Introduction

The mechanism of magnetic field detection has not been conclusively established in sea turtles or other animals (Lohmann and Johnsen 2000).  One hypothesis is that cells of the nervous system contain or are closely associated with crystals of biogenic single-domain magnetite and produce varying signals based on the physical forces applied to the crystals by the direction and/or magnitude of the Earth's magnetic field (Kirschvink and Gould 1981). 

If magnetite particles are involved in magnetic field detection, then reversing the dipole moment of the particles might alter their interaction with the Earth's magnetic field.  This in turn might affect the nature of the signal transduction and thus alter the behavior of an animal using the geomagnetic field for orientation or navigation.  The dipole moments of magnetite crystals can be reversed by applying a magnetic pulse, provided that the field is aligned correctly, strong enough, and sufficiently brief that the particles cannot rotate within the field (Kirschvink 1983).   Pulses of this type have been found to alter the orientation behavior of several species of migratory birds, resulting in either a shift in orientation direction (Wiltschko et al. 1994, Beason et al. 1995, Beason et al. 1997, Munro et al. 1997, Wiltschko et al. 1998, Wiltschko et al. 2002) or random orientation (Wiltschko and Wiltschko 1995b, Beason et al. 1995).  These results suggest that at least some part of the avian magnetoreception system involves permanently magnetized material, presumably magnetite. 

As a first step toward investigating whether magnetoreception in sea turtles is based at least partly on permanently magnetic material, I compared the orientation behavior of turtles subjected to strong, pulsed fields with that of turtles that were treated similarly but did not receive a magnetic pulse.   The results indicate that pulsed fields alter magnetic orientation behavior, a finding consistent with the hypothesis that at least part of the sea turtle magnetoreception system involves magnetite.

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). 

Procedure

For each trial, a hatchling was placed into a Lycra harness that encircled the carapace but did not inhibit swimming movements (Salmon and Wyneken, 1987).  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 south quadrant of the arena, the arena cover was lowered, the black plastic covering was secured over the tank, and the data acquisition computer was started.  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.

Turtles were assigned to one of two groups.  Immediately prior to being placed into the arena, turtles in one group were exposed to a series of 5 brief magnetic pulses (25,000 µT, 4 msec rise time).   The coil was aligned with the N-S axis of the geomagnetic field, but the polarity of the pulsed fields generated by the coil was opposite that of the Earth’s field.  Turtles were held within the solenoid so that the first, third and fifth pulses were directed south-posterior (as defined by Beason et al. 1995), and the second and fourth were south-left (Fig. 2).  This procedure was designed to affect magnetic particles aligned along at least two different body axes.  A series of pulses was used because previous results with birds suggested that multiple pulses produce a greater effect on orientation than a single pulse (Beason et al.1995).  Control turtles were held in the solenoid in the same manner as turtles exposed to the pulsed field, but no pulses were generated during this time.

Using these procedures, turtles from one or two nests were tested each night.  In all cases, one turtle from each nest was tested with the magnetic pulse treatment and one turtle from the same nest was tested without the magnetic treatment.  Turtles receiving different treatments (pulsed or non-pulsed) were tested alternately.

Data analysis and statistics

The data-acquisition computer calculated a mean heading for each turtle based on all data collected during the first 60 min. of the trial (when turtles were swimming toward the light source) and during the final 60 min of the trial (when turtles were swimming in darkness).  A Rayleigh test was used to determine whether each group (control turtles and pulsed turtles) was significantly oriented during the light and dark periods.  In addition, the distributions of the two groups were compared using Watson's U² test (Batschelet 1981) for both the light and dark periods .

Results and Discussion

            Control turtles and turtles exposed to the magnetic pulses both swam consistently toward a light in the east (Fig. 6.3A,C).  The ability to maintain a course toward a light has long been used as a behavioral assay for assessing whether hatchlings are developmentally competent to maintain headings and are motivated to swim steadily (Salmon and Wyneken, 1987, Lohmann, 1991, Light et al. 1993, Lohmann and Lohmann 1994a,b, 1996, Lohmann et al. 2001, Irwin and Lohmann 2003).  The fact that both groups of turtles established and maintained courses toward light indicates that the magnetic pulses did not prevent turtles from maintaining a heading or alter in any obvious way their motivation to orient or swim. 

Whereas no differences between the behavior of control and magnetically pulsed turtles could be discerned when the groups were exposed to a light source, significant differences in orientation occurred when the turtles were tested in darkness under conditions in which they are known to rely on magnetic orientation (Lohmann 1991, Light et al. 1993, Irwin and Lohmann 2003).  Control turtles continued to swim approximately eastward (Fig. 6.3B), a response that presumably reflects a magnetic directional preference acquired on the basis of light cues (Lohmann and Lohmann 1994a).  In contrast, turtles that had been exposed to magnetic pulses were not significantly oriented as a group (Fig. 6.3D).  The distributions of the two groups of turtles were significantly different.    

 Thus, the orientation behavior of turtles exposed to the magnetic pulses differed from that of controls under conditions in which turtles orient magnetically, but not when turtles orient using visual cues.  This argues that the pulsed fields were unlikely to have exerted a general effect on physiology, health, activity level, or motivation that influences all orientation behavior.  Instead, the magnetic pulses appear to have affected processes specific to magnetic orientation.

One hypothesis to explain how animals detect magnetic fields is that crystals of biogenic magnetite are involved in the magnetic transduction process (Kirschvink and Gould 1981, Kirschvink et al. 2001).  If so, then pulsed fields capable of altering the dipole moment of the crystals might, in principle, affect the behavior of an animal that is relying on magnetic field information (Kirschvink 1983).  Our results are consistent with several previous studies in which applying magnetic pulses resulted in altered magnetic orientation behavior (Wiltschko et al. 1994, Beason et al. 1995, Beason et al. 1997, Munro et al. 1997, Wiltschko et al. 1998, Wiltschko et al. 2002, Marhold 1997, Wiltschko and Wiltschko 1995b, Beason et al. 1997). 

Sea turtles emerge from their nests with the ability to derive both directional (Lohmann 1991) and positional (Lohmann and Lohmann 1996, Lohmann et al. 2001) information from the Earth's magnetic field.   In principle, the pulsed fields might have interfered with one or both of these abilities.  One possibility is that the pulsed fields disrupted the turtles’ magnetic compass sense, so that they could not orient in the normal way.  Alternatively or additionally, the pulsed fields might have impaired the ability of the turtles to perceive magnetic elements such as inclination (Lohmann and Lohmann 1994b) or intensity (Lohmann and Lohmann 1996)  correctly, so that an accurate assessment of positional information was not possible.  Additional research will be needed to distinguish among these possibilities.  Regardless, however, our results are consistent with the hypothesis that magnetite is involved in at least part of the turtle magnetoreception system.

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