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The influence of the sun's position and elevated cues on the orientation of hatchling sea turtles
Anim . Behav., 1970,18, 648-651
THE INFLUENCE OF THE SUN'S POSITION AND ELEVATED CUES ON THE ORIENTATION OF HATCHLING SEA TURTLES BY N . MROSOVSKY
Departments of Psychology and Zoology, University of Toronto
runs approximately East-West . The turtle arena test was used to assess orientation . In this test turtles are released in the centre of an open circle on their natural nesting beaches ; on reaching the edge of the circle they fall into a subdivided trench . Detailed descriptions are available elsewhere (Mrosovksy & Shettleworth 1968) . In the present experiments the animals were tipped randomly in the centre of the arena and 12 min were allowed for the test. Turtles still within the arena at this time were not used in the calculation of orientation vectors (see Batschelet 1965) . There were two groups, both comprising green turtles, Chelonia mydas, and hawksbill turtles, Eretmochelys imbricata, tested simultaneously . The first group was tested in the evening (17 .00 hours, on 30 May 1968) . The animals were less than 1 day out of the nest . The following morning (06 .50 hours) a second group of turtles out of the same nests was tested . On both occasions the sky was virtually cloudless . The water was not visible at turtle eye level from any point within the arena. The second group of turtles was naive, the first had been given one previous test in the arena, but since repeated testing has little effect on seafinding orientation (Mrosovsky & Shettleworth 1968), this point is ignored in the subsequent discussion .
The ability of turtles to reach the sea from the nest has recently been explained at the behavioural level in terms of a tropotactic reaction to light (Mrosovsky 1967 ; Ehrenfeld 1968 ; Mrosovsky & Shettleworth 1968) . Many beaches where turtles nest have a relatively simple topography with an open stretch of sand backed by trees and vegetation . This gives a brightness difference between the open seaward horizon and the darker tree line and land mass . With a tropotactic reaction the illumination from the open seaward horizon would guide the turtles in the correct direction even if the water was out of sight initially, because the inputs to both eyes would be equal when they were facing approximately toward the sea. With a purely tropotactic reaction any light stimulus should have some effect on the orientation . However, Ehrenfeld (1968) concluded, on the basis of data reported by Ehrenfeld & Carr (1967), that the position of the celestial bodies had no demonstrable effect on seafinding orientation . If this is so then there must be some systems for inhibiting reactions to certain lights, a reasonable possibility since many animals with tropotactic reactions do not only and invariably respond in this manner (Fraenkel & Gunn 1961) . On the other hand, given the poor visual acuity of marine turtles when on land (Walls 1942 ; Ehrenfeld & Koch 1967), and the large brightness differences between seaward and landward horizons on many natural nesting beaches, it might well be an unnecessary complication to have ways of discarding stimuli from the sun and the moon . Further tests were made therefore to discover if certain visual stimuli are indeed without major influence on the direction taken by hatchling turtles on their way to the sea.
Results and Discussion Figure 1 shows the mean vectors and individual results for both groups . The turtles tended to veer off toward the sun instead of taking the most direct path to the sea . There is little overlap between the distributions for morning and evening tests and for both species the differences between the tests at the two times of day are significant (P<0 .05, Mann-Whitney test, two-tailed) . The results show that the position of the sun does influence the orientation of hatchling sea turtles as would be expected if their reaction to light was tropotactic . The definite nature of these results compared to the smaller effects in Ehrenfeld & Carr's (1967) experiments is probably due to the fact that the present work was
Experiment 1 Methods This experiment compared the orientation of turtles released in the morning and in the evening when the sun was in different positions . The tests took place in Surinam, on the Wia-Wia Nature Reserve at Bigisanti where the coast 648
MROSOVSKY : ORIENTATION OF HATCHLING SEA TURTLES
Fig. 1 . Mean orientation vectors (arrows within circles) and individual orientation (dots) of green and hawksbill turtles released with the sun shining from different directions (arrows outside circles) . The numbers and dot within the circles represent animals that had not reached the edge of the arena when the test ended. The numbers round the edge of the circles label the segments of the arena ; segment 1 was nearest to the sea .
