Motion perception and visual signal design in Anolis lizards

Abstract

Anolis lizards communicate with displays consisting of motion of the head and body. Early portions of long-distance displays require movements that are effective at eliciting the attention of potential receivers. We studied signal-motion efficacy using a two-dimensional visual-motion detection (2DMD) model consisting of a grid of correlation-type elementary motion detectors. This 2DMD model has been shown to accurately predict Anolis lizard behavioural response. We tested different patterns of artificially generated motion and found that an abrupt 0.38 shift of position in less than 100 ms is optimal. We quantified motion in displays of 25 individuals from five species. Four species employ near-optimal movement patterns. We tested displays of these species using the 2DMD model on scenes with and without moderate wind. Display movements can easily be detected, even in the presence of windblown vegetation. The fifth species does not typically use the most effective display movements and display movements cannot be discerned by the 2DMD model in the presence of windblown vegetation. A number of Anolis species use abrupt up-and-down head movements approximately 10 mm in amplitude in displays, and these movements appear to be extremely effective for stimulating the receiver visual system.

Predation-associated modulation of movement-based signals by a Bahamian lizard

“…These signals are given spontaneously by males to advertise their presence as they patrol their territories, as well as being directed toward other individuals during courtship and the settlement of territorial disputes (38)(39)(40). Theoretical predictions, supported by empirical data, have shown that the physical properties of head-bob displays in A. sagrei, particularly the square wave-like temporal pattern generated by rapid changes in head position, are highly conspicuous to visually oriented receivers, even when those receivers are inattentive (41,42). The amplitude of these movements determines the maximum distance from which the sensory system of a conspecific is stimulated effectively by the signal-what is termed the "active space."…”

Quantifying Ecological Constraints on Motion Signaling

“…Most notably would be variation in terms of amplitude and movement direction. Angular speed differences between signal and noise have been the subject of recent attention (Peters and Evans, 2003;Ord et al, 2007Ord et al, , 2010Fleishman and Pallus, 2010;Pallus et al, 2010), while differences between signal and noise in terms of the direction of movement also might be a useful avenue of inquiry, although it has not received as much attention in the literature (but see Peters et al, 2008;How et al, 2009). Movement amplitude and direction information could be extracted from the raw data we have collected.…”

“…Furthermore, Peters and Evans (2003) employed the same methodology to compare velocity signatures from lizard signals and plant sequences in order to estimate the relative conspicuousness of each motor pattern in the lizard display. This led to a much better understanding of motion signal structure and opened the path to many studies on the influence of the environment on lizard signaling behavior (Leal and Fleishman, 2004;Ord et al, 2007Ord et al, , 2010Peters et al, 2007Peters et al, , 2008Ord and Stamps, 2008;Fleishman and Pallus, 2010).…”

“…The use of motion detector algorithms for quantifying animal signals is now the standard way to compare display movements and plant motion noise (Zanker and Zeil, 2005;Fleishman and Pallus, 2010;Pallus et al, 2010). However, all these studies utilize video footage from a single camera view that necessarily assumes the camera is ideally positioned to sample the display.…”

“…However, all these studies utilize video footage from a single camera view that necessarily assumes the camera is ideally positioned to sample the display. In studies in which environmental noise is also captured from the same camera view and subsequently analyzed (Ord et al, 2007;Fleishman and Pallus, 2010), it is also assumed that the sampled plant motion is sufficient. We suggest that this approach has been informative but restricts our understanding of the context in which motion signals are generated.…”