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Synopsis 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. 

AbstractA diversity of defence colourations that shift over time provides protection against natural enemies. Adaptations for camouflage depend on an organism’s interactions with the natural environment (predators, habitat), which can change ontogenetically. Wallace’s flying frogs (Rhacophorus nigropalmatus) are cryptic emerald green in their adult life stage, but juveniles are bright red and develop white spots on their back 1 month after metamorphosis. This latter conspicuous visual appearance might function as antipredator strategy, where frogs masquerade as bird or bat droppings so that predators misidentified them as inedible objects. To test this idea, we created different paraffin wax frog models—red with white spots, red without white spots, green, and unpainted—and placed them in equal numbers within a > 800 m²rainforest house at the Vienna Zoo. This environment closely resembles the Bornean rainforest and includes several free-living avian predators of frogs. We observed an overall hit rate of 15.5%. A visual model showed that the contrast of red, green and control models against the background colouration could be discriminated by avian predators, whereas green models had less chromatic difference than red morphs. The attack rate was significantly greater for red but was reduced by half when red models had white spots. The data therefore supports the hypothesis that the juvenile colouration likely acts as a masquerade strategy, disguising frogs as animal droppings which provides similar protection as the cryptic green adult colour. We discuss the ontogenetic colour change as a possible antipredator strategy in relation to the different habitats used at different life stages. Significance statementPredation pressure and the evolution of antipredator strategies site at the cornerstone of animal-behaviour research. Effective antipredator strategies can change in response to different habitats that animals use during different life stages. We study ontogenetic shifts in colour change as dynamic antipredator strategy in juvenile and adult Wallace’s flying frogs. We show that the unusual colour pattern of juveniles (bright red with small white spots) likely functions as a masquerade of animal droppings. Specifically, we show that white dotting, which can be associated with animal faeces, acts as the main visual feature that turns an otherwise highly conspicuous individual into a surprisingly camouflaged one. To our knowledge, this is the first experimental exploration of a vertebrate masquerading as animal droppings. 

Gene duplication is an important process of molecular evolutionary change, though identifying these events and their functional implications remains challenging. Studies on gene duplication more often focus on the presence of paralogous genes within the genomes and less frequently explore shifts in expression. We investigated the evolutionary history of calsequestrin (CASQ), a crucial calcium-binding protein in the junctional sarcoplasmic reticulum of muscle tissues. CASQ exists in jawed vertebrates as subfunctionalized paralogs CASQ1 and CASQ2 expressed primarily in skeletal and cardiac muscles, respectively. We used an enhanced sequence dataset to support initial duplication of CASQl in a jawed fish ancestor prior to the divergence of cartilaginous fishes. Surprisingly, we find CASQ2 is the predominant skeletal muscle paralog in birds, while CASQ1 is either absent or effectively nonfunctional. Changes in the amino acid composition and electronegativity of avian CASQ2 suggest enhancement to calcium-binding properties that preceded the loss of CASQ1. We identify this phenomenon as CASQ2 “synfunctionalization,” where one paralog functionally replaces another. While additional studies are needed to fully understand the dynamics of CASQ1 and CASQ2 in bird muscles, the long and consistent history of CASQ subfunctions outside of birds indicate a substantial evolutionary pressure on calcium-cycling processes in muscle tissues, likely connected to increased avian cardiovascular and metabolic demands. Our study provides an important insight into the molecular evolution of birds and shows how gene expression patterns can be comparatively studied across phylum-scale deep time to reveal key evolutionary events 

Negotiating social dynamics among allies and enemies is a complex problem that often requires individuals to tailor their behavioral approach to a specific situation based on environmental and/or social factors. One way to make these contextual adjustments is by arranging behavioral output into intentional patterns. Yet, few studies explore how behavioral patterns vary across a wide range of contexts, or how allies might interlace their behavior to produce a coordinated response. Here, we investigate the possibility that resident female and male downy woodpeckers guard their breeding territories from conspecific intruders by deploying defensive behavior in context-specific patterns. To study whether this is the case, we use correlation networks to reveal how suites of agonistic behavior are interrelated. We find that residents do organize their defense into definable patterns, with female and male social mates deploying their behaviors non-randomly in a correlated fashion. We then employ spectral clustering analyses to further distill these responses into distinct behavioral motifs. Our results show that this population of woodpeckers adjusts the defensive motifs deployed according to threat context. When we combine this approach with behavioral transition analyses, our results reveal that pair coordination is a common feature of territory defense in this species. However, if simulated intruders are less threatening, residents are more likely to defend solo, where only one bird deploys defensive behaviors. Overall, our study supports the hypothesis that nonhuman animals can pattern their behavior in a strategic and coordinated manner, while demonstrating the power of systems approaches for analyzing multiagent behavioral dynamics. 

Zhao et al. recently reported results which, they claim, suggest that sexual selection produces the multimodal displays seen in little torrent frogs (Amolops torrentis) by co-opting limb movements that originally evolved to support parasite defense (Zhao et al., 2022). Here, we explain why we believe this conclusion to be premature.