done on a beach running in an East-West direction while their experiments took place at Tortuguero, Costa Rica, where the beach runs approximately North-South . In the latter case when the sun or moon are low over the horizon they will be either well over the sea or well over the land . When they are very low over the land they will be hidden by trees, and when above the tree line will exert a relatively even effect on turtles heading toward the water compared to the present experimental conditions when the sun was nearly at right angles to the most direct seaward path . Even with the sun in this position most of the turtles, by the time they reached the edge of the arena, were heading in directions that would have taken them to the sea . This attests to the usefulness of a tropotactic reaction to light in seafinding for turtles, even in rather unusual conditions . Experiment 2 The previous experiment, taken together with observations on the effects of cloud conditions (Mrosovsky & Shettleworth 1968), suggests that brightness cues from all kinds of light sources contribute to the information pool on
649
which the turtle bases its seaward orientation . This does not imply that all information is given equal weight in the integration processes . The main brightness differences on nesting beaches are near the horizon between the dark tree line and the open sea . It is possible that stimuli from well above the horizon might have relatively little effect on the directional responses of turtles and that the influence of celestial bodies might be reduced in this way at most times of day. An experiment was therefore designed to determine whether cues above the level of a normal tree line could have strong effects on orientation . Methods A group of naive hawksbill turtles, less than 1 day out of the nest, was released in the centre of an arena surrounded by a wall of opaque black tar paper 85±5 cm high. This cut off any visual cues from the shore, trees and sea at turtle eye level. By the seaward segments of the arena an additional higher wall, 204±15 cm, was erected, thus introducing into an otherwise uniformly surrounded area a dark cue between about 10° and 22° 30" angular elevation from the centre of the arena (see Fig . 2 for further dimensions) . The walls were set back from the edge of the arena to prevent them casting any shadows on the area where the animals were moving about. As a further precaution the tests were made on a night with the sky relatively clear of clouds but moonless (21 .40 hours, 30 May 1968) . After a 12-min test the same animals were collected and given a second test in the same arena, only this time without the additional extra high wall by the seaward segments . Results and Discussion Figure 2 shows that with the extra elevated cue in the seaward direction the turtles headed in the opposite direction on the whole . With no such cue present, and an even wall all round, the mean orientation vector was reduced to virtually zero. In the first test many of the turtles did not reach the edge of the arena . Most of these were moving close to the landward segments of the arena when the test ended and had more time been allowed would doubtless have strengthened the orientation vector in that direction . Their slow movement was probably due to overheating. Warming is known to inhibit activity of hatchling turtles (Mrosovsky 1968) . In the present case the turtles had been kept on the day previous to the test indoors in sultry conditions . By the time of their second test
65 0
ANIMAL BEHAVIOUR, 18, 4
receptor density and some central weighting that make stimuli from certain directions, or of certain colours, relatively more important than others in determining orientation . Moreover how the tropotactic reaction is organized with respect to inputs from where the visual fields of the two eyes overlap remains to be analysed . Nevertheless the present experiments suggest that visual aspects of seafinding behaviour in hatchling turtles may depend only on a tropotactic reaction to light and that any visual stimulus in the field of view has some effect on directional responses . Although in some circumstances this permits the sun and moon to distract turtles from the shortest route to the sea, increasing exposure time to predators, perhaps this is a necessary sacrifice for a system that gives the certainty of reaching the water in almost all conditions. Summary 1. Hatchling sea turtles, Chelonia mydas and Eretmochelys imbricata, reacted to the position of the sun during tests of seafinding ability by deviating from the shortest route to the water . 2. Cutting off cues from close to the horizon disrupted orientation . Introduction of artificial cues elevated above the normal horizon into an otherwise uniform visual situation re-established orientation . 3. The results suggest that any kind of visual stimulus has some effect on the seafinding orientation of hatchling turtles and that this orientation depends on a relatively uncomplicated type of tropotactic reaction . Fig. 2 . Mean orientation vectors (arrows within circles) and individual orientation (dots) of green and hawksbill turtles released at night with an elevated cue in the seaward direction (top) and with only a uniform wall all around (bottom) . The numbers and dot within the circles represent animals that had not reached the edge of the arena when the test ended . The numbers round the edge of the circles label the segments of the arena ; segment 1 was nearest to the sea.
