Given a fixed number of photoreceptors, an amniote eye adapted to a dim light environment should have (1) a large pupil diameter (aperture of the optical system) relative to posterior nodal distance (distance from near the center of the lens to the retina) to increase retinal image brightness and (2) a large pupil diameter relative to retinal area to maximize the number of photons entering the eye (Schmitz and Motani, 2010). Physiological optics predicts several modifications of the optical system of amniote eyes that would increase visual light sensitivity.
The selective benefit of light-sensitive eyes varies with the light intensity of the preferred time of activity over the 24-hr cycle, because natural irradiance levels in terrestrial environments span eight to nine orders of magnitude (Lythgoe, 1979 Land, 1981 Martin, 1983). In particular, the ability of eyes to produce bright images even in dim light conditions is considered a driving factor in the morphological evolution of eyes (Kirk, 2006 Hall and Ross, 2007 Hall, 2008a, 2008b Schmitz and Wainwright, 2011). For animals that rely primarily on vision to collect information about their environment, selection reflecting the contrasting characteristics of photopic (brightly lit) and scotopic (dimly lit) settings can ultimately result in different characteristics of the eye. Because diurnal and nocturnal animals face different selective regimes, their sensory physiologies can differ significantly. DAP also plays an important role in temporal and spatial resource partitioning in an ecosystem (Schoener, 1974 Kronfeld-Schor and Dayan, 2003). The periods of the day or night during which an animal is active influence the resources available to it, its foraging habits, and the potential predators it may encounter. © 2018 Wiley Periodicals, Inc.ĭiel activity pattern (DAP) is a key aspect of an animal's ecology and behavior. Our results indicate that these analyses offer several viable options for predicting DAP in the fossil record, but such analyses should be conducted in a phylogenetic context whenever possible. We find that although there are some interfering phylogenetic factors, nocturnal and non-nocturnal sciurids can be differentiated from one another with over 80% accuracy using all methods investigated here attempts to differentiate crepuscular animals from nocturnal and diurnal species proved much less successful. The latter two methods do not require a priori assignment of DAP and therefore reflect the situation in a fossil data set. We investigated the possibility of predicting DAP in sciurids (Mammalia: Rodentia: Sciuridae) using orbit measurements and other cranial dimensions, and a variety of quantitative methods, including phylogenetic flexible discriminant analysis, classification trees, and logistic regression. However, it would be useful if bony orbit dimensions could be used to determine DAP, particularly for mammals, which have no scleral ring, and nonmammalian synapsids, which infrequently preserve scleral rings.
Osteological characters, such as scleral ring dimensions, are also reliable proxies, but bony orbit dimensions alone have proven less reliable because soft tissues other than the eyeball can affect orbit size and shape. Eyeball dimensions are good predictors of DAP because they relate directly to light sensitivity of the eye. Various anatomical correlates have therefore been used to attempt to classify DAP. Diel activity pattern (DAP) is a key aspect of an animal's ecology, but it is difficult to infer when behavior cannot be directly observed, as in the fossil record.
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