Temporal variation in acoustic and visual signalling as a function of stream background noise in the Bornean foot-flagging frog, Staurois parvus

Physiological Basis of Convergent Evolution in Animal Communication Systems

To humans, the diverse array of display behaviors that animals use for communication can easily seem peculiar or bizarre. While ample research delves into the evolutionary principles that shape these signals' effectiveness, little attention is paid to evolutionary patterning of signal design across taxa, particularly when it comes to the potential convergent evolution of many elaborate behavioral displays. By taking a mechanistic perspective, we explore the physiological and neurobiological mechanisms that likely influence the evolution of communication signals, emphasizing the utilization of pre-existing structures over novel adaptations. Central to this investigation are the concepts of perceptual bias and ritualization that we propose contribute to the convergence of elaborate display designs across species. Perceptual bias explains a phenomenon where pre-existing perceptual systems of receivers, used for innate behaviors such as food and predator recognition, select for certain traits of a communication signal from a signaler. Ritualization occurs when traits with no functional role in communication are co-opted through selection and transformed into a new communicative signal. Importantly, susceptibility for ritualization can be brought about through physiological modifications that occurred early in evolutionary time. In this way, perceptual bias can be a selective force that causes the co-option of non-communicative traits into a new communication signal through ritualization involving pre-existing modifications to physiological systems. If the perceptual bias, non-communicative signal, and physiological modifications that increase susceptibility to ritualization are highly conserved, then we may see the convergent evolution of the new communication signal with unrelated taxa facing similar sensory constraints. We explore this idea here using the foot-flagging frog system as a theoretical case study.

How new communication behaviors evolve: Androgens as modifiers of neuromotor structure and function in foot-flagging frogs

How diverse animal communication signals have arisen is a question that has fascinated many. Xenopus frogs have been a model system used for three decades to reveal insights into the neuroendocrine mechanisms and evolution of vocal diversity. Due to the ease of studying central nervous system control of the laryngeal muscles in vitro, Xenopus has helped us understand how variation in vocal communication signals between sexes and between species is produced at the molecular, cellular, and systems levels. Yet, it is becoming easier to make similar advances in non-model organisms. In this paper, we summarize our research on a group of frog species that have evolved a novel hind limb signal known as 'foot flagging.' We have previously shown that foot flagging is androgen dependent and that the evolution of foot flagging in multiple unrelated species is accompanied by the evolution of higher androgen hormone sensitivity in the leg muscles. Here, we present new preliminary data that compare patterns of androgen receptor expression and neuronal cell density in the lumbar spinal cord - the neuromotor system that controls the hind limb - between foot-flagging and non-foot-flagging frog species. We then relate our work to prior findings in Xenopus, highlighting which patterns of hormone sensitivity and neuroanatomical structure are shared between the neuromotor systems underlying Xenopus vocalizations and foot-flagging frogs' limb movement and which appear to be species-specific. Overall, we aim to illustrate the power of drawing inspiration from experiments in model organisms, in which the mechanistic details have been worked out, and then applying these ideas to a non-model species to reveal new details, further complexities, and fresh hypotheses.

Consistency in responses to conspecific advertisement calls with various signal-to-noise ratios in both sexes of the Anhui tree frog

Environmental noise has a significant negative impact on acoustic communication in most situations, as it influences the production, transmission, and reception of acoustic signals. However, how animals respond to conspecific sounds when there is interference from environmental noise, and whether males and females display convergent behavioral responses in the face of noise masking remain poorly understood. In this study, we investigated the effects of conspecific male advertisement calls with different signal-to-noise ratios on male-male competition and female choice in the Anhui tree frog Rhacophorus zhoukaiyae using playback and phonotaxis experiments, respectively. The results showed that (1) female Anhui tree frogs preferentially selected the conspecific calls with higher SNR compared to calls with lower SNR; (2) males preferentially responded vocally to the conspecific calls with higher SNR compared to calls with lower SNR; and (3) males' competitive strategies were flexible in the face of noise interference. These results suggest that preferences of both sexes converge in outcome, and that male competitive strategies may depend on predictable female preferences. This study will provide an important basis for further research on decision-making in animals.

