J Comp Physiol A (2003) 189: 447–459 DOI 10.1007/s00359-003-0423-1 ORIGINAL PAPER R. A. Peters Æ C. S. Evans Design of the Jacky dragon visual display: signal and noise characteristics in a complex moving environment Received: 16 November 2002 / Revised: 31 March 2003 / Accepted: 11 April 2003 / Published online: 20 May 2003 Ó Springer-Verlag 2003 Abstract Visual systems are typically selective in their (Nakayama and Loomis 1974). For example, recogni- response to movement. This attribute facilitates the tion of prey items is much easier when sudden movement identification of functionally important motion events. compromises crypsis that depends upon appearance Here we show that the complex push-up display pro- (Regan and Beverley 1984), and selective processing of duced by male Jacky dragons (Amphibolurus muricatus) approaching objects facilitates detection of predators is likely to have been shaped by an interaction between (Schiff et al. 1962). However, only a small proportion of typical signalling conditions and the sensory properties environmental movement is relevant. Filtering mecha- of receivers. We use novel techniques to define the nisms ensure that the limited capacity of the visual sys- structure of the signal and of a range of typical moving tem is not overwhelmed by other information. backgrounds in terms of direction, speed, acceleration Considerable progress has been made in explaining and sweep area. Results allow us to estimate the relative the design of static signals. The strategy in such studies conspicuousness of each motor pattern in the stereo- has been to combine the objective measurement of typed sequence of which displays are composed. The structure with knowledge of both the light environment introductory tail-flick sweeps a large region of the visual in which the animal is seen, and of sensory processes field, is sustained for much longer than other compo- (Endler 1990, 1992). Selectivity in the response of visual nents, and has velocity characteristics that ensure it will systems to motion is likely to have shaped the evolution not be filtered in the same way as wind-blown vegeta- of movement-based animal signals in an analogous tion. These findings are consistent with the idea that the fashion. In particular, the sensory properties of receivers tail-flick has an alerting function. Quantitative analyses will define the optimal design for conspicuousness of movement-based signals can hence provide insights (Guilford and Dawkins 1991; Endler 1992). To convey into sensory processes, which should facilitate identifi- information effectively, animals must maximise the cation of the selective forces responsible for structure. chance of being detected. This can be achieved by sig- Results will complement the detailed models now nalling at particular times of the day, when environ- available to account for the design of static visual sig- mental conditions enhance signal intensity (Endler nals. 1991), when receivers’ sensory systems are most sensitive (Aho et al. 1988), or when the signals of other species are Keywords Lizards Æ Movement-based signals Æ Visual absent (Greenfield 1988). Each of these strategies effec- communication Æ Visual ecology tively increases the signal-to-noise ratio (Endler 1992). The likelihood of detection is also increased if signals contrast with the background against which they are typically performed (Endler 1991; Fleishman 1992). The Introduction distinctive colour of some static visual signals facilitates rapid recognition by conspecifics (Bernard and Rem- Sensitivity to visual motion helps animals to select ington 1991). Similarly, movement-based signals are functionally-important stimuli for further analysis most effective when they stimulate the visual system in a way that irrelevant visual motion does not (Fleishman 1992). R. A. Peters (&) Æ C. S. Evans Direction can be calculated from two relatively Animal Behaviour Laboratory, Department of Psychology, Macquarie University, 2109 Sydney, NSW, Australia imprecise position measurements and is thought to be E-mail: [email protected] the first motion variable neurally coded in visual systems Fax: +61-2-98509231 (Barlow et al. 1964). The directional selectivity of 448 individual neurons provides a basic mechanism, which is We build upon earlier work describing visual ecology typically supplemented by integration over a larger in the context of movement-based signals (Zeil and population (Grzywacz et al. 1994). Processing of object Zanker 1997; Fleishman 1992). Specifically, our goal speed is probably based upon the spatial and temporal was to explain why the tail-flick is used as the intro- properties of directionally selective neuron activity (Si- ductory component to the push-up display and to moncelli and Heeger 1998). Direction and speed are identify how constraints imposed by signalling in a fundamental parameters from which other important complex visual environment might have contributed to motion variables (e.g. acceleration) can be derived. the design of this motion-based signal. Detailed Previous efforts to characterize movement-based