84
The Biology of Sea Turtles, Vol. II
3.2.2.3 Behavior
Behavior studies on sea turtles performed in the aqueous setting are limited because
of the difficulties associated with training turtles to respond to specific stimuli.
Fehring (1972), however, used the sea turtle’s ability to detect colors to develop a
hue discrimination behavioral study. Broadband hues were used (deep blue, magenta,
and red-orange) to determine whether loggerhead sea turtles ( C. caretta ) could be
trained to use hue in search for food. The research study was not designed to test
for an inherent hue preference, but rather was designed to test whether the turtles
could be trained to pick one hue over another. Each animal was given a choice of
two hues and, through training, was taught that only one of these hues would provide
a food reward. Fehring found that these animals were easily trained, with relatively
few errors, and thus concluded that sea turtles are able to use their ability to
distinguish colors to find food (1972).
3.2.3 V ISUAL A CUITY
3.2.3.1 Topographical Organization of the Retina
Retinal morphology and topography research can describe the potential resolving
power of an eye under differing illumination conditions. Within the retina itself,
two factors can affect the ability of an animal to resolve items under varying light
conditions: convergence of photoreceptor cells onto ganglion cells, and the topographical organization of photoreceptor cells along the surface of the retina (Walls,
1942; Davson, 1972; Ali and Klyne, 1985). Within the photoreceptor layer, the
sea turtle has two types of cells: rods and cones. For most vertebrates, and sea
turtles are no exception, the general function of the rod photoreceptor is to
maximize sensitivity of the eye to dim stimuli, whereas the general function of
the cone photoreceptor is to resolve details of a visual object (Copenhaver, 1964;
Davson, 1972; Stell, 1972). Convergence of photoreceptor cells upon ganglion
cells, otherwise termed summation, can prove to be both beneficial and disadvantageous. When the stimulus is weak (under dim light conditions), more than one
rod photoreceptor cell converging onto a single ganglion cell will subsequently
increase the strength of the neural signal, allowing the stimulus to be recognized.
However, when summation occurs between cone photoreceptor cells and ganglion
cells, the information relayed to the optic tectum is not characteristic of one cone,
but rather a summation of many, resulting in reduced spatial resolution (Walls,
1942; Davson, 1972).
Topographical distribution of cone photoreceptor cells also can be an indication
of the resolution ability of an animal. The retinas of many vertebrates have regions
of higher cell densities, often called an area centralis or visual streak, which
provides a region of increased visual acuity. The area centralis can vary in shape
and location along the retina among species, and this variation is often indicative
of behavior and life history attributes of the animal (Walls, 1942; Brown, 1969;
Heuter, 1991).
1123 book.book Page 84 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
3.2.2.3 Behavior
Behavior studies on sea turtles performed in the aqueous setting are limited because
of the difficulties associated with training turtles to respond to specific stimuli.
Fehring (1972), however, used the sea turtle’s ability to detect colors to develop a
hue discrimination behavioral study. Broadband hues were used (deep blue, magenta,
and red-orange) to determine whether loggerhead sea turtles ( C. caretta ) could be
trained to use hue in search for food. The research study was not designed to test
for an inherent hue preference, but rather was designed to test whether the turtles
could be trained to pick one hue over another. Each animal was given a choice of
two hues and, through training, was taught that only one of these hues would provide
a food reward. Fehring found that these animals were easily trained, with relatively
few errors, and thus concluded that sea turtles are able to use their ability to
distinguish colors to find food (1972).
3.2.3 V ISUAL A CUITY
3.2.3.1 Topographical Organization of the Retina
Retinal morphology and topography research can describe the potential resolving
power of an eye under differing illumination conditions. Within the retina itself,
two factors can affect the ability of an animal to resolve items under varying light
conditions: convergence of photoreceptor cells onto ganglion cells, and the topographical organization of photoreceptor cells along the surface of the retina (Walls,
1942; Davson, 1972; Ali and Klyne, 1985). Within the photoreceptor layer, the
sea turtle has two types of cells: rods and cones. For most vertebrates, and sea
turtles are no exception, the general function of the rod photoreceptor is to
maximize sensitivity of the eye to dim stimuli, whereas the general function of
the cone photoreceptor is to resolve details of a visual object (Copenhaver, 1964;
Davson, 1972; Stell, 1972). Convergence of photoreceptor cells upon ganglion
cells, otherwise termed summation, can prove to be both beneficial and disadvantageous. When the stimulus is weak (under dim light conditions), more than one
rod photoreceptor cell converging onto a single ganglion cell will subsequently
increase the strength of the neural signal, allowing the stimulus to be recognized.
However, when summation occurs between cone photoreceptor cells and ganglion
cells, the information relayed to the optic tectum is not characteristic of one cone,
but rather a summation of many, resulting in reduced spatial resolution (Walls,
1942; Davson, 1972).
Topographical distribution of cone photoreceptor cells also can be an indication
of the resolution ability of an animal. The retinas of many vertebrates have regions
of higher cell densities, often called an area centralis or visual streak, which
provides a region of increased visual acuity. The area centralis can vary in shape
and location along the retina among species, and this variation is often indicative
of behavior and life history attributes of the animal (Walls, 1942; Brown, 1969;
Heuter, 1991).
1123 book.book Page 84 Monday, November 11, 2002 11:11 AM