Bottlenose Dolphin (Tursiops truncatus) Double-Slit Pupil Asymmetries Enhance Vision by ProQuest

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Geometries of the iris, retinal cell distributions, and the optical characteristics of the lens and cornea have evolved to optimize the visual adaptations of the bottlenose dolphin (Tursiops truncatus) to the oceanic environment. Under high ambient light conditions, the operculum of the iris shields the lens and forms two asymmetrical slit pupils. Under these conditions, light entering the eye is channeled and focused onto the two areas of the retina having a finer retinal mosaic of ganglion cells (typically associated with higher image resolution). The paths of light determined by tracing rays in the reverse direction through these pupils coincide with a dolphin's behaviorally observed preferred viewing directions. These rays aid in determining the interdependence between the graininess of the retinal mosaic and resolution spot sizes in the object space. For oblique forward and downward viewing directions in air, the larger temporal pupil admits light which passes through the weakly refractive margin of a bifocal lens, counterbalancing the optically strong cornea in air. In water, light passing through the optically strong lens core is focused from a wide lateral and downward field-of-vision. Although other explanations for comparable aerial and underwater vision remain plausible, a dolphin eye model incorporating a bifocal lens offers an explanation consistent with ophthalmoscopic refractive state measurements. The model is also consistent with visual acuity study results conducted in air and in water under both high and low ambient light levels. From insight gained after applying a common data analysis technique to visual acuity studies conducted by other researchers and tracing oblique rays through the asymmetric double-slit pupils, a re-examination of explanatory hypotheses for the paradoxical observations of comparable aerial and underwater vision is presented. Based in part on these findings and supportive evidence from dolphin vision researchers, the unique

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									Aquatic Mammals 2009, 35(2), 269-280, DOI 10.1578/AM.35.2.2009.269




            Bottlenose Dolphin (Tursiops truncatus) Double-Slit
                    Pupil Asymmetries Enhance Vision
                                        Lorenzo A. Rivamonte
              Engineering Division, U.S. Army TMDE Activity, Redstone Arsenal, AL 35898-5340, USA;
                                   E-mail: lorenzo.andre.rivamonte@us.army.mil


                     Abstract                             Key Words: vision, bifocal lens, iris asymmetries,
                                                          bottlenose dolphin, Tursiops truncatus, opercu-
Geometries of the iris, retinal cell distributions,       lum, bioacoustic-imaging
and the optical characteristics of the lens and
cornea have evolved to optimize the visual adap-                              Introduction
tations of the bottlenose dolphin (Tursiops trun-
catus) to the oceanic environment. Under high             Dolphins echolocate under water to locate and
ambient light conditions, the operculum of the iris       identify things of interest in their ecosystem.
shields the lens and forms two asymmetrical slit          Vision can be used for similar purposes in both
pupils. Under these conditions, light entering the        air and water. Bioacoustic-imaging and vision
eye is channeled and focused onto the two areas of        are probably integrated and co-processed in real
the retina having a finer retinal mosaic of ganglion      time as indicated in a study where active echo-
cells (typically associated with higher image reso-       location increased when ambient lighting was
lution). The paths of light determined by tracing         abruptly decreased (Akamatsu et al., 1992). In
rays in the reverse direction through these pupils        another example, it was observed that a dolphin’s
coincide with a dolphin’s behaviorally observed           approach to a maze-obstructed underwater target
preferred viewing directions. These rays aid in           was much faster and less dependent on active
determining the interdependence between the               echolocation when not deprived of vision as a sen-
graininess of the retinal mosaic and resolution           sory input (Azzali, 1992). Captive dolphins famil-
spot sizes in the object space. For oblique forward       iar with their aquatic environment decrease active
and downward viewing directions in air, the larger        echolocation and seem to depend more on sight.
temporal pupil admits light which passes through          This provides excellent opportunities to observe
the weakly refractive margin of a bifocal lens,           the dolphin’s preferred viewing directions both in
counterbalancing the optically strong cornea in           air and under water. Aerial vision may also take
air. In water, light passing through the optically        on added importance as enrichment interactions
strong lens core is focused from a wide lateral           with humans most often take place in air (Pryor,
and downward field-of-vision. Although other              1975).
explanations for comparable aerial and underwa-               Explanations for paradoxical observations of
ter vision remain plausible, a dolphin eye model          comparable aerial and underwater vision remain a
incorporating a bifocal lens offers an explanation        subject of debate. After reexamining the observa-
consistent with ophthalmoscopic refractive state          tions and proposed theories from dolphin vision
measurements. The model is also consistent with           studies, a consolidation of more likely explana-
visual acuity study results conducted in air and in       tions for the capabilities and nature of dolphin
water under both high and low ambient light levels.       vision is presented. Comparisons of similarly
From insight gained after applying a common data          structured behavioral acuity studies often require
analysis technique to visual acuity studies con-          a reanalysis of the data with a common statisti-
ducted by other researchers and tracing oblique           cal technique and threshold criterion. A classic
rays through the asymmetric double-slit pupils, a         psychometric function as used in the Madsen
re-examination of explanatory hypotheses for the          (1976) color vision and spectral sensitivity study
paradoxical observations of comparable aerial and         is described and applied to the behavioral visual
underwater vision is presented. Based in part on          acuity studies. The bottlenose dolphin (Tursiops
these findings and supportive evidence from dol-          truncatus) bifocal lens model (Rivamonte, 1976)
phin vision researchers, the unique distinguishing        is reviewed for light passing obliquely through the
characteristics of dolphin vision are summarized.         double-slit pupil in bright sunlight, and corrected
270                                                Rivamonte


to illustrate the refraction of light by the front sur-
face of the cornea.

              
								
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