Animal signals evolve to be effective in the environment in which they are emitted [1–3], and therefore the diversity we see in animal communication systems is partially driven by the environment in which the animal lives [4–5]. An effective signal requires reliable detection by an intended receiver following transmission through ecologically complex environments [1]. It is evident that environmental structures and habitat-specific characteristics can affect signal transmission [1, 4, 6–7], such that different habitats dictate the optimal signal for reliable detection [8]. Dynamically changing abiotic and biotic noise in the environment also interferes with effective communication between individuals, and animals are required to produce signals that could compete with these irrelevant sensory inputs [3, 9–10]. Many animals modify their signalling behaviour in response to changing conditions [11–15], such that these moment-to-moment adjustments to signalling strategies represent a form of behavioural plasticity to maintain signal efficacy [16–17]. Clearly, habitat-specific transmission properties are important in determining the optimal structure of a signal, and it is therefore imperative that we quantify signal efficacy in the context of the relevant noise landscape to understand in more detail the evolution of animal communication strategies.
One way to examine how the environment influences signals is by comparing changes in signal efficacy in different habitats. Earlier research into quantifying acoustic characteristics of different habitat types provided the foundation for understanding diversity in the physical structure of avian songs [6, 18]. Cross-habitat examination of signal efficacy between different habitats also helped to explain intraspecific divergence of calling behaviours in anuran species [19]. Similarly, by contrasting signals in different habitats, signal divergence was described for lizard populations utilising colour-based signals [20], as well as closely related insect species utilising seismic communication strategies [21]. Structural differences in movement-based signals have also been observed in many lizard species occupying both similar and distinctive habitats [17, 22–24]. Bloch and Irschick [25] found temporal differences in the display sequences of the green anole, Anolis carolinensis, suggesting population divergence due to population density and habitat use. Similarly, structural differences in the core display of the Jacky dragon, Amphibolurus muricatus, between three different populations might be a consequence of behavioural plasticity in response to variation in habitat structure [26]. Clearly, movement-based signals also show variation consistent with local adaptation explanations, but detailed cross-habitat comparisons of signal structure has rarely been demonstrated. Aside from the technical difficulties associated with quantifying the environmental conditions and animal displays [27], there are important legislative and ethical restrictions that prevent translocating animals between habitats that they do not naturally inhabit.
Plant movements are the primary source of motion noise that affects movement- based signal communication [14, 28–31]. However, the motion noise landscape is difficult to examine in a controlled, systematic way, as it differs vastly from one microhabitat to another as a result of the interaction between variable plant characteristics, habitat topography and microhabitat structure [32]. To our knowledge, the recent work by Ramos and Peters [33–34] represents the only attempts to consider the relative effectiveness of movement-based signals of lizards in multiple habitats. In the first of these studies, evidence is presented for local adaptation, showing that two sympatric lizard species who utilise different movements in their displays, nonetheless produced similar motion speeds [33]. In the second paper, the authors showed structural differences between allopatric populations of the same species [34]. In both studies, habitat structure was considered to be the guiding force leading to convergence or divergence in structure respectively. The approach taken in both studies, however, was to film displays in local environments, and then the movement of plants at a separate time, in a controlled manner, and to then contrast resultant motion speed data [35]. Although these are novel and informative insights, we have developed new techniques based around sophisticated three-dimensional (3D) animations that affords us the opportunity to consider movement-based signals embedded in noise [27].
The aim of the present study was to consider directly the relative effectiveness of the movement-based displays of multiple lizard species in multiple habitats using 3D animation. We reused the subject of earlier work [27, 36], the Jacky lizard, A. muricatus, and created three additional model lizards and their respective habitats: the long-nosed dragon, Gowidon longirostris, the Mallee dragon, Ctenophorus fordi, and the tawny dragon, C. decresii (Figure S1,S2). In addition to quite distinct signals, these four species also occupy different habitat types, from densely vegetated coastal heath environments to sparsely vegetated rocky outcrops and semi-arid Mallee woodland with spinifiex understories (Figure S2). The associated noise landscape in these habitats also varies as a result of vegetation density and plant species present. Signals adapted to a noisier environment tend to perform better when placed in a less noisy environment [33–34]. Therefore, we predicted that the signal of A. muricatus would perform better in the other three habitats as it occupies a densely vegetated and noisy environment. Similarly, we would also expect the habitat of A. muricatus to have the greatest influence on signal performance. The habitats of G. longirostris and C. decresii are very similar, so we predicted that these will influence signal performance of all species in a similar manner, and that their displays would perform equivalently. Ctenophorus fordi, on the other hand, would produce relatively weak signals as this species utilises the simplest motor pattern compared to the other three species.
Habitat structure is a crucial determinant of signal structure across species and modalities and here we extend this literature to include the movement-displays of lizards from different habitats. The diverse habitats are each inhabited by only one of the focal species, and experimental translocation of absent species would not be permissible. While we have simulated these circumstances, 3D animation technology is sufficiently advanced that these are excellent proxies for filming the same circumstances in nature (if it were possible). Our animations are detailed and founded on the natural systems they represent, while our analytical approach is quantitative but descriptive, and shows clearly that habitat structure is an important contributing factor for movement-based signalling systems.