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First published online January 17, 2007
Journal of Experimental Biology 210, 395-402 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02676
Active space of a movement-based signal: response to the Jacky dragon (Amphibolurus muricatus) display is sensitive to distance, but independent of orientation
1 Department of Biological Sciences, Macquarie University, Sydney,
Australia
2 Centre for the Integrative Study of Animal Behaviour, Macquarie
University, Sydney, Australia
* Author for correspondence at present address: Visual Sciences, Research School of Biological Sciences, Australian National University, Canberra, ACT 0200, Australia (e-mail: richard.peters{at}anu.edu.au)
Accepted 15 November 2006
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Summary |
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Key words: active space, Jacky dragon, Amphibolurus muricatus, signal design, movement-based signal
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Introduction |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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) and the sensory
systems of receivers (Ryan et al.,
1990). The environment affects signal transmission by adding noise
and altering signal properties (Endler,
1992; Marten and Marler,
1977), whereas receiver characteristics dictate the structural
correlates of conspicuousness (e.g. Leal
and Fleishman, 2004), audibility (e.g.
Klump et al., 1986) and
localizability (Wood et al.,
2000). These processes have been studied most in bioacoustics;
distortion occurs in both the frequency domain (selective attenuation and
filtering) and in the time domain (reverberation) (reviewed by
Hopp et al., 1998;
Bradbury and Vehrencamp, 1998).
Such effects are, in principle, predictable
(Morton, 1982), and can
provide ranging cues (Whitehead,
1987).
Signals are designed to stimulate the sense organs of intended receivers
(Capranica, 1965). In many
signalling systems, sensory thresholds define the range-limit for reliable
detection. The active space of the signal is defined by the distance at which
the probability of detection drops to chance
(Brumm et al., 2004;
Dupuch et al., 2004;
Janik, 2000;
Linn and Roelofs, 1989;
Lohr et al., 2003). For
example, pioneering work by Brenowitz measured the acoustic properties of
red-winged blackbird (Agelaius phoeniceus) song in the field and
determined that the active space of these vocalizations corresponded with the
distance to the edge of the neighbouring territory
(Brenowitz, 1982). More
recently, studies have demonstrated that signalers can alter the structure of
a vocalization to compensate for increased noise, thereby maintaining active
space (Brumm et al., 2004;
Cynx et al., 1998).
Rather less is known about analogous phenomena in visual signals, although
experiments with bees have demonstrated that the probability of detecting
colour is influenced by both visual angle
(Giurfa et al., 1996) and
distance (Hempel de Ibarra et al.,
2001). Hempel de Ibarra et al. also identified likely advantages
to combining colours with certain spectral properties and in certain spatial
arrangements, based on neural mechanisms of colour perception
(Hempel de Ibarra et al.,
2001). For movement-based visual signals, sensory properties, such
as the minimum detectable amplitude of motion
(Fleishman, 1992), are
similarly likely to determine active space. However, the physical properties
of this class of signal have received little systematic study (for reviews,
see Fleishman, 1986;
Peters and Evans, 2003a;
Zeil and Zanker, 1997), and
there has been no formal analysis of changes in structure as a function of
distance.
Our strategy for analyzing movement-based signals combines quantitative
analyses of the structure of signals and noise
(Peters and Evans, 2003a) with
signal-detection experiments (Peters and
Evans, 2003b). We use a native Australian agamid lizard, the Jacky
dragon (Amphibolurus muricatus), as a model system. Jacky dragon
displays are performed in response to conspecific intruders and comprise five
distinct motor patterns, delivered in an obligatory sequence
(Peters and Ord, 2003).
Signals have to be robust to variation in background noise. One strategy to
ensure reliable detection is to include particularly conspicuous introductory
elements designed to draw the attention of the receiver to the motor patterns
that follow. Such alerting components typically have high intensity, delivered
over a short duration [auditory: Wiley and Richards
(Wiley and Richards, 1982);
visual: Fleishman (Fleishman,
1988)]. However, the Jacky dragon employs a different approach
(Peters and Evans, 2003a), in
which the introductory tail-flick is characterized by longer duration at
reduced average speed.
