Robotic Frog Research Articles

Design of the Jump Mechanism for a Biomimetic Robotic Frog.

Frogs are vertebrate amphibians with both efficient swimming and jumping abilities due to their well-developed hind legs. They can jump over obstacles that are many or even tens of times their size on land. However, most of the current jumping mechanisms of biomimetic robotic frogs use simple four-bar linkage mechanisms, which has an unsatisfactory biomimetic effect on the appearance and movement characteristics of frogs. At the same time, multi-joint jumping robots with biomimetic characteristics are subject to high drive power requirements for jumping action. In this paper, a novel jumping mechanism of a biomimetic robotic frog is proposed. Firstly, the structural design of the forelimb and hindlimb of the frog is given, and the hindlimb of the robotic frog is optimized based on the design of a single-degree-of-freedom six-bar linkage. A simplified model is established to simulate the jumping motion. Secondly, a spring energy storage and trigger mechanism is designed, including incomplete gear, one-way bearing, torsion spring, and so on, to realize the complete jumping function of the robot, that is, elastic energy storage and regulation, elastic energy release, and rapid leg retraction. Thirdly, the experimental prototype of the biomimetic robotic frog is fabricated. Finally, the rationality and feasibility of the jumping mechanism are verified by a jumping experiment. This work provides a technical and theoretical basis for the design and development of a high-performance amphibious biomimetic robotic frog.

How signals interact in multimodal displays: Insights from a robotic frog.

Caldart, M. V., M. B. dos Santos & G. Machado (2021). Function of a multimodal signal: a multiple hypothesis test using a robot frog Journal of Animal Ecology, https://doi.org/10.1111/1365-2656.13620. Animals can communicate using signals perceived by different sensory systems, and many combine multiple sensory modalities in their display repertoires. Why these multimodal displays evolve and how they function to transmit information between individuals are crucial questions in behavioural and evolutionary research. Most empirical studies addressing these questions assume, even if implicitly, that signals of different modalities have independent effects on receiver responses. Nonetheless, the potential for interactions between signals as an explanation for the function of multimodal displays has been recognized for over two decades. Caldart et al. (2021) use a robotic frog and a receiver-based approach to test four alternative hypotheses for the function of multimodal (acoustic+visual) displays in the stream-dwelling frog Crossodactylus schmidti. Their results lend support to an inter-signal interaction mechanism, whereby inclusion of visual signals modifies the context in which an acoustic display is interpreted. In contrast, the results in Caldart et al. (2021) are less consistent with the hypotheses that emphasize the quality-related information encoded in different signal modalities and a hypothesis that focuses on signal transmission across heterogeneous environments. These results showcase how experimental manipulation of different signal modalities and tests of multiple alternative hypotheses are key to clarifying the function of multimodal displays.

Fully 3D printed multi-material soft bio-inspired frog for underwater synchronous swimming

Inspired by the relatively simple morphology of Anura such as Rana Esculenta (a semi aquatic frog), we propose the design, fabrication, characterization of a soft biomimetic robotic frog based on multilayer structural design of Shape Memory Alloy (SMA) and ultra-flexible material. Dual thrust generation approach has been proposed to achieve the synchronous swimming motion of frog, utilizing four SMA (BMF 150) muscle wires. The frog robot is named as Exploratory Frog (EXPOG) and is fabricated using multiheaded 3D printing system. Whereas the body of the EXPOG is printed using polylactic acid (PLA), which is connected with functionally responsive limbs (50 mm length and average 6 mm gradient thickness). Each limb consists of two muscles (SMA-1 and SMA-2). SMA-1 works as the main thrust creating unit, which is powered by 5V, 250 mA whereas SMA-2 creates back thrust force for antagonistic motion. The proposed kinematic model was simulated in COMSOL to verify the surface velocity and vortex generation in water using the frog's strokes. The design motivated from the rigorous modelling and real frog dynamics analysis, enabled the as-developed EXPOG swim synchronously in significant level of consistency with the real frog. Electrical and mechanical characterizations have been performed. Moreover, experimental data was further processed using TRACKER for angle, displacement, and velocity analysis. The EXPOG (weighing 57 g) can swim at different controllable frequencies (0.5 – 2 Hz) which enables it to swim at the speed up to 15 mm/s (0.3 BLS) on the surface of deep water (150 cm) with excellent weight balance.