Acknowledgments These experiments were made possible through the courtesy of the Surinam Forest Service . The author thanks Dr J . P . Schulz and Mr G . Plak for their generous help and Miss S . J . Shettleworth for commenting on the manuscript . Support came from the National Research Council of Canada .
they had been exposed for some time to a cool outdoor breeze . A sufficient number of animals completed the tests to demonstrate that although the main cues on the natural beachscape guiding the hatchlings seaward are close to the horizon (see also Daniel & Smith 1947 ; Ehrenfeld & Carr 1967), when these are eliminated the animals still respond strongly to higher cues . Of course there may be factors such as variations in retinal
REFERENCES Batschelet, E. (1965). Statistical methods for the analysis of problems in animal orientation and certain biological rhythms. Am . Inst. biol. Sci. Monogr. Washington. Daniel, R. S. & Smith, K . U . (1947) . The sea-approach of the neonate loggerhead turtle (Caretta caretta). J. comp. physio!. Psycho!., 40, 413-420. Ehrenfeld, D. W . (1968). The role of vision in the seafinding orientation of the green turtle (Chelonia mydas) . II . Orientation mechanism and range of spectral sensitivity . Anim. Behav., 16, 281-287.
MROSOVSKY : ORIENTATION OF HATCHLING SEA TURTLES Ehrenfeld, D . W . & Carr, A . (1967). The role of vision in the sea-finding orientation of the green turtle (Chelonia mydas) . Anim. Behav ., 15, 25-36 . Ehrenfeld, D. W . & Koch, A. L . (1967). Visual accommodation in the green sea turtle . Science, N. Y., 155, 827-828 . Fraenkel, G. S. & Gunn, D . L . (1961) . The Orientation of Animals. New York : Dover Publications . Mrosovsky, N . (1967). How turtles find the sea . Sci. JI, 3,52-57 .
651
Mrosovsky, N . (1968) . Nocturnal emergence of hatchling sea turtles : control by thermal inhibition of activity . Nature, Lond., 220, 1338-1339. Mrosovsky, N. & Shettleworth, S . J . (1968). Wavelength preferences and brightness cues in the water finding behaviour of sea turtles . Behaviour, 32, 211-257. Walls, G. (1942) . The Vertebrate Eye and its Adaptive Radiation, p. 437 . Bloomfield Hills, Michigan : Cranbrook Institute of Science. (Received 26 January 1970 ; MS. number : 948)
THE INFLUENCE OF THE SUN'S POSITION AND ELEVATED CUES ON THE ORIENTATION OF HATCHLING SEA TURTLES BY N . MROSOVSKY
Departments of Psychology and Zoology, University of Toronto
runs approximately East-West . The turtle arena test was used to assess orientation . In this test turtles are released in the centre of an open circle on their natural nesting beaches ; on reaching the edge of the circle they fall into a subdivided trench . Detailed descriptions are available elsewhere (Mrosovksy & Shettleworth 1968) . In the present experiments the animals were tipped randomly in the centre of the arena and 12 min were allowed for the test. Turtles still within the arena at this time were not used in the calculation of orientation vectors (see Batschelet 1965) . There were two groups, both comprising green turtles, Chelonia mydas, and hawksbill turtles, Eretmochelys imbricata, tested simultaneously . The first group was tested in the evening (17 .00 hours, on 30 May 1968) . The animals were less than 1 day out of the nest . The following morning (06 .50 hours) a second group of turtles out of the same nests was tested . On both occasions the sky was virtually cloudless . The water was not visible at turtle eye level from any point within the arena. The second group of turtles was naive, the first had been given one previous test in the arena, but since repeated testing has little effect on seafinding orientation (Mrosovsky & Shettleworth 1968), this point is ignored in the subsequent discussion .
The ability of turtles to reach the sea from the nest has recently been explained at the behavioural level in terms of a tropotactic reaction to light (Mrosovsky 1967 ; Ehrenfeld 1968 ; Mrosovsky & Shettleworth 1968) . Many beaches where turtles nest have a relatively simple topography with an open stretch of sand backed by trees and vegetation . This gives a brightness difference between the open seaward horizon and the darker tree line and land mass . With a tropotactic reaction the illumination from the open seaward horizon would guide the turtles in the correct direction even if the water was out of sight initially, because the inputs to both eyes would be equal when they were facing approximately toward the sea. With a purely tropotactic reaction any light stimulus should have some effect on the orientation . However, Ehrenfeld (1968) concluded, on the basis of data reported by Ehrenfeld & Carr (1967), that the position of the celestial bodies had no demonstrable effect on seafinding orientation . If this is so then there must be some systems for inhibiting reactions to certain lights, a reasonable possibility since many animals with tropotactic reactions do not only and invariably respond in this manner (Fraenkel & Gunn 1961) . On the other hand, given the poor visual acuity of marine turtles when on land (Walls 1942 ; Ehrenfeld & Koch 1967), and the large brightness differences between seaward and landward horizons on many natural nesting beaches, it might well be an unnecessary complication to have ways of discarding stimuli from the sun and the moon . Further tests were made therefore to discover if certain visual stimuli are indeed without major influence on the direction taken by hatchling turtles on their way to the sea.