These contrasts in the design of introductory components warranted closer
inspection. We began by considering the properties responsible for signal
efficacy in the Jacky dragon tail-flick
(Peters and Evans, 2003b). We
found that duration was more important than speed in evoking an orienting
response, with long signals being most successful. Given the efficacy benefit
of increased duration, we speculated that the tail-flick might be most suited
to this role because it probably has lower energetic costs than other display
motor patterns and is less likely to compromise escape. An alternative
explanation is suggested by potential differences in active space. The
tail-flick sweeps a three-dimensional (3D) region around the animal and is
less constrained than the display movements that follow, which are largely
restricted to one plane of movement, along the long axis of the body. The
active space of the tail-flick may thus be greater than that of the rest of
the display, conferring efficacy that is robust to variation in receiver
orientation. This effect could produce a selective advantage independent of
duration, or the two parameters could be linked, for example as a consequence
of physical or biomechanical constraints.
In the present study, we investigated variation in the probability of
response to different signalling motor patterns as a function of distance and
angle of view. This is the first experimental analysis of this type for any
class of dynamic visual signals. We adapted a strategy analogous to those used
in acoustic studies (for a review, see
Dooling et al., 2000) and
techniques developed in our previous work in the Jacky dragon system
(Peters and Evans, 2003b).
Signal properties were manipulated using video playback of highly detailed 3D
animations. We created an animation of each motor pattern under consideration
and then systematically varied the angle of view and apparent distance. We
predicted that response to the tail-flick would be less affected by changes in
view and increased apparent distance between signaler and receiver than the
other motor patterns in the display.
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Materials and methods |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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Design
Our goal was to examine the effect of viewing angle and distance on the
detection probability of different motor patterns that make up the Jacky
dragon push-up display. Digital video playback of 3D animation is the only
technique that permits precise definition of both stimulus morphology and
movement. Since our stimuli contained no uncontrolled structural variation of
the sort that compels sampling when natural recordings are used for playback,
we chose to use a single exemplar of each motor pattern. This approach avoided
introducing variance that would reduce statistical power and increase the
likelihood of Type II error, or require a substantially increased sample size.
We acknowledge that this provides only a direct test of active space variation
in the signals produced by a typical lizard. It is conceivable that different
response functions might be obtained with individuals substantially smaller or
larger than the one chosen; exploration of this sort of
morphology–movement interaction is outside the scope of the present
study. Our approach is consistent with that adopted in the large corpus of
successful playback studies in bioacoustics (reviewed by
Gerhardt and Huber, 2002),
which underpin current theories of animal communication.
The duration of display motion is likely to be a key parameter mediating
probability of detection (Peters and
Evans, 2003b); it was therefore necessary to standardize this
across stimuli. Four out of the five motor patterns that comprise the Jacky
dragon display are both brief (approximately 400 ms) and highly stereotyped.
In contrast, the tail-flick is variable and can last as long as 12 s
(Peters and Evans, 2003a). We
reasoned that presenting each motor pattern in isolation was likely to produce
a floor effect in which none of the stimuli evoked a response, or would
require repetition of the motor pattern, which would be highly unnatural. We
therefore considered logical breaks in the sequence of motor patterns based on
Markov analysis (Peters and Ord,
2003), and selected the backward and forward foreleg waves
(transition P=1.0) as one pair, and the push-up and body-rock
components (transition P=1.0) as another. The duration of each of
these motor pattern pairs was 840 ms (21 PAL video frames). An equal number of
frames from a tail-flick sequence, which contained two high-amplitude flicks,
was used for comparison.
Next, we considered how to divide the space around the displaying lizard
into a manageable number of regions (Fig.
1A). The space to the front of the lizard is most relevant, as
Jacky dragons rarely display when they are oriented directly away from the
intended receiver. Furthermore, display structure is unlikely to vary much as
a consequence of being viewed directly from one side or the other
(Fig. 1A; filled and empty
circles). An exception to this is the foreleg wave component. As noted
elsewhere (Peters and Ord,
2003), the choice of foreleg appears to be arbitrary. It is not
restricted to the side nearest the receiver, nor do lizards show an individual
left- or right-side bias of the type described in anurans
(Vallortigara et al., 1998).
To maximize the likelihood of detection in the present experiment, we modeled
the foreleg wave using the limb on the receiver's side of the signalling
animal's body. We thus restricted our interest to the front side nearest to
the receiver, sampling angles orthogonal to the displaying lizard (90°),
oblique (45°) and front-on (0°;
Fig. 1B). Our manipulation of
distance was limited to the range over which we have observed lizards
exchanging displays in a large, outdoor enclosure (315 m2; D. Van
Dyk, unpublished data). We chose 6 m as the maximum range of a signalling
interaction, and added two closer distances of 3 m and 1 m.