Biologically inspired swimming robotic frog based on pneumatic soft actuators

Research on soft robots and swimming robots has been widely reported and demonstrated. However, none of these soft swimming robots can swim flexibly and efficiently using legs, just like a frog. This paper demonstrates a self-contained, untethered swimming robotic frog actuated by 12 pneumatic soft actuators, which can swim in the water for dozens of minutes by mimicking the paddling gait of the natural frog. We designed two kinds of pneumatic soft actuators as the joints on the robotic frog’s legs, which allows the legs to be lighter and more compact. It is found that such soft actuators have great potential in developing amphibious bionic robots, because they are fast-responding, inherently watertight and simple in structural design. The kinematic analysis in swimming locomotion was conducted for the prototype robotic frog, and the locomotion trajectory of each leg was planned based on the analysis of the paddling gait of frogs. Combined with the deformation model of the soft actuators, the robotic frog’s legs are controlled by coordinating the air pressure of each joint actuator. The robotic frog’s body is compact and the total mass is 1.29 kg. Different paddling gaits were tested to investigate swimming performance. The results show that the robotic frog has agile swimming ability and high environmental adaptability. The robotic frog can swim forward more than 0.6 m (3.4 times the body length) in one paddling gait cycle(6 s), whose average swimming velocity is about 0.1 m s−1, and the minimum turning radius is about 0.15 m (less than 1 body length).

Multimodal weighting differences by bats and their prey: probing natural selection pressures on sexually selected traits

Multimodal communication has received increasing attention in recent years. While much is understood about how intended receivers (such as potential mates) respond to multimodal displays, less is known about how eavesdropping predators perceive and interpret these cues. The male tungara frog, Physalaemus pustulosus, is a neotropical anuran that attracts females with an acoustic call and a dynamically inflating/deflating vocal sac. However, the tungara frog's multimodal courtship display also attracts eavesdropping predators, such as fringe-lipped bats, Trachops cirrhosus. We utilized robotic frog models to expose fringe-lipped bats to multimodal tungara frog courtship displays. The models varied in call amplitude and/or the presence of vocal sac cues. In a two-choice test, we show that fringe-lipped bats more often attack higher-amplitude calls. Additionally, coupling the inflating vocal sac cues to the lower-amplitude frog call increased the probability that a bat would attack this less attractive call. Previous studies have demonstrated that vocal sac cues do not increase the attractiveness of low-amplitude calls to female P. pustulosus. Thus, although natural selection, through the bats, and sexual selection, through the female frogs, exert counter-selection forces on the male's sexual display, the strength of these forces are not symmetrical. We discuss possible explanations for why this might be the case. This study underlines the importance of understanding the contribution of both intended and unintended receivers on signal evolution, and it helps explain how selection pressures might vary across sensory modalities.

Perceived Synchrony of Frog Multimodal Signal Components IsInfluenced by Content and Order.

Multimodal signaling is common in communication systems. Depending on the species, individual signal components may be produced synchronously as a result of physiological constraint (fixed) or each component may be produced independently (fluid) in time. For animals that rely on fixed signals, a basic prediction is that asynchrony between the components should degrade the perception of signal salience, reducing receiver response. Male túngara frogs, Physalaemus pustulosus, produce a fixed multisensory courtship signal by vocalizing with two call components (whines and chucks) and inflating a vocal sac (visual component). Using a robotic frog, we tested female responses to variation in the temporal arrangement between acoustic and visual components. When the visual component lagged a complex call (whine + chuck), females largely rejected this asynchronous multisensory signal in favor of the complex call absent the visual cue. When the chuck component was removed from one call, but the robofrog inflation lagged the complex call, females responded strongly to the asynchronous multimodal signal. When the chuck component was removed from both calls, females reversed preference and responded positively to the asynchronous multisensory signal. When the visual component preceded the call, females responded as often to the multimodal signal as to the call alone. These data show that asynchrony of a normally fixed signal does reduce receiver responsiveness. The magnitude and overall response, however, depend on specific temporal interactions between the acoustic and visual components. The sensitivity of túngara frogs to lagging visual cues, but not leading ones, and the influence of acoustic signal content on the perception of visual asynchrony is similar to those reported in human psychophysics literature. Virtually all acoustically communicating animals must conduct auditory scene analyses and identify the source of signals. Our data suggest that some basic audiovisual neural integration processes may be at work in the vertebrate brain.

