Scaling of visual acuity with body size in mammals and birds

Abstract

Summary1. Kirschfeld (1976) suggested that visual acuity is directly proportional to body length across a wide range of animal species. A survey of eye size, visual acuity and body size of birds and mammals that is consistent with Kirschfeld's suggestion is reported. Hypoallometry (scaling factor < 1) for eye size vs body size combines with hyperallometry (scaling factor > 1) for acuity vs eye size to produce roughly linear scaling between acuity and body size. 2. Kirschfeld (1976) also suggested that the distance at which important objects are typically viewed is a linear function of body length. 'Subjective distance' (viewing distance/body length) was therefore thought to be independent of body size across species. However, for prey detection by mammalian and avian predators, it is doubtful that subjective visual distances are size independent because prey size and visual acuity both scale by factors > 0·5 with predator body size; hence, detection distance should scale with size by a factor > 1. Scaling analyses also suggest that subjective visual distances for intraspecific social interactions are size dependent. 3. A positive association between body size and viewing distance has implications for the scaling of coat pattern features. In environments with fractal visual backgrounds (in which perceived sizes of background pattern elements do not change as distance from the background changes), larger animal species should have larger coat patches than smaller species if they are adapted to be cryptic at greater viewing distances than smaller species are.

Quantitative estimates of visual performance features in fossil birds

“…2a) reveals slightly positive allometry of PND. The allometric slope (OLS slope 5 1.14, 95% CI 5 1.02-1.24; RMA slope 5 1.17, CI 5 1.06-1.28; consult Table 6 for a summary of regression statistics) is similar to that found by Kiltie (2000). Both OLS and RMA slope are different from 1.0 (p OLS 5 0.02, p RMA < 0.001).…”

“…Behavioral measure-ments of acuity usually rely on repeated twochoice discrimination tasks, which help to determine the smallest resolvable visual angle. Kiltie (2000) found that values between anatomically modeled and behaviorally determined visual acuity agreed well.…”

“…The slight deviation from the expected isometry is surprising. A possible explanation for the positive allometry considers photoreceptor scaling as indicated by Kiltie (2000). One important assumption of my optical prediction of isometry was that the size of photoreceptors does not change with increasing eye size.…”

“…Eyeball axial length has been used previously to estimate PND (e.g., Hughes, 1977); however, with the exception of Kiltie (2000), allometric scaling was not considered. The correlation between A and LD has largely been unexplored in the literature.…”

Visual perception and social foraging in birds

“…This reflects the need of birds for higher acuity at longer focal distance and the resultant higher image magnification (Box 2). Assuming similar photoreceptor densities, VISUAL ACUITY increases with eye size, and eye size with body size [12,13]. Thus, larger species have a greater ability to identify objects at any given distance.…”