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Stimulus preparation Background of windblown plants
Jacky dragon displays are typically seen against windblown plants that
generate irrelevant movement in the visual field. The ability of lizard motor
patterns to engage visual processing is thus best assessed after the subject
has spent a period spent viewing such background motion. We recorded plant
sequences at the field site where these lizards had been caught. A perch site
was identified and used as the focal point for video recording, so that we
could subsequently embed the computer-generated lizard in the natural scene.
Three 10-min sequences were recorded, with a fixed focal length, at distances
of 1, 3 and 6 m between the perch and the camera. All sequences were filmed in
summer within a one-hour period, on a bright day with no cloud cover. We
placed a 300 mm rule in the frame at the beginning of each clip so that we
could subsequently scale the animated lizard to be precisely proportional to
the background. Plant sequences were edited to 6 min in duration and exported
as numbered JPEG sequences.
Lizard animations
Lizard animations were created using LightWave 3D v7 (NewTek Inc., San
Antonio, TX, USA). In a previous study, we created a 3D computer-generated
Jacky dragon tail using shaping tools within the modeler module of LightWave
(Peters and Evans, 2003b).
However, ground-up synthesis of a complete Jacky dragon is more challenging.
To accurately reproduce morphology, we began with a 3D laser-scan of a
taxidermically prepared Jacky dragon. This produced a detailed wire-frame
model that represented the shape of the lizard with a mesh of 100 000
polygons. Digital photographs of lizard skin were then used to create a
texture map in Adobe Photoshop v6 (Adobe Systems Inc., San Jose, CA, USA);
this then overlaid the 3D wire-frame model.
Animation procedures have been described previously
(Peters and Evans, 2003b).
Briefly, we added a skeleton of artificial `bones'. These are invisible
elements that facilitate adjustment of a model in which there is both global
displacement and local movement of some sectors, relative to others. To
animate the lizard, we used a `rotoscoping' technique. This involves
superimposing the computer-generated model over sequential digital video
frames from an actual lizard display sequence, and then adjusting the limbs
and body in all three planes (x, y and z) to match. Weight
maps associated with separate regions of the wire-frame model ensured that
manipulation of the invisible bones caused deformation of the wire-frame in
the appropriate region of the body surface. Although this technique is
laborious, it produces a much more faithful reproduction of natural movements
than alternative methods that employ `tweening' algorithms to interpolate
between keyframes.
We created separate animations for the tail-flick, foreleg waves and push-up/body-rock. Finished animations had a duration of 6 min. The lizard remained stationary for the first 3 min, then produced one of the three motor patterns, repeating this 60 and 120 s later.
Manipulation of viewing angle and distance was achieved by adjusting the
virtual `camera' in LightWave. Adjusting the location and rotation of the
camera in the scene changes the camera's view and the resultant animation,
without adjusting the position of the lizard model
(Fig. 1A,B). We positioned the
camera within the scene using footage of the straight rule in the unused part
of the background sequence (see above) to determine the scale. The relative
angular sizes of stimulus motor patterns
(Table 1) exceed Fleishman's
(Fleishman, 1986) estimate of
the minimum required to engage the visual grasp reflex of another lizard
species (Genus: Anolis; 0.22° of visual angle).
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The final stage of the animation process was to add the appropriate background of windblown plants and render each sequence. Digital video footage of vegetation movement was first converted to a series of still frames. These were then loaded sequentially as the background over which the computer-generated lizard was superimposed, creating an uncompressed JPEG image for each of the 9000 frames in each sequence (Fig. 2). We used Final Cut Pro 3 (Apple Computer) to transform these still images into digital video files at full PAL resolution (720x576 pixels). Control sequences were generated in the same way, but with the lizard remaining motionless throughout.
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). All stimuli were presented on a Sony PVM-14M2A colour
monitor (resolution >600 lines; screen size 34 cm, measured diagonally).
Stimulus sequences were stored on a LaCie d2 external hard drive connected to
an Apple G3 iMac running Final Cut Pro 3. Video was streamed as a digital
signal to a Sony GV-D300E mini-DV deck, using an IEEE 1394 Firewire interface.
The S-video analogue output from the deck was then connected to the Sony
monitor.