Noise affects mate choice based on visual information via cross-sensory interference

Animal communication is often hampered by noise interference. Noise masking has primarily been studied in terms of its unimodal effect on sound information provision and use, while little is known about its cross-modal effect and how animals weigh unimodal and multimodal courtship cues in noisy environments. Here, we examined the cross-modal effects of background noise on female visual perception of mate choice and female preference for multimodal displays (sound + vocal sac) in a species of treefrog. We tested female mate choices using audio/video playbacks in the presence and absence of noise (white noise band-filtered to match or mismatch female sensitive hearing range, heterospecific chorus). Surprisingly, multimodal displays do not improve receiver performance in noise. The heterospecific chorus and white noise band-filtered to match female sensitive hearing ranges, significantly reduced female responses to the attractive visual stimuli in addition to directly impairing auditory information use. Meanwhile, the cross-modal impacts of background noise are influenced to some extent by whether the noise band matches female sensitive hearing range and the difficulty of distinguishing tasks. Our results add to the evidence for cross-modal effects of noise and are the first to demonstrate that background noise can disrupt female responses to visual information related to mate choice, which may reduce the communication efficiency of audiovisual signals in noisy environments and impose fitness consequences. This study has key ecological and evolutionary implications because it illustrates how noise influences mate choice in wildlife via cross-sensory interference, which is crucial in revealing the function and evolution of multimodal signals in noisy environments as well as informing evidence-based conservation strategies for forecasting and mitigating the multimodal impacts of noise interference on wildlife.

Can you hear/see me? Multisensory integration of signals does not always facilitate mate choice

Females of many species choose mates using multiple sensory modalities. Multimodal noise may arise, however, in dense aggregations of animals communicating via multiple sensory modalities. Some evidence suggests multimodal signals may not always improve receiver decision-making performance. When sensory systems process input from multimodal signal sources, multimodal noise may arise and potentially complicate decision-making due to the demands on cognitive integration tasks. We tested female túngara frog, Physalaemus (=Engystomops) pustulosus, responses to male mating signals in noise from multiple sensory modalities (acoustic and visual). Noise treatments were partitioned into three categories: acoustic, visual, and multimodal. We used natural calls from conspecifics and heterospecifics for acoustic noise. Robotic frogs were employed as either visual signal components (synchronous vocal sac inflation with call) or visual noise (asynchronous vocal sac inflation with call). Females expressed a preference for the typically more attractive call in the presence of unimodal noise. However, during multimodal signal and noise treatments (robofrogs employed with background noise), females failed to express a preference for the typically attractive call in the presence of conspecific chorus noise. We found that social context and temporal synchrony of multimodal signaling components are important for multimodal communication. Our results demonstrate that multimodal signals have the potential to increase the complexity of the sensory scene and reduce the efficacy of female decision making.

Parasite defensive limb movements enhance acoustic signal attraction in male little torrent frogs

Many animals rely on complex signals that target multiple senses to attract mates and repel rivals. These multimodal displays can however also attract unintended receivers, which can be an important driver of signal complexity. Despite being taxonomically widespread, we often lack insight into how multimodal signals evolve from unimodal signals and in particular what roles unintended eavesdroppers play. Here, we assess whether the physical movements of parasite defense behavior increase the complexity and attractiveness of an acoustic sexual signal in the little torrent frog (Amolops torrentis). Calling males of this species often display limb movements in order to defend against blood-sucking parasites such as frog-biting midges that eavesdrop on their acoustic signal. Through mate choice tests we show that some of these midge-evoked movements influence female preference for acoustic signals. Our data suggest that midge-induced movements may be incorporated into a sexual display, targeting both hearing and vision in the intended receiver. Females may play an important role in incorporating these multiple components because they prefer signals which combine multiple modalities. Our results thus help to understand the relationship between natural and sexual selection pressure operating on signalers and how in turn this may influence multimodal signal evolution.

Function of a multimodal signal: A multiple hypothesis test using a robot frog

Multimodal communication may evolve because different signals may convey information about the signaller (content-based selection), increase efficacy of signal processing or transmission through the environment (efficacy-based selection), or modify the production of a signal or the receiver's response to it (inter-signal interaction selection). To understand the function of a multimodal signal (aggressive calls + toe flags) emitted by males of the frog Crossodactylus schmidti during territorial contests, we tested two hypotheses related to content-based selection (quality and redundant signal), one related to efficacy-based selection (efficacy backup), and one related to inter-signal interaction selection (context). For each hypothesis we derived unique predictions based on the biology of the study species. In a natural setting, we exposed resident males to a robot frog simulating aggressive calls (acoustic stimulus) and toe flags (visual stimulus), combined and in isolation, and measured quality-related traits from males and local levels of background noise and light intensity. Our results provide support to the context hypothesis, as toe flags (the context signal) are insufficient to elicit a receiver's response on their own. However, when toe flags are emitted together with aggressive calls, they evoke in the receiver qualitatively and quantitatively different responses from that evoked by aggressive calls alone. In contrast, we found no evidence that toe flags and aggressive calls provide complementary or redundant information about male quality, which are key predictions of the quality and redundant signal hypotheses respectively. Finally, the multimodal signal did not increase the receiver's response across natural gradients of light and background noise, a key prediction of the efficacy backup hypothesis. Toe flags accompanying aggressive calls seem to provide contextual information that modify the receiver's response in territorial contests. We suggest this contextual information is increased motivation to escalate the contest, and discuss the benefits to the signallers and receivers of adding a contextual signal to the aggressive display. Examples of context-dependent multimodal signals are rare in the literature, probably because most studies focus on single hypotheses assuming content- or efficacy-based selection. Our study highlights the importance of considering multiple selective pressures when testing multimodal signal function.

