How lacertids resolve spatial details: visual acuity in the common wall lizard (Podarcis muralis)

Visual acuity, the ability to discern static spatial details, is crucial for understanding how organisms perceive their environment. Lacertid lizards are diurnal, feed on small prey, and communicate using multiple visual signals, including postures, dynamic displays and conspicuous colour patches. However, their visual acuity remains unstudied, leaving a critical gap in our understanding of their visual capabilities. Visual acuity is often estimated behaviourally using an optomotor apparatus, which quantifies a reflex-orienting movement called the optomotor response (OMR), or histologically by evaluating retinal ganglion cell (RGC) densities. We combined these two techniques to estimate visual acuity in a lacertid lizard, Podarcis muralis. OMR assays estimated an acuity of 2.05 cpd, while RGC counts revealed a peak density (>7000 cells mm-2) within the area centralis, corresponding to 1.56 cpd. RGC topographic maps revealed additional areas of high cell density in the dorso-nasal and dorso-temporal regions, while the area centralis was located slightly ventrally in the centro-temporal retina. This RGC distribution suggests adaptations to resolve stimuli in the centre and lower parts of the lizard's visual field, which may enhance predator and prey detection on the ground. Using our estimated acuities, we calculated that a lizard can detect a conspecific from 10 m and a 2 mm-long object from 40 cm away. In addition, we generated AcuityView images (R package) showing how this species might discern conspecific colour patches at different distances. These images suggest that beyond 10 cm, the surroundings become increasingly blurred, suggesting a role for static visual signals specifically in close-range communication.