Results and Discussion Figure 1 shows the mean vectors and individual results for both groups . The turtles tended to veer off toward the sun instead of taking the most direct path to the sea . There is little overlap between the distributions for morning and evening tests and for both species the differences between the tests at the two times of day are significant (P<0 .05, Mann-Whitney test, two-tailed) . The results show that the position of the sun does influence the orientation of hatchling sea turtles as would be expected if their reaction to light was tropotactic . The definite nature of these results compared to the smaller effects in Ehrenfeld & Carr's (1967) experiments is probably due to the fact that the present work was
Experiment 1 Methods This experiment compared the orientation of turtles released in the morning and in the evening when the sun was in different positions . The tests took place in Surinam, on the Wia-Wia Nature Reserve at Bigisanti where the coast 648
MROSOVSKY : ORIENTATION OF HATCHLING SEA TURTLES
Fig. 1 . Mean orientation vectors (arrows within circles) and individual orientation (dots) of green and hawksbill turtles released with the sun shining from different directions (arrows outside circles) . The numbers and dot within the circles represent animals that had not reached the edge of the arena when the test ended. The numbers round the edge of the circles label the segments of the arena ; segment 1 was nearest to the sea .
done on a beach running in an East-West direction while their experiments took place at Tortuguero, Costa Rica, where the beach runs approximately North-South . In the latter case when the sun or moon are low over the horizon they will be either well over the sea or well over the land . When they are very low over the land they will be hidden by trees, and when above the tree line will exert a relatively even effect on turtles heading toward the water compared to the present experimental conditions when the sun was nearly at right angles to the most direct seaward path . Even with the sun in this position most of the turtles, by the time they reached the edge of the arena, were heading in directions that would have taken them to the sea . This attests to the usefulness of a tropotactic reaction to light in seafinding for turtles, even in rather unusual conditions . Experiment 2 The previous experiment, taken together with observations on the effects of cloud conditions (Mrosovsky & Shettleworth 1968), suggests that brightness cues from all kinds of light sources contribute to the information pool on
649
which the turtle bases its seaward orientation . This does not imply that all information is given equal weight in the integration processes . The main brightness differences on nesting beaches are near the horizon between the dark tree line and the open sea . It is possible that stimuli from well above the horizon might have relatively little effect on the directional responses of turtles and that the influence of celestial bodies might be reduced in this way at most times of day. An experiment was therefore designed to determine whether cues above the level of a normal tree line could have strong effects on orientation . Methods A group of naive hawksbill turtles, less than 1 day out of the nest, was released in the centre of an arena surrounded by a wall of opaque black tar paper 85±5 cm high. This cut off any visual cues from the shore, trees and sea at turtle eye level. By the seaward segments of the arena an additional higher wall, 204±15 cm, was erected, thus introducing into an otherwise uniformly surrounded area a dark cue between about 10° and 22° 30" angular elevation from the centre of the arena (see Fig . 2 for further dimensions) . The walls were set back from the edge of the arena to prevent them casting any shadows on the area where the animals were moving about. As a further precaution the tests were made on a night with the sky relatively clear of clouds but moonless (21 .40 hours, 30 May 1968) . After a 12-min test the same animals were collected and given a second test in the same arena, only this time without the additional extra high wall by the seaward segments . Results and Discussion Figure 2 shows that with the extra elevated cue in the seaward direction the turtles headed in the opposite direction on the whole . With no such cue present, and an even wall all round, the mean orientation vector was reduced to virtually zero. In the first test many of the turtles did not reach the edge of the arena . Most of these were moving close to the landward segments of the arena when the test ended and had more time been allowed would doubtless have strengthened the orientation vector in that direction . Their slow movement was probably due to overheating. Warming is known to inhibit activity of hatchling turtles (Mrosovsky 1968) . In the present case the turtles had been kept on the day previous to the test indoors in sultry conditions . By the time of their second test
