Avoidance of a moving threat in the common chameleon (Chamaeleo chamaeleon): rapid tracking by body motion and eye use

Abstract

A chameleon (Chamaeleo chamaeleon) on a perch responds to a nearby threat by moving to the side of the perch opposite the threat, while bilaterally compressing its abdomen, thus minimizing its exposure to the threat. If the threat moves, the chameleon pivots around the perch to maintain its hidden position. How precise is the body rotation and what are the patterns of eye movement during avoidance? Just-hatched chameleons, placed on a vertical perch, on the side roughly opposite to a visual threat, adjusted their position to precisely opposite the threat. If the threat were moved on a horizontal arc at angular velocities of up to 85°/s, the chameleons co-rotated smoothly so that (1) the angle of the sagittal plane of the head relative to the threat and (2) the direction of monocular gaze, were positively and significantly correlated with threat angular position. Eye movements were role-dependent: the eye toward which the threat moved maintained a stable gaze on it, while the contralateral eye scanned the surroundings. This is the first description, to our knowledge, of such a response in a non-flying terrestrial vertebrate, and it is discussed in terms of possible underlying control systems.

The (Under)Use of Eye-Tracking in Evolutionary Ecology

“…Software and analytical approaches are paramount to improving eye-tracking techniques; for example, an eye-tracking accuracy of <0.1º has been reported in archer fish using an innovative computational triangulation technique which can correct for the refractive properties of glass, water and air [22]. On land, head movement compensation algorithms have been found to improve the deviation in eyetracking signals relative to targets during mobile eye-movements [23] and carefully synching of data is required in during binocular tracking in species with independent eye-movements such as chameleons and mantis shrimp [24,25].…”

“…Eye-tracking studies have already provided us with some interesting insights into how animals interact with the spatial world, including tracking and scanning behaviours in species with eye architecture which is very different to the human [24,25,51,52] [see Box 1]. However, the difficulty of using mobile eye-tracking in the field has meant that few eye-tracking studies have been done in non-humans during natural locomotion.…”

“…Eye-tracking studies using species with independent eye-movements, such as the peacock mantis shrimp (Odontodactylus scyllarus) [24] and chameleons (Chamaeleo chamaeleon) [25] are consolidating the idea that the independence of eyemovements is task-dependant. For example, chameleons respond to threats at intermediate distances by repositioning on a perch at the opposite side to the threat and compressing their abdomens.…”

“…For example, chameleons respond to threats at intermediate distances by repositioning on a perch at the opposite side to the threat and compressing their abdomens. Importantly, concurrent eye-tracking whilst measuring these responses to threat has found that the leading eye maintains fixation on the threat image, whilst the other (following) eye continues to scan the environment [25]. Such coordinated movements may act to update visual space and contribute to the precise repositioning seen in chameleons.…”

The Ecological View of Selective Attention

“…This reptilian species has a remarkable capability to move its two eyes independently of each other ( Harkness, 1977 ; Ott, 2001 ). For example, one eye can maintain fixation on a potentially threatening object while the other continues to search the environment ( Lev-Ari et al, 2016 ). It seems that the chameleon’s parallel, independent scanning of the two hemifields is a violation of attention principles.…”