Antipredator defences in motion: animals reduce predation risks by concealing or misleading motion signals

Motion is a crucial part of the natural world, yet our understanding of how animals avoid predation whilst moving remains rather limited. Although several theories have been proposed for how antipredator defence may be facilitated during motion, there is often a lack of supporting empirical evidence, or conflicting findings. Furthermore, many studies have shown that motion often 'breaks' camouflage, as sudden movement can be detected even before an individual is recognised. Whilst some static camouflage strategies may conceal moving animals to a certain extent, more emphasis should be given to other modes of camouflage and related defences in the context of motion (e.g. flicker fusion camouflage, active motion camouflage, motion dazzle, and protean motion). Furthermore, when motion is involved, defence strategies are not necessarily limited to concealment. An animal can also rely on motion to mislead predators with regards to its trajectory, location, size, colour pattern, or even identity. In this review, we discuss the various underlying antipredator strategies and the mechanisms through which they may be linked to motion, conceptualising existing empirical and theoretical studies from two perspectives - concealing and misleading effects. We also highlight gaps in our understanding of these antipredator strategies, and suggest possible methodologies for experimental designs/test subjects (i.e. prey and/or predators) and future research directions.

Movement-based signalling by four species of dragon lizard (family Agamidae) from the Kimberley region of Western Australia

Communication signals underpin the social lives of animals, from species recognition to mate selection and territory defense. Animal signals are diverse in structure between and within species, with the diversity reflecting interacting factors of shared evolutionary history, constraints imposed on senders and receivers and the ecological context in which signalling takes place. The dragon lizards of Australia (family Agamidae) are known for their movement-based visual displays and are useful models for how ecology influences behaviour. However, we know little about the communication strategies of many species. Our aim here was to provide new knowledge on some of these species, focusing on the north-west of Western Australia. We filmed within-species pairwise interactions of Diporiphora superba, D. bennetti, D. sobria and Ctenophorus isolepis isolepis. We describe and quantify for the first time push-up displays by D. superba and C. isolepis isolepis and tail waving displays of D. bennetti. Only D. sobria did not generate movement-based visual signals. We have confirmed that more species engage in such behaviour than previously reported, but further work is required to document the full repertoire of these species. The implications of our work are discussed in the context of signal structure, function and environmental context.

Male courtship reduces the risk of female aggression in web-building spiders but varies in structure

Male courtship serves multiple functions in addition to inducing females to accept them as a mate. In predatory species, male courtship can function to reduce the risk of sexual cannibalism. This is particularly important in web-building spiders in which males risk being mistaken for prey when they enter the female’s predatory trap—the web—in order to commence courtship. Male spiders generate vibrations by shuddering in the female’s web. Shudder vibrations can delay female aggression, even toward prey struggling in the web. We predicted that shudder vibrations are highly conserved across species of web-building spider as males all face the same constraint of not being mistaken for prey by females. We examined how conserved shudder vibrations are across web-building spiders by testing whether female Trichonephila plumipes delay aggressive behavior toward real prey struggling in the web during playback of conspecific or heterospecific (Argiope keyserlingi) male shudder vibrations. We found that while conspecific shudder vibrations do indeed delay female predatory behavior, heterospecific male shudder vibrations do not. There is evidence of shudder or shudder-like vibrations in male courtship behavior across web-building spider families, but these vary in structure. This suggests that despite strong constraints on courtship signal design to separate predatory responses from sexual responses, there is additional selection driving the divergence of signals across distantly related spider species.

Territorial Displays of the Ctenophorus decresii Complex: A Story of Local Adaptations

Closely related species make for interesting model systems to study the evolution of signaling behavior because they share evolutionary history but have also diverged to the point of reproductive isolation. This means that while they may have some behavioral traits in common, courtesy of a common ancestor, they are also likely to show local adaptations. The Ctenophorus decresii complex is such a system, and comprises six closely related agamid lizard species from Australia: C. decresii, C. fionni, C. mirrityana, C. modestus, C. tjanjalka, and C. vadnappa. In this study, we analyze the motion displays of five members of the C. decresii complex in the context of their respective habitats by comparing signal structure, habitat characteristics and signal contrast between all species. Motor pattern use and the temporal sequence of motor patterns did not differ greatly, but the motion speed distributions generated during the displays were different for all species. There was also variation in the extent to which signals contrasted with plant motion, with C. vadnappa performing better than the other species at all habitats. Overall, this study provides evidence that members of the C. decresii complex exhibit local adaptations in signaling behavior to their respective habitat, but they also maintain some morphological and behavioral traits in common, which is likely a consequence from the ancestral state.

Motion: enhancing signals and concealing cues

Animal colour patterns remain a lively focus of evolutionary and behavioural ecology, despite the considerable conceptual and technical developments over the last four decades. Nevertheless, our current understanding of the function and efficacy of animal colour patterns remains largely shaped by a focus on stationary animals, typically in a static background. Yet, this rarely reflects the natural world: most animals are mobile in their search for food and mates, and their surrounding environment is usually dynamic. Thus, visual signalling involves not only animal colour patterns, but also the patterns of animal motion and behaviour, often in the context of a potentially dynamic background. While motion can reveal information about the signaller by attracting attention or revealing signaller attributes, motion can also be a means of concealing cues, by reducing the likelihood of detection (motion camouflage, motion masquerade and flicker-fusion effect) or the likelihood of capture following detection (motion dazzle and confusion effect). The interaction between the colour patterns of the animal and its local environment is further affected by the behaviour of the individual. Our review details how motion is intricately linked to signalling and suggests some avenues for future research.

This Review has an associated Future Leader to Watch interview with the first author.

Summary: While motion can reveal information about the signaller, motion can also be a means of concealing cues by reducing the likelihood of detection or the likelihood of capture following detection.

Simulations with Australian dragon lizards suggest movement-based signal effectiveness is dependent on display structure and environmental conditions

Habitat-specific characteristics can affect signal transmission such that different habitats dictate the optimal signal. One way to examine how the environment influences signals is by comparing changes in signal effectiveness in different habitats. Examinations of signal effectiveness between different habitats has helped to explain signal divergence/convergence between populations and species using acoustic and colour signals. Although previous research has provided evidence for local adaptations and signal divergence in many species of lizards, comparative studies in movement-based signals are rare due to technical difficulties in quantifying movements in nature and ethical restrictions in translocating animals between habitats. We demonstrate herein that these issues can be addressed using 3D animations, and compared the relative performance of the displays of four Australian lizard species in the habitats of each species under varying environmental conditions. Our simulations show that habitats differentially affect signal performance, and an interaction between display and habitat structure. Interestingly, our results are consistent with the hypothesis that the signal adapted to the noisier environment does not show an advantage in signal effectiveness, but the noisy habitat was detrimental to the performance of all displays. Our study is one of the first studies for movement-based signals that directly compares signal performance in multiple habitats, and our approach has laid the foundation for future investigations in motion ecology that have been intractable to conventional research methods.