Magnetic Distortion Interferes With Orientation Behavior

Introduction

Several approaches have been used to investigate magnetic orientation behavior in animals.  One technique is to use large magnetic coil systems to generate relatively uniform fields over a considerable area (Merritt et al. 1983, Kirschvink 1992).  When the magnetic field is changed artificially, researchers can determine if the orientation of animals inside the coil shows a corresponding shift (Wiltschko and Wiltschko 1995).  A second method of investigation is to attach a small magnet or electromagnetic coil directly to an animal in an attempt to disrupt or alter magnetic orientation behavior.  If the imposed field produced is strong enough to interfere with the Earth’s field and redundant cues are not available, changes in orientation behavior (Keeton 1971, Walcott and Green 1974, Mathis and Moore 1988) or the performance of a conditioned response (Walker and Bitterman 1989, Haugh et al. 2001) can sometimes be observed.

To date, all laboratory experiments investigating magnetic orientation behavior in sea turtles have involved large magnetic coil systems (e.g. Lohmann 1991, Light et al. 1993, Lohmann and Lohmann 1994b, 1996a, Goff et al. 1998).  To investigate whether small magnets can be used to disrupt orientation behavior of sea turtles, I monitored two groups of hatchlings under laboratory conditions in which they are known to orient magnetically.  The results demonstrate that magnetic orientation behavior in hatchling turtles can be disrupted by attaching a small magnet to the carapace. 

Methods

Animals

Hatchling loggerheads were obtained from nests at a beach hatchery several hours before they would otherwise have emerged naturally and were immediately placed into lightproof coolers.  They were then transported to a laboratory and maintained in darkness.  Each hatchling was tested only once and released on the beach following experiments each night.

 Experimnetal Arena

Experiments were conducted in a black, plastic, circular pool filled with water and enclosed by a removable lightproof cover (Fig. 1).  A light-emitting diode was attached to the inside wall of the pool directly east of the center of the arena.  The LED could be turned on and off as needed (see below). 

Figure 1. Experimental arena

Procedure

For each trial, a hatchling was placed in a Lycra harness that encircled the carapace but did not inhibit swimming movements (Salmon and Wyneken 1987).  Turtles were assigned to one of two groups.  One set of turtles (n=13) had a 0.24g, 14mm SpinBar® magnet attached by Velcro to the harness on the dorsal side of the turtle approximately 1 cm posterior to the nucal scute (Fig. 2).  The magnet produced a field greater than Earth-strength over the entire body of the turtle. The other set of turtles (n=15) had a magnetically inert brass bar of equivalent size and weight attached in the same manner and location as the magnet. 

Figure 2. Turtle wearing the harness

The harness was attached by monofilament line to a wooden tracker arm that was affixed to a rotary digital encoder mounted above the center of the pool (Fig. 1). The tracker arm could rotate freely within the horizontal plane and thus tracked the movement of the turtle as it swam.  Information was relayed, via the digital encoder, to a data acquisition computer that continuously monitored the heading of the turtle throughout each trial. 

At the beginning of each trial, the LED in the east side of the tank was turned on.  A harnessed hatchling was then released in the arena.  The arena cover was lowered over the tank and the data acquisition computer was started.  The computer recorded the magnetic heading of the turtle every 10 sec throughout the trial.  The turtle was allowed to swim toward the light for 60 min.  The light was then turned off.  After a 3 min adjustment period, the orientation of the turtle was monitored as it swam in darkness during the next 60 min.

Data analysis and statistics

The data-acquisition computer calculated the mean heading for each turtle based on all data collected during the final 60 min of the trial (i.e., the period beginning 3 min after the light was turned off). The orientation of each group of turtles was analyzed using a Rayleigh test and the distributions of the two groups were compared using Watson's U2 test (Batschelet 1981).

Results and Discussion

Turtles with brass bars attached to their carapace were significantly oriented as a group with a mean heading of 77.5°.  This eastward orientation closely resembles that observed in several previous studies in which similar procedures were used (Lohmann 1991, Light et al. 1993, Lohmann and Lohmann 1994a) and probably represents a magnetic directional preference acquired on the basis of light cues provided at the start of each trial (Lohmann and Lohmann 1994a).   In contrast, turtles with magnets attached were not significantly oriented as a group (Fig. 3B).   The orientation of the two groups was statistically different.  Because the size and weight of the brass bars and magnets were nearly identical, the results imply that the magnetic field produced by the bar magnets disrupted the ability of the turtles to maintain consistent orientation under the test conditions.  

Figure 3. Results
Hatchling sea turtles have been shown to derive both directional information (Lohmann 1991) and positional information (Lohmann and Lohmann 1994b, 1996b, Lohmann et al. 2001) from the Earth's magnetic field.  In principle, the magnets attached to the turtles might have interfered with either or both of these abilities.  One possibility is that the field disrupted the turtles’ magnetic compass sense, so that they could not hold a reliable course.  Alternatively or additionally, however, the field from the magnet might have distorted the field line inclination and intensity around the turtles, so that an accurate assessment of positional information was not possible.  The present data are insufficient to distinguish among these different possibilities.

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This experiment was published as: 
Irwin, W. P. and Lohmann, K. J. (2003). Magnet-induced disorientation in hatchling loggerhead sea turtles. J Exp Biol 206: 497-501.
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