We used a Panasonic WV-CP240 CCTV camera, with a WV-LZ62/2 lens, and a VHS
video deck to record lizard responses. All stimuli were presented with the
soundtrack muted, but a marker tone (inaudible to the subject lizard) was
dubbed directly to the VHS deck so that responses could later be scored
relative to stimulus onset. We performed all playback tests between 08:00 h
and 12:00 h, which corresponds with the period of peak activity in our captive
population (Ord, 2001).
The study was performed in November and December 2004. We divided testing into six blocks of five days each, with a two-day interval between blocks. Lizards saw one stimulus sequence per day, each in a unique sequence. To control diel variation in behaviour, we standardized presentation time for each subject.
Statistical analyses
Our dependent measure was the orienting response, which we operationally
defined as an observable movement of the head toward the monitor. As we were
primarily interested in the propensity of each signal to attract attention, we
considered only response probabilities to the initial display at the third
minute. The data were analysed with non-parametric tests of significance for
repeated measures. Friedman's analyses of variance (ANOVAs) were used when
comparing more than two conditions, whereas Wilcoxon signed-ranks tests were
used for pair-wise comparisons (Siegel and
Castellan, 1988). For ANOVAs, we calculated partial
2 from the parametric ANOVA model as an estimate of effect
size. Partial 2 can be interpreted as the amount of variation
explained by a parameter after controlling for that explained by the other
parameters (Cohen, 1988). The
effect size for pair-wise comparisons, h, was calculated as the
absolute difference between probability values following arcsine
transformation (Cohen, 1988):
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First, a Friedman's ANOVA was performed to assess variation in the probability of response over all stimuli. Second, we grouped responses to stimuli by each of the three structural parameters (motor pattern, angle of view and distance) and tested for main effects using Friedman's ANOVAs. These were followed by Wilcoxon signed-ranks tests for pair-wise comparisons between conditions. Our third series of analyses again used Friedman's ANOVAs and Wilcoxon signed-ranks tests, but considered interactions between two variables collapsed across the third variable; for example, we examined the effect of distance within angle of view, pooling data across motor patterns.
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Results |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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2=51.004, d.f.=17,
P=0.002).
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2=0.102, d.f.=2, P=0.950) or angle of view
(2=1.910, d.f.=2, P=0.385). However, there was a
strong effect of distance (2=15.574, d.f.=2,
P=0.0001). Wilcoxon signed-ranks tests revealed significantly higher
response probability to 1 m than both 3 m (z=2.898, P=0.004,
h=0.17) and 6 m (z=3.198, P=0.001,
h=0.22), whereas the probability of responses did not differ between
displays at 3 and 6 m (z=0.574, P=0.566,
h=0.05). Our final set of analyses considered interactions between two variables, while pooling data across the third variable. Results from Friedman's ANOVAs are summarized in Table 2. We found no significant variation in response probability to different motor patterns when considered within angle of view or viewing distance. Similarly, angle of view did not affect response probability when considered within motor pattern or the distance category. However, the effect of distance was robust. This was clearly significant at each angle of view. When we considered the effect of distance and motor pattern, we found a pronounced effect for tail-flicks and push-up/body-rocks, but only a non-significant trend for foreleg waves.
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Discussion |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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; Linn and Roelofs,
1989). We attribute this effect to the reduction in visual angle
subtended by display movements at 3 m and 6 m
(Fleishman, 1992)
(Table 1). Perhaps surprisingly, we were unable to detect any variation in responsiveness due to angle of view. It is possible that those structural properties of display motor patterns that influence detection do not vary appreciably with orientation, which might reflect selection for efficacy under the possible viewing conditions. Alternatively, this finding could be a Type II error, attributable to assay insensitivity. Although we cannot rule out such an interpretation, it is important to note that our response measure and sample size were sufficient to detect a robust effect of distance. We conclude that any putative effect of receiver viewing angle must be small, relative to that of signaler range.
Our results reveal that the active spaces of the movements that make up Jacky dragon displays are statistically indistinguishable. We had predicted an interaction between motor pattern, angle of view and distance, because the 3D sweep of the tail-flick contrasts with the essentially single-plane movement of the push-up/body-rock and foreleg waves. There was a slight hint of an interaction between motor pattern and distance in the failure to detect a range effect with foreleg waves (Table 2). However, these data are highly variable, in contrast to the orderly changes in efficacy found with the other two motor patterns (Fig. 3), and we are reluctant to interpret them further.