A Common Endocrine Signature Marks the Convergent Evolution of an Elaborate Dance Display in Frogs

AbstractUnrelated species often evolve similar phenotypic solutions to the same environmental problem, a phenomenon known as convergent evolution. But how do these common traits arise? We address this question from a physiological perspective by assessing how convergence of an elaborate gestural display in frogs (foot-flagging) is linked to changes in the androgenic hormone systems that underlie it. We show that the emergence of this rare display in unrelated anuran taxa is marked by a robust increase in the expression of androgen receptor (AR) messenger RNA in the musculature that actuates leg and foot movements, but we find no evidence of changes in the abundance of AR expression in these frogs' central nervous systems. Meanwhile, the magnitude of the evolutionary change in muscular AR and its association with the origin of foot-flagging differ among clades, suggesting that these variables evolve together in a mosaic fashion. Finally, while gestural displays do differ between species, variation in the complexity of a foot-flagging routine does not predict differences in muscular AR. Altogether, these findings suggest that androgen-muscle interactions provide a conduit for convergence in sexual display behavior, potentially providing a path of least resistance for the evolution of motor performance.

Androgen Receptor Modulates Multimodal Displays in the Bornean Rock Frog (Staurois parvus)

Multimodal communication is common in the animal kingdom. It occurs when animals display by stimulating two or more receiver sensory systems, and often arises when selection favors multiple ways to send messages to conspecifics. Mechanisms of multimodal display behavior are poorly understood, particularly with respect to how animals coordinate the production of different signals. One important question is whether all components in a multimodal display share an underlying physiological basis, or whether different components are regulated independently. We investigated the influence of androgen receptors (ARs) on the production of both visual and vocal signal components in the multimodal display repertoire of the Bornean rock frog (Staurois parvus). To assess the role of AR in signal production, we treated reproductively active adult males with the antiandrogen flutamide (FLUT) and measured the performance of each component signal in the multimodal display. Our results show that blocking AR inhibited the production of multiple visual signals, including a conspicuous visual signal known as the "foot flag," which is produced by rotating the hind limb above the body. However, FLUT treatment caused no measurable change in vocal signaling behavior, or in the frequency or fine temporal properties of males' calls. Our study, therefore, suggests that activation of AR is not a physiological prerequisite to the coordination of multiple signals, in that it either does not regulate all signaling behaviors in a male's display repertoire or it does so only in a context-dependent manner.

Convergence to the tiniest detail: vocal sac structure in torrent-dwelling frogs

Cascades and fast-flowing streams impose severe restrictions on acoustic communication, with loud broadband background noise hampering signal detection and recognition. In this context, diverse behavioural features, such as ultrasound production and visual displays, have arisen in the evolutionary history of torrent-dwelling amphibians. The importance of the vocal sac in multimodal communication is being increasingly recognized, and recently a new vocal sac visual display has been discovered: unilateral inflation of paired vocal sacs. In the diurnal stream-breeding Hylodidae from the Atlantic forest, where it was first described, this behaviour is likely to be enabled by a unique anatomical configuration of the vocal sacs. To assess whether other taxa share this exceptional structure, we surveyed torrent-dwelling species with paired vocal sacs across the anuran tree of life and examined the vocal sac anatomy of exemplar species across 18 families. We found striking anatomical convergence among hylodids and species of the distantly related basal ranid genera Staurois, Huia, Meristogenys and Amolops. Ancestral character state reconstruction identified three new synapomorphies for Ranidae. Furthermore, we surveyed the vocal sac configuration of other anuran species that perform visual displays and report observations on what appears to be unilateral inflation of paired vocal sacs, in Staurois guttatus – an extremely rare behaviour in anurans.