Effects of Habitat Light Intensity on Mammalian Eye Shape

Abstract

Many aspects of mammalian visual anatomy vary with activity pattern, reflecting the divergent selective pressures imposed by low light and high light visual environments. However, ambient light intensity can also differ substantially between and within habitats due to differences in foliage density. We explored the effects of interhabitat and intrahabitat variation in light intensity on mammalian visual anatomy. Data on relative cornea size, activity pattern, and habitat type were collected from the literature for 209 terrestrial mammal species. In general, mammalian relative cornea size significantly varied by habitat type. In within-order and across-mammal analyses, diurnal and cathemeral mammals from forested habitats exhibited relatively larger corneas than species from more open habitats, reflecting an adaptation to increase visual sensitivity in forest species. However, in all analyses, we found no habitat-type effect in nocturnal species, suggesting that nocturnal mammals may experience selection to maximize visual sensitivity across all habitats. We also examined whether vertical strata usage affected relative cornea size in anthropoid primates. In most analyses, species occupying lower levels of forests and woodlands did not exhibit relatively larger corneas than species utilizing higher levels. Thus, unlike differences in intensity between habitat types, differences in light intensity between vertical forest strata do not appear to exert a strong selective pressure on visual morphology. These results suggest that terrestrial mammal visual systems reflect specializations for habitat variation in light intensity, and that habitat type as well as activity pattern have influenced mammalian visual evolution. Anat Rec, 294:905-914, 2011. V V C 2011 Wiley-Liss, Inc.

Does Structural Complexity Determine the Morphology of Assemblages? An Experimental Test on Three Continents

“…Eye morphology has previously been shown to relate to time of activity (i.e. nocturnal, crepuscular or diurnal, [33], [34], [35], [36]), while eye position (measured as distance to the mandibles) may be determined by the degree of predatory behaviour [25], [37]. We show here that habitat complexity also determines the success of species, based on the position of their eyes.…”

Visual Acuity in Mammals: Effects of Eye Size and Ecology

“…For example, Veilleux and Lewis [2011] recently found that mammalian eye shape varied predictably with ambient light intensity in different habitat types. In particular, mammals from more open habitats exhibited smaller relative cornea sizes compared to species from closed canopy forests.…”

Eye Morphology and Retinal Topography in Hummingbirds (Trochilidae: Aves)

“…Therefore, it appears that, in terms of gross morphology, the eyes of the 5 hummingbird species investigated here are adapted for diurnal vision. In general, ratios that measure the size of the cornea relative to the eye, such as the C:T ratio, have proven to be highly correlated with activity pattern in birds [Hall and Ross, 2007;Lisney et al, 2012bLisney et al, , 2013b, nonavian reptiles [Hall, 2008], and mammals [Hughes, 1977;Kirk, 2004Kirk, , 2006Veilleux and Lewis, 2011]. Higher ratios equate to an enlarged cornea relative to the size of the eye, which serves to increase the amount of light that can enter the eye when the pupil is maximally dilated and is thus hugely advantageous for nocturnal species active at low light levels [Walls, 1942;Land and Nilsson, 2002;Kirk, 2006].…”

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