Our design systematically sampled a portion of the space around a signalling lizard (Fig. 1, filled circles). It is logical to expect stimuli defined by these points in the horizontal plane to have effects comparable with those that would be evoked by corresponding stimuli on the other side of the signaler (Fig. 1, empty circles). To keep the total size of the stimulus set manageable with the limited number of subjects available, we also restricted manipulation of camera position to a single plane. As Jacky dragons are semi-arboreal and typically display from raised perches, it will be important to consider variation in display height in future experiments. It is conceivable that responses to the motor pattern examined in this study might be differentially affected by the vertical position of receivers, relative to signalers.
We manipulated apparent distance by moving the camera component of the 3D
layout, thus varying the angular size of the animated lizard. Such variation
is potentially ambiguous; it might be perceived as change in absolute size,
rather than in range (Zeil,
2000). Potential depth cues include motion parallax, binocular
stereopsis and accommodation. It is important to note that the background
sequences we used scaled appropriately with simulated distance
(Fig. 2), thus ensuring that
the visual angle subtended by the lizard animation was proportional to the
visual motion of the surrounding plants. This relative scaling was designed to
introduce additional depth cues.
Notwithstanding such considerations, the findings of this study do not
support our initial prediction that the structural properties of the
introductory tail-flick would confer efficacy superior to those of the motor
patterns that follow it, at least in terms of active space. It thus seems
unlikely that active space alone is sufficient to explain the almost invariant
sequence of motor patterns found in natural Jacky dragon displays
(Peters and Ord, 2003).
Results are instead consistent with previous experimental findings suggesting
that duration is the principal determinant of signal efficacy
(Peters and Evans, 2003b). We
have also shown that signalling movements contrast reliably with those of
wind-blown vegetation, when viewed at the same viewing distance
(Peters and Evans, 2003a).
Alternative explanations for why the tail-flick serves an introductory role
include reduced conspicuousness to predators and lower energetic costs.
Increased display activity has been shown to reduce survivorship in another
lizard, Sceloporus jarrovi
(Marler and Moore, 1988;
Moore and Marler, 1987),
although it is difficult to know whether predators are selectively attentive
to visual displays or if increased activity levels in general render the
lizard more vulnerable to attack. Experimental investigations of the relative
conspicuousness to predators of different motor patterns will be required to
resolve this question. A useful approach is suggested by recent field studies
investigating the `survival' of artificial moths varying in levels of
camouflage (Schaefer and Stobbe,
2006; Stevens and Cuthill,
2006). Analogous tests of movement-based signals could be
performed using programmable robots
(Martins et al., 2005;
Patricelli et al., 2002). In
addition to reduced conspicuousness to predators, it is likely that the
tail-flick motor pattern requires less energy. The energetic cost of display
behaviour has been investigated in several systems [e.g. fiddler crabs, genus
Uca, Matsumasa and Murai
(Matsumasa and Murai, 2005);
lizards, Cnemidophorus exsanguis, C. sonorae, Sceloporus virgatus and
S. jarrovi, Pough and Andrews
(Pough and Andrews, 1985)].
These studies consistently demonstrate a cost to the signaler. However, common
techniques such as measuring oxygen consumption in a respirometer, or
determining lactate levels, do not allow for comparisons between multiple
components produced in a rapid sequence within a single display as in the
Jacky dragon; infrared cameras that permit measurement of heat levels in
real-time may provide a useful proxy for energy expenditure.
The diversity of animal signals we see today reflects a history of
selective forces and a variety of constraints
(Endler and Basolo, 1998;
Ord et al., 2001;
Ord et al., 2002). Constraints
can be inherent to the signaler, as in the case of physical or morphological
limitations (Podos, 2001), as
well as those imposed by the sensory system of receivers
(Aho et al., 1988;
Fleishman, 1992), which must
perceive and process the signal. External factors such as the transmission
environment contribute by defining the optimal structure for minimizing
masking by background noise (Brumm et al.,
2004). Similarly, the need to avoid detection by predators
influences both signal design (Marler,
1955; Stoddard,
1999; Wood et al.,
2000) and the timing of signalling
(Endler, 1987). A greater
understanding of any communication system can be achieved through careful
consideration of these factors; meta-analyses across communication systems and
sensory modalities allow for the refinement of prevailing models of signal
evolution.
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Acknowledgments |
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