liz- description of structural differences between signal ard signals have used display action patterns to compare components and wind-blown vegetation effectively de- the properties of simple movements, such as the head fines the task faced by the sensory system and should nod (e.g. Martins et al. 1998). These analyses identify hence provide insights about mechanisms of visual differences between display profiles, but only allow weak processing in these lizards. inferences about the relative conspicuousness of signals. Pioneering work by Fleishman demonstrated the value of a more quantitative approach; comparisons using Materials and methods Fourier analysis show that the displays of Anolis auratus have more power at high frequencies than background Apparatus vegetation movement (reviewed in Fleishman 1992). Similarly, Zeil and Zanker (1997) described the motion Video recordings were made using a Canon XL1 digital video characteristics of claw waving in Fiddler crabs (genus camcorder (optical resolution 625 lines) mounted on a tripod. We used a shutter speed of 1/250 s, an aperture of F8 and Sony Uca) using two-dimensional motion detectors to capture DVM60EX digital tape (550 lines recorded resolution). The dis- the direction and strength of signals in a range of spatio- tance between the camera and the subject was constant. Full details temporal frequency bands. In a recent paper, we have of the recording setup for lizard displays are provided in Ord et al. developed Zeil and Zanker’s (1997) idea of using com- (2002). putational motion analysis algorithms to measure the structure of movement-based animal signals (Peters et al. 2002). We used a motion analysis algorithm that Video sequences estimates velocity based on local changes in image Image sequences depicted wind-blown vegetation and the five intensity, allowing us to track the direction and speed of components of the Jacky dragon push-up display, which were each movement on a fine temporal scale. analysed separately (Fig. 1). Sequence duration was measured in ) Most previous work on lizard visual signals has fo- frames at the rate of 25 s 1 (PAL standard). Table 1 presents a cused on displays that are composed of relatively simple, summary of the sequences used in analyses. repeated motor patterns. In contrast, the push-up (aggressive) display of the Jacky dragon involves five Wind-blown vegetation distinct motor patterns delivered in rapid succession, and subsets of this sequence may be repeated within a Five plant species were selected to represent the habitat in which display bout. The temporal order of components can be lizards were originally captured. Species were chosen that typically form the background for basking Jacky dragons: Acacia longifolia, described using Markov chain analysis (e.g. Hailman Grevillea linearfolia, Kunzea ambigua, Pteridium esculentum and et al. 1985, 1987), which allows comparisons of gross Xanthorrhea arborea. Video-recording was carried out in the structure (Peters and Ord 2003). This is a useful first summer of 2001. We measured wind-speed while filming using a step, but we are particularly interested in the details of hand-held anemometer (Dick Smith Model Q1411) and obtained sequences of each plant species over the range from 0.70 ms)1 to signal design. Here we examine the properties of indi- 2.0 ms)1. Analysis of wind-speed data recorded at our field site by vidual display motor patterns, focusing on the degree to the Australian Bureau of Meteorology reveals that this video which they are each conspicuous against a typical footage sampled plant movement from light to typical wind con- background of wind-blown vegetation. ditions (Fig. 2). We used an optic flow algorithm (Peters et al. 2002) to generate ’velocity signatures’—scatterplots repre- Push-up display of the Jacky dragon senting the direction and speed of movement—for each display component. The shapes of the velocity signatures We used video footage from an archival collection recorded during were then summarised with standard ellipses, which al- a study reported elsewhere (Ord et al. 2002). Each sequence showed lowed an initial descriptive analysis comparing the signal a male on a platform made from rough-sawn timber with a uniform and static background. Male Jacky dragon displays consist of a with background movement. Subsequent analyses stereotyped series of discrete motor patterns (Fig. 1): introductory sought to specify more precisely how the various se- tail-flicks are followed by backward and forward arm-waves, a quences differed. First, we collapsed across time to push-up and then a body rock (a wave travelling anterior to pos- examine direction and speed. Second, direction of terior down the body). Some, or all, of the components may then be repeated. A total of 53 display sequences from four different male movement was ignored to measure variation in speed lizards were used in the following analyses of movement charac- over time. Finally, we concentrated on the spatial loca- teristics (Table 1). Transition probabilities between display com- tion of motion, ignoring time, direction and speed.