`Fixed-axis' magnetic orientation by an amphibian: non-shoreward-directed compass orientation, misdirected homing or positioning a magnetite-based map detector in a consistent alignment relative to the magnetic field?
John B. Phillips1,*,
S. Chris Borland2,
Michael J. Freake3,
Jacques Brassart4 and
Joseph L. Kirschvink4
1 Biology Department, Virginia Tech University, Blacksburg, VA 24061,
USA
2 Information in Place, Inc., 501N. Morton St., Suite 206, Bloomington, IN
47404, USA
3 Dept of Natural Sciences, Lee University, 1120 Ocoee St., Cleveland, TN
37311, USA
4 Division of Geological and Planetary Sciences, California
Institute of Technology, MS 170-25, Pasadena, CA 91125, USA
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Fig. 1. Wavelength-dependence of shoreward compass orientation (A,B) and homing
orientation (C,D) by newts housed in outdoor tanks under full-spectrum light
prior to testing (data from Phillips and
Borland, 1994 ). In shoreward orientation tests, (A) newts tested
under 400 nm and 450 nm light oriented in the correct shoreward direction and
were indistinguishable from controls tested under full-spectrum light (not
shown), while (B) newts tested under 550 nm and 600 nm light exhibited
significant magnetic orientation that was rotated approximately 90°
counterclockwise of the shore direction. In homing tests, (C) newts tested
under 400 nm and 450 nm light exhibited homeward orientation, while (D) newts
tested under 550 nm and 600 nm light failed to show a consistent direction of
orientation relative to home (NS, not significant). In the homing tests, newts
tested under 550 nm and 600 nm light were also randomly distributed with
respect to the direction of shore in the outdoor training tanks and with
respect to magnetic north (not shown). Data points are magnetic bearings of
individual newts tested in one of four symmetrical magnetic field alignments
(see Materials and methods) plotted relative to the magnetic bearing of the
artificial shore in the outdoor training tank (A,B) or relative to the
magnetic direction of the newts' home ponds (C,D). In both the shoreward and
homing tests, data are pooled from newts collected from ponds that differed in
home direction by approximately 90°, and held prior to testing in tanks
with three different shore directions; see Phillips and Borland
(1994).
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Fig. 2. Hypothesized response on the hybrid magnetoreception mechanism under
full-spectrum and long-wavelength light
(Phillips and Borland, 1994).
(A) In the proposed hybrid magnetoreception system, the magnetic compass
(double-headed solid arrow) is used to align the map detector (single-headed
open arrow) with respect to the axis of the magnetic field (north at top of
figure) and, thus, to obtain more accurate measurements of one or more
magnetic field components used for the map component of homing. In turn, the
map detector, which is sensitive to the polarity of the magnetic field, is
used to distinguish between the two ends of the magnetic axis when the newt is
carrying out the compass component of homing, replacing the inclination (`dip
angle'), which newts use when exhibiting shoreward magnetic compass
orientation (Phillips, 1986a).
(B,C) Under long-wavelength light, the directional response of the magnetic
compass is rotated by 90° (Phillips
and Borland, 1992a). (B) When newts are carrying out the compass
component of homing, the 90° rotation of the magnetic compass' response
would cause the axis indicated by the magnetic compass to be perpendicular to
the polarity of the magnetic field indicated by the map detector, preventing
newts from using the hybrid system to determine compass direction. [Previous
homing studies have shown that newts held in the outdoor tanks under
full-spectrum light and tested under long-wavelength light are disoriented,
suggesting that they do not fall back on the inclination compass for the
compass component of homing when polarity information is ambiguous
(Fig. 1B; and see
Phillips and Borland, 1994)].
(C) When newts are carrying out the map component of homing, the 90°
rotation of the magnetic compass' response under long-wavelength light would
cause newts to position the map detector perpendicular to the alignment in
which it is normally positioned to take map readings, and, therefore, prevent
them from obtaining map information.
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Fig. 3. Magnetic bearings plotted relative to home direction after housing in
outdoor tanks under long-wavelength light (data from
Table 1). (A) Magnetic bearings
of newts housed under long-wavelength light and tested under full-spectrum
light failed to show a significant direction of orientation relative to home
(13°, r=0.11, P>0.10; Rayleigh test). (B) The same
was true of newts housed and tested under long-wavelength light (351°,
r=0.19, P>0.10). Diamonds represent newts collected in
ponds to the east-southeast (ESE) of the testing site, and circles represent
newts collected from ponds to the south-southwest (SSW) of the testing site.
NS, not significant.
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Fig. 4. Magnetic bearings plotted relative to shore direction after housing in
outdoor tanks under long-wavelength light (data from
Table 1). (A) Magnetic bearings
of newts housed under long-wavelength light and tested under full-spectrum
light failed to show a significant direction of orientation relative to shore
(8°, r=0.11, P>0.10). (B) The same was true of newts
housed and tested under long-wavelength light (76°, r=0.04,
P>0.10). Diamonds represent newts collected in ponds to the
east-southeast (ESE) of the testing site, and circles represent newts
collected from ponds to the south-southwest (SSW) of the testing site. NS, not
significant.
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Fig. 5. Magnetic bearings plotted relative to magnetic north (mN) after housing in
outdoor tanks under long-wavelength light (data from
Table 1). (A) Magnetic bearings
of newts housed under long-wavelength light and tested under full-spectrum
light exhibited significant bimodal orientation (black double-headed arrow)
along a northeast—southwest magnetic axis (38-218°, r=0.40,
P<0.02; Rayleigh test). (B) Newts housed and tested under
long-wavelength light exhibited similar bimodal orientation (26-206°,
r=0.56, P<0.001). The two distributions were not
significantly (NS) different (U2=0.087,
P>0.10; Watson U2-test). Triangles at the
edges of the two distributions show the home directions for the
south-southwest (SSW; circles, filled triangles) and east-southeast (ESE;
diamonds, open triangles) groups. Open circles and diamonds represent the
magnetic bearings of newts from which natural remanent magnetism (NRM)
measurements were later obtained (Brassart
et al., 1999).
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Fig. 6. Distribution of natural remanent magnetism (NRM) declinations (data from
Brassart et al., 1999).
Horizontal alignment of the NRM relative to the front of the newt's head (NRM
declination) for 18 newts used in behavioral studies (see Tables
1,2).
Diamonds represent newts from the east-southeast (ESE) group, and circles
represent newts from the south-southwest (SSW) group. NS, not significant.
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Fig. 7. Distribution of NRM20 bearings. (A) Distribution of NRM20 bearing for newts
tested under full-spectrum light. (B) Distribution of NRM20 bearings for newts
tested under long-wavelength light. Each NRM20 bearing provides an estimate of
the alignment of a newt's natural remanent magnetism (NRM) when it contacted
the 20 cm criterion circle. NRM20 bearings were calculated by adding a newt's
NRM declination (Fig. 6) to its
magnetic bearing at the 20 cm criterion circle
(Fig. 5); see test. NS, not
significant; mN, magnetic north.
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Fig. 8. Possible relationship between orientation and scoring time of newts tested
under long-wavelength light. Histogram shows the number of newts scoring in
different time intervals. Three clusters of scoring times are evident.
Circular diagrams are the distributions of magnetic bearings of newts from
each of the three clusters (separated by dashed lines). mN, magnetic
north.
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© The Company of Biologists Ltd 2002