Bats perceptually weight prey cues across sensory systems when hunting in noise.

Anthropogenic noise can interfere with environmental information processing and thereby reduce survival and reproduction. Receivers of signals and cues in particular depend on perceptual strategies to adjust to noisy conditions. We found that predators that hunt using prey sounds can reduce the negative impact of noise by making use of prey cues conveyed through additional sensory systems. In the presence of masking noise, but not in its absence, frog-eating bats preferred and were faster in attacking a robotic frog emitting multiple sensory cues. The behavioral changes induced by masking noise were accompanied by an increase in active localization through echolocation. Our findings help to reveal how animals can adapt to anthropogenic noise and have implications for the role of sensory ecology in driving species interactions.

Interactions between complex multisensory signal components result in unexpected mate choice responses

Multimodal (multisensory) signalling is common in many species and often facilitates communication. How receivers integrate individual signal components of multisensory displays, especially with regard to variance in signal complexity, has received relatively little attention. In nature, male tungara frogs, Physalaemus pustulosus, produce multisensory courtship signals by vocalizing and presenting their inflating and deflating vocal sac as a visual cue. Males can produce a simple call (whine only) or a complex call (whine + one or more chucks). In a series of two-choice experiments, we tested female preferences for variation in acoustic call complexity and amplitude (unimodal signals). We then tested preferences for the same calls when a dynamic robotic frog was added to one call, generating a multimodal stimulus. Females preferred a complex call to a simple call; when both calls contained at least one chuck, additional numbers of chucks did not further increase attractiveness. When calls contained zero or one chuck, the visual stimulus of the robofrog increased call attractiveness. When calls contained multiple chucks, however, the visual component failed to enhance call attractiveness. Females also preferred higher amplitude calls and the addition of the visual component to a lower amplitude call did not alter this preference. At relatively small amplitude differences, however, the visual signal increased overall discrimination between the calls. These results indicate that the visual signal component does not provide simple enhancement of call attractiveness. Instead, females integrate multisensory components in a nonlinear fashion. The resulting perception and behavioural response to complex signals probably evolved in response to animals that communicate in noisy environments.

Multimodal signaling improves mating success in the green tree frog (Hyla cinerea), but may not help small males

Many anuran amphibians are challenged with the detection of courtship signals in noisy chorus environments. Anurans and other animals partially solve this discrimination challenge by employing auditory mechanisms such as grouping sounds by frequency, time, or spatial location. Animals are also known to employ visual cues as a mechanism of improving auditory signal detection. In this study, we examined the effect of acoustic and visual stimuli on female mate choice preferences in the green tree frog, Hyla cinerea. We used a series of two choice playback tests and added a robotic frog, with an inflatable vocal sac, to test interactions among visual and acoustic signal components. Females preferred vocalizations with faster call rates (i.e., high energy cost) and lower call frequencies (i.e., larger males). When call properties were held equal, females discriminated against an acoustic only stimulus in favor of the combined acoustic/visual multimodal signal. A visual component did not, however, increase the attractiveness of an otherwise unattractive (high-frequency) acoustic signal. Thus, female green tree frogs integrate the visual display into the acoustic communication system and males that are visually accessible can increase their probability of mating success. Visual accessibility, however, is unlikely to improve mating success for small males (high-frequency callers). Animal communication signals are often complex and communicated in multiple sensory channels (e.g., auditory + visual). Female choice is known to be an important mechanism driving signal evolution. Thus, for complex mating signals, a first step in understanding their evolution is to test how females respond to various combinations of components. Here, we tested female mate choice in the green tree frog, H. cinerea, using a combination of audio playbacks and a robotic frog as the visual component. When the audio signal was standardized, females preferred a signal enhanced by a robotic frog. The robotic frog did not increase female responses to an unattractive call (indicative of a small male), however. These results suggest that visual accessibility can improve a male’s chance of mating, but this advantage is context dependent and does not extend to smaller males.

Cross-modal integration and non-linear relationships: What can frogs tell us about solving cocktail party problems?

