but lowered their aggressive thresholds in response to aggressive calls. I consider the latter response to be an example of sensitization, a behavior that has been documented infrequently in vocalizing anurans. Sensitization is a plausible mechanism responsible for the high levels of aggressive calling observed in this species. Given the high costs of aggressive calling, however, it is unclear why a mechanism that increases aggressive call output would be maintained.

Visual cues elicit courtship signals in a nocturnal anuran

In lekking species, the allocation of effort into mate attraction signals is not uniform over time, and signalers may expend the greatest effort when potential mates are nearby. Close-range courtship interactions are critical determinants of male fitness and the study of these interactions can therefore answer important questions in sexual selection. In anurans, attention has largely focused on long-range mate attraction mediated by acoustic signaling. However, many species also engage in courtship behaviors at close range, and the cues that elicit these behaviors are unknown but likely to be non-acoustic. I performed an experiment in which I assessed the role of female visual cues in eliciting courtship calls by males of the nocturnal treefrog Hyla versicolor. Males that could see an approaching female were more likely to give courtship calls than those that could not. These results provide some of the first evidence for an effect of vision on calling behavior in a nocturnal anuran and demonstrate that multiple sensory modalities are involved in the final stages of mate attraction.

Cognition in contests: mechanisms, ecology, and evolution

Animal contests govern access to key resources and are a fundamental determinant of fitness within populations. Little is known about the mechanisms generating individual variation in strategic contest behavior or what this variation means for population level processes. Cognition governs the expression of behaviors during contests, most notably by linking experience gained with decision making, but its role in driving the evolutionary ecological dynamics of contests is only beginning to emerge. We review the kinds of cognitive mechanisms that underlie contest behavior, emphasize the importance of feedback loops and socio-ecological context, and suggest that contest behavior provides an ideal focus for integrative studies of phenotypic variation.

Sources of variability in advertisement and aggressive calling in competitive interactions in the grey treefrog, Hyla versicolor

Understanding the sources and magnitudes of variation in signal traits is an important first step towards understanding the forces that may act on signal evolution. In anuran amphibians, acoustic communication plays a major role in the processes of mate attraction and aggression. The significance of variation in characteristics of the mate-attracting advertisement calls is relatively well understood, whereas another call type, the aggressive call, has received much less attention. I recorded both advertisement and aggressive calls given by male grey treefrogs, Hyla versicolor, in staged competitive interactions. From these recordings, I assessed the effects of physical and abiotic influences on signal production by measuring the correlations between the call characteristics of both call types and variables associated with size, body condition and body temperature. I also estimated within- and between-male variation in call characteristics. Temperature affected diverse characteristics of both call types, whereas the effects of morphology were limited primarily to call-frequency variables. Spectral characteristics of both call types were largely static within males, whereas gross-temporal characteristics were much more variable. Within each call type, many of the call characteristics were correlated with one another, whereas between call types, there were relatively few strong correlations between call characteristics. I discuss these results in terms of their implications for signal evolution, mate choice and assessment in contests.

Effects of multiple-speaker playbacks on aggressive calling behavior in the treefrog Dendropsophus ebraccatus

In addition to producing signals, males of chorusing species also act as receivers by adjusting properties of their vocalizations in response to those of other nearby individuals. Although it is likely that males are responsive to more than one other individual, most playback studies investigating male response have involved dyads in which vocal responses are measured to stimuli presented from a single speaker. In this study, I explored changes in both the propensity to give aggressive calls and the temporal properties of those calls in response to the playback of multiple aggressive call stimuli in the treefrog Dendropsophus ebraccatus. I found that males were sensitive to both the number of simulated aggressive callers and their specific call characteristics. Males generally gave a highly aggressive response to the first stimulus presented, but their response to the modification of this stimulus by the addition or subtraction of a simulated competitor depended on the degree of aggressiveness of the stimuli. Males tended to decrease their aggressive responses when either a more aggressive call was silenced or a less aggressive call was added and to increase their aggressive responses in the opposite situation. Aggressive calling in this species is clearly affected by complex changes in the social environment and I suggest that future studies explore these issues in other species to improve the understanding of communication interactions.