65 0
ANIMAL BEHAVIOUR, 18, 4
receptor density and some central weighting that make stimuli from certain directions, or of certain colours, relatively more important than others in determining orientation . Moreover how the tropotactic reaction is organized with respect to inputs from where the visual fields of the two eyes overlap remains to be analysed . Nevertheless the present experiments suggest that visual aspects of seafinding behaviour in hatchling turtles may depend only on a tropotactic reaction to light and that any visual stimulus in the field of view has some effect on directional responses . Although in some circumstances this permits the sun and moon to distract turtles from the shortest route to the sea, increasing exposure time to predators, perhaps this is a necessary sacrifice for a system that gives the certainty of reaching the water in almost all conditions. Summary 1. Hatchling sea turtles, Chelonia mydas and Eretmochelys imbricata, reacted to the position of the sun during tests of seafinding ability by deviating from the shortest route to the water . 2. Cutting off cues from close to the horizon disrupted orientation . Introduction of artificial cues elevated above the normal horizon into an otherwise uniform visual situation re-established orientation . 3. The results suggest that any kind of visual stimulus has some effect on the seafinding orientation of hatchling turtles and that this orientation depends on a relatively uncomplicated type of tropotactic reaction . Fig. 2 . Mean orientation vectors (arrows within circles) and individual orientation (dots) of green and hawksbill turtles released at night with an elevated cue in the seaward direction (top) and with only a uniform wall all around (bottom) . The numbers and dot within the circles represent animals that had not reached the edge of the arena when the test ended . The numbers round the edge of the circles label the segments of the arena ; segment 1 was nearest to the sea.
Acknowledgments These experiments were made possible through the courtesy of the Surinam Forest Service . The author thanks Dr J . P . Schulz and Mr G . Plak for their generous help and Miss S . J . Shettleworth for commenting on the manuscript . Support came from the National Research Council of Canada .
they had been exposed for some time to a cool outdoor breeze . A sufficient number of animals completed the tests to demonstrate that although the main cues on the natural beachscape guiding the hatchlings seaward are close to the horizon (see also Daniel & Smith 1947 ; Ehrenfeld & Carr 1967), when these are eliminated the animals still respond strongly to higher cues . Of course there may be factors such as variations in retinal
REFERENCES Batschelet, E. (1965). Statistical methods for the analysis of problems in animal orientation and certain biological rhythms. Am . Inst. biol. Sci. Monogr. Washington. Daniel, R. S. & Smith, K . U . (1947) . The sea-approach of the neonate loggerhead turtle (Caretta caretta). J. comp. physio!. Psycho!., 40, 413-420. Ehrenfeld, D. W . (1968). The role of vision in the seafinding orientation of the green turtle (Chelonia mydas) . II . Orientation mechanism and range of spectral sensitivity . Anim. Behav., 16, 281-287.
MROSOVSKY : ORIENTATION OF HATCHLING SEA TURTLES Ehrenfeld, D . W . & Carr, A . (1967). The role of vision in the sea-finding orientation of the green turtle (Chelonia mydas) . Anim. Behav ., 15, 25-36 . Ehrenfeld, D. W . & Koch, A. L . (1967). Visual accommodation in the green sea turtle . Science, N. Y., 155, 827-828 . Fraenkel, G. S. & Gunn, D . L . (1961) . The Orientation of Animals. New York : Dover Publications . Mrosovsky, N . (1967). How turtles find the sea . Sci. JI, 3,52-57 .
651
Mrosovsky, N . (1968) . Nocturnal emergence of hatchling sea turtles : control by thermal inhibition of activity . Nature, Lond., 220, 1338-1339. Mrosovsky, N. & Shettleworth, S . J . (1968). Wavelength preferences and brightness cues in the water finding behaviour of sea turtles . Behaviour, 32, 211-257. Walls, G. (1942) . The Vertebrate Eye and its Adaptive Radiation, p. 437 . Bloomfield Hills, Michigan : Cranbrook Institute of Science. (Received 26 January 1970 ; MS. number : 948)