Courtship in most anuran amphibians occurs in noisy environments, analogous to human communication at cocktail parties. Female frogs express strong mating preferences for particular properties of male vocalizations, but how they identify individual callers within the noisy chorus environment remains unclear. One possible mechanism is cross-modal integration, whereby females attend to both acoustic and visual cues (male vocal sac inflation). In choice experiments, we used a robotic frog with an inflating vocal sac, combined with acoustic playbacks, to test the role of cross-modal integration in female tüngara frogs. In nature, male tüngara frogs produce a two-note courtship call and the vocal sac inflates synchronously during production of both notes. We tested female mating preferences when we artificially varied the temporal synchrony of the vocal sac inflation relative to the two call notes. Some combinations elicited a strong preference from females, some combinations generated a strong aversive response, and other combinations were neutral. These data show that females conduct cross-modal assessments of male callers. The temporal combinations that elicited positive, negative, or neutral responses were not predictive in a linear fashion, however, suggesting that the integration of visual cues may strongly modulate auditory perception in females.

Determining the influence of muscle operating length on muscle performance during frog swimming using a bio-robotic model

Frogs are capable of impressive feats of jumping and swimming. Recent work has shown that anuran hind limb muscles can operate at lengths longer than the ‘optimal length’. To address the implications of muscle operating length on muscle power output and swimming mechanics, we built a robotic frog hind limb model based upon Xenopus laevis. The model simulated the force–length and force–velocity properties of vertebrate muscle, within the skeletal environment. We tested three muscle starting lengths, representing long, optimal and short starting lengths. Increasing starting length increased maximum muscle power output by 27% from 98.1 W kg−1 when muscle begins shortening from the optimal length, to 125.1 W kg−1 when the muscle begins at longer initial lengths. Therefore, longer starting lengths generated greater hydrodynamic force for extended durations, enabling faster swimming speeds of the robotic frog. These swimming speeds increased from 0.15 m s−1 at short initial muscle lengths, to 0.39 m s−1 for the longest initial lengths. Longer starting lengths were able to increase power as the muscle's force–length curve was better synchronized with the muscle's activation profile. We further dissected the underlying components of muscle force, separating force–length versus force–velocity effects, showing a transition from force–length limitations to force–velocity limitations as starting length increased.

Inter-signal interaction and uncertain information in anuran multimodal signals

Abstract Disentangling the influence of multiple signal components on receivers and elucidating general processes influencing complex signal evolution are difficult tasks. In this study we test mate preferences of female squirrel treefrogs Hyla squirella and female túngara frogs Physalaemus pustulosus for similar combinations of acoustic and visual components of their multimodal courtship signals. In a two-choice playback experiment with squirrel treefrogs, the visual stimulus of a male model significantly increased the attractivness of a relatively unattractive slow call rate. A previous study demonstrated that faster call rates are more attractive to female squirrel treefrogs, and all else being equal, models of male frogs with large body stripes are more attractive. In a similar experiment with female túngara frogs, the visual stimulus of a robotic frog failed to increase the attractiveness of a relatively unattractive call. Females also showed no preference for the distinct stripe on the robot that males commonly bear on their throat. Thus, features of conspicuous signal components such as body stripes are not universally important and signal function is likely to differ even among species with similar ecologies and communication systems. Finally, we discuss the putative information content of anuran signals and suggest that the categorization of redundant versus multiple messages may not be sufficient as a general explanation for the evolution of multimodal signaling. Instead of relying on untested assumptions concerning the information content of signals, we discuss the value of initially collecting comparative empirical data sets related to receiver responses.

Multimodal signal variation in space and time: how important is matching a signal with its signaler?

Multimodal signals (acoustic+visual) are known to be used by many anuran amphibians during courtship displays. The relative degree to which each signal component influences female mate choice, however, remains poorly understood. In this study we used a robotic frog with an inflating vocal sac and acoustic playbacks to document responses of female túngara frogs to unimodal signal components (acoustic and visual). We then tested female responses to a synchronous multimodal signal. Finally, we tested the influence of spatial and temporal variation between signal components for female attraction. Females failed to approach the isolated visual cue of the robotic frog and they showed a significant preference for the call over the spatially separate robotic frog. When presented with a call that was temporally synchronous with the vocal sac inflation of the robotic frog, females did not show a significant preference for this over the call alone; when presented with a call that was temporally asynchronous with vocal sac inflation of the robotic frog, females discriminated strongly against the asynchronous multimodal signal in favor of the call alone. Our data suggest that although the visual cue is neither necessary nor sufficient for attraction, it can strongly modulate mate choice if females perceive a temporal disjunction relative to the primary acoustic signal.