Sensory Biology of Sea Turtles
89
blindfolds were placed on the turtles to determine whether they could orient without
visual input. Bilaterally blindfolded turtles were unable to find the sea at all (Daniel
and Smith, 1947; Carr and Ogren, 1960; van Rhijn, 1979), and unilaterally blindfolded sea turtles circled toward the uncovered eye, suggesting that the sea turtle
finds the sea using tropotactic behavior (comparing intensities in both eyes and
moving accordingly) (Ehrenfeld, 1968; Mrosovsky and Shettleworth, 1968; Mrosovsky, 1972; Mrosovsky et al., 1979). These hatchling sea turtles are attracted to,
and move toward, the brightest direction.
Shape identification, or the ability of a sea turtle to visualize objects on the
beach, has also been investigated in the context of sea-finding behavior. The reaction
by hatchlings to a horizon obstructed by objects found on or surrounding the beach
has been documented in many studies (Parker, 1922; Limpus, 1971; Salmon et al.,
1992). Salmon and Wyneken (1994) found that sea-finding for sea turtles depends
on three rules when orienting toward the sea: (1) sea turtles move toward brighter
regions, (2) sea turtles move away from high beach silhouettes (such as foliage or
sand dunes), and (3) when these two cues are inconsistent, sea turtles move in relation
to elevation (beach silhouettes), not brightness. Ehrenfeld and Carr (1967) tested
the extent to which green sea turtles ( C. mydas ) visualize objects on the beach when
making decisions about which direction to crawl. Adult turtles were fitted with an
eye-covering apparatus that was designed to hold wax paper filters. The wax paper
filter acted to soften sharp images by scattering light. The results showed that if the
turtles were allowed to acclimate to the wax paper filter for 10 min, then their seafinding ability was not hampered by a diffuse vision. The result of this research
implies that C. mydas adults are not using sharp visual acuity to find water, but
rather diffuse beach silhouettes.
Brightness level, a known stimulus to which sea turtles respond, is often a result
of the wavelength characteristics of that stimulus. Therefore, wavelength preferences
of turtles on the beach have also been investigated as a tool for finding the sea after
hatching or a nesting event. Ehrenfeld and Carr (1967) found that adult female green
sea turtles ( C. mydas ) wearing colored filters (red, green, and blue) were still able
to find water better than those turtles that were blindfolded. However, some colors
worked better than others. For example, sea turtles wearing a green filter performed
as well as the control group (nonblindfolded turtles). However, turtles wearing the
red filter showed a sharp decrease in performance, indicating a possible upper limit
to spectral sensitivity.
Mrosovsky and Shettleworth (1968) found that green hatchling sea turtles
had a preference for short wavelengths, even if the intensity of the longer
wavelengths was stronger. Mrosovsky (1972) found that red wavelengths had
very little effect on green sea turtles except when very bright, but turtles were
attracted to blue light even at low energy levels. These studies indicate that green
turtles have a preference for shorter wavelength light. Witherington and Bjorndal
(1991) tested loggerhead ( C. caretta ) and green ( C. mydas ) sea turtle hatchlings
for color preference in air using a V-maze, two-choice design. When placed in
the maze, both species chose 360 (near-ultraviolet), 400 (violet), and 500 (bluegreen) nm wavelengths over a constant light source, but did not choose 600
(yellow-orange) or 700 (red) nm wavelengths. Loggerheads actually moved away
1123 book.book Page 89 Monday, November 11, 2002 11:11 AM
89
blindfolds were placed on the turtles to determine whether they could orient without
visual input. Bilaterally blindfolded turtles were unable to find the sea at all (Daniel
and Smith, 1947; Carr and Ogren, 1960; van Rhijn, 1979), and unilaterally blindfolded sea turtles circled toward the uncovered eye, suggesting that the sea turtle
finds the sea using tropotactic behavior (comparing intensities in both eyes and
moving accordingly) (Ehrenfeld, 1968; Mrosovsky and Shettleworth, 1968; Mrosovsky, 1972; Mrosovsky et al., 1979). These hatchling sea turtles are attracted to,
and move toward, the brightest direction.
Shape identification, or the ability of a sea turtle to visualize objects on the
beach, has also been investigated in the context of sea-finding behavior. The reaction
by hatchlings to a horizon obstructed by objects found on or surrounding the beach
has been documented in many studies (Parker, 1922; Limpus, 1971; Salmon et al.,
1992). Salmon and Wyneken (1994) found that sea-finding for sea turtles depends
on three rules when orienting toward the sea: (1) sea turtles move toward brighter
regions, (2) sea turtles move away from high beach silhouettes (such as foliage or
sand dunes), and (3) when these two cues are inconsistent, sea turtles move in relation
to elevation (beach silhouettes), not brightness. Ehrenfeld and Carr (1967) tested
the extent to which green sea turtles ( C. mydas ) visualize objects on the beach when
making decisions about which direction to crawl. Adult turtles were fitted with an
eye-covering apparatus that was designed to hold wax paper filters. The wax paper
filter acted to soften sharp images by scattering light. The results showed that if the
turtles were allowed to acclimate to the wax paper filter for 10 min, then their seafinding ability was not hampered by a diffuse vision. The result of this research
implies that C. mydas adults are not using sharp visual acuity to find water, but
rather diffuse beach silhouettes.
Brightness level, a known stimulus to which sea turtles respond, is often a result
of the wavelength characteristics of that stimulus. Therefore, wavelength preferences
of turtles on the beach have also been investigated as a tool for finding the sea after
hatching or a nesting event. Ehrenfeld and Carr (1967) found that adult female green
sea turtles ( C. mydas ) wearing colored filters (red, green, and blue) were still able
to find water better than those turtles that were blindfolded. However, some colors
worked better than others. For example, sea turtles wearing a green filter performed
as well as the control group (nonblindfolded turtles). However, turtles wearing the
red filter showed a sharp decrease in performance, indicating a possible upper limit
to spectral sensitivity.
Mrosovsky and Shettleworth (1968) found that green hatchling sea turtles
had a preference for short wavelengths, even if the intensity of the longer
wavelengths was stronger. Mrosovsky (1972) found that red wavelengths had
very little effect on green sea turtles except when very bright, but turtles were
attracted to blue light even at low energy levels. These studies indicate that green
turtles have a preference for shorter wavelength light. Witherington and Bjorndal
(1991) tested loggerhead ( C. caretta ) and green ( C. mydas ) sea turtle hatchlings
for color preference in air using a V-maze, two-choice design. When placed in
the maze, both species chose 360 (near-ultraviolet), 400 (violet), and 500 (bluegreen) nm wavelengths over a constant light source, but did not choose 600
(yellow-orange) or 700 (red) nm wavelengths. Loggerheads actually moved away
1123 book.book Page 89 Monday, November 11, 2002 11:11 AM