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Last updated: 25 June 2008

Use of Magnetic Map Information by a Short-Distance Migrant

Newts deprived of directional visual, olfactory, magnetic and inertial cues during displacement from their home ponds ('route deprivation') have been shown to exhibit accurate homeward orientation when given access to both magnetic and non-magnetic cues at the testing site, indicating that they can use map information to determine the home direction (Phillips et al. 1995). More recent experiments investigated the possibility that magnetic inclination is used to derive one coordinate of a unicoordinate or bicoordinate map (Fischer et al. 2001, Phillips et al. 2002). Newts were displaced from their home ponds 42 km north-northeast to a testing site (home direction = 207 degrees) where the inclination was slightly steeper than the inclination at their home ponds. When exposed to a +2 degree change in inclination resulting in a value that should occur in the same general direction as our testing site, but at a greater distance from the home pond, the newts were homeward oriented (Fig 1A) and indistinguishable from controls (Fig 1B). In contrast, when newts were exposed to a -2 degree change in inclination resulting in a value that should occur on the other side of the home ponds, they oriented in the opposite direction (Fig 1E). Identical changes in inclination had no effect on shoreward magnetic compass orientation, which does not involve the use of map information (see Fig 4 in Fischer et al. 2001), suggesting that the change in orientation in Fig 1E was due to an effect on the map rather than the compass component of homing.

In subsequent experiments (Phillips et al. 2002), newts were exposed to smaller changes in magnetic inclination to behaviorally "titrate" the home value (see Phillips 1996). As predicted, the transition from homeward to anti-homeward orientation coincided with the value of inclination measured in the vicinity of the newts' home ponds (Fig 1B-D). Moreover, newts exposed to a -0.15 + 0.03 degree decrease in inclination, which coincided with the home value of -0.17 + 0.04 degree , failed to exhibit a consistent direction of orientation (Fig 1C). Disorientation is consistent with the newts having a unicoordinate map that would require other information (e.g., familiar landmarks) to specify their east-west position relative to home, or a bicoordinate map with the value of the second coordinate also coinciding with the home value.

Figure 1. Effects of changes in magnetic inclination on the homing orientation of newts (Data from Fischer et al., 2001; Phillips et al. 2002). A) Newts exposed to a 2 + 0.2 degree increase in inclination oriented in the home direction. B) Newts exposed to the ambient magnetic field at the testing site (controls) also oriented in the home direction. C) Newts exposed to a decrease in inclination of -0.15 + 0.03 degree , which approximated the home value, failed to exhibit a consistent direction of orientation. D) Newts exposed to a decrease in inclination of -0.48 + 0.04 degree were significantly oriented opposite the home direction. E) Newts exposed to a -2.0 + 0.2 degree decrease also oriented ~opposite the home direction. Methods: Data points are magnetic bearings of newts tested in one of four alignments of an Earth-strength magnetic field (magnetic north at North, East, South or West). Prior to testing, newts were held in outdoor tanks with an artificial shore to the North (diamond-shaped symbols) or West (round symbols). Arrows inside the circles indicate mean vectors that were significant by the Rayleigh test (p < 0.05). Mean vector length, r, is proportional to the strength of orientation (radius of circle corresponds to r = 1). Dashed lines show the 95% confidence interval for the mean vector bearings. The black triangle outside each circle indicates the home direction. Values on the left are the inclination changes relative to the ambient value at the testing site (B).

In subsequent experiments (Phillips et al. 2002), newts were exposed to smaller changes in magnetic inclination to behaviorally "titrate" the home value (see Phillips 1996). As predicted, the transition from homeward to anti-homeward orientation coincided with the value of inclination measured in the vicinity of the newts' home ponds (Fig 1B-D). Moreover, newts exposed to a -0.15 + 0.03 degree decrease in inclination, which coincided with the home value of -0.17 + 0.04 degree , failed to exhibit a consistent direction of orientation (Fig 1C). Disorientation is consistent with the newts having a unicoordinate map that would require other information (e.g., familiar landmarks) to specify their east-west position relative to home, or a bicoordinate map with the value of the second coordinate also coinciding with the home value.

Use of Magnetic Map Information by a Long-Distance Migrant

Behavioral experiments with experienced adult migratory birds have also provided evidence for the geomagnetic field's involvement in deriving map information (Fischer et al. 2002). Adult Tasmanian silvereyes were tested in Armidale, NSW near the mid point of their fall migration up the southeastern coast of Australia. In the ambient field of Armidale, the birds exhibited seasonally-appropriate north-northeasterly (NNE) orientation. The birds were then divided into two groups and exposed to magnetic intensity and inclination values that would normally be encountered near the beginning (simulated south displacement, "SimS") or end (simulated north displacement, "SimN") of their fall migration. Consistent with the predictions of the magnetic map hypothesis, adult migrants exposed to the SimS condition continued to exhibit NNE orientation, while those exposed to the SimN condition failed to exhibit a consistent direction of orientation (Fig 2).

Figure 2. Effects of changes in magnetic intensity and inclination on the migratory orientation of adult Tasmanian silvereyes. A) SimS birds tested in a magnetic field with values of approximately 60,200 nT (total intensity) and -65 degree (inclination) oriented to the NNE (38 degrees, r = 0.75, p < 0.01, one-sample Hotelling's test); and did not differ significantly from the responses of these birds in the control condition (p > 0.10, two-sample Hotelling's test; control data, data included in B). B) Controls tested in the ambient magnetic field with values of approximately 55,000 nT (total intensity) and -62 degree (inclination) exhibited seasonally appropriate orientation to the NNE (22 degrees, r = 0.52, p < 0.001, one-sample Hotelling's test). C) Responses of SimN birds tested in a magnetic field with values of approximately 49,950 nT (total intensity) and -58.5 degree (inclination) were randomly distributed (p > 0.10; one-sample Hotelling's test) and differed significantly from their responses in the control condition (p < 0.01, two-sample Hotelling's test; data included in B). Responses of silvereyes in SimN and SimS (A vs C) also differed significantly (p < 0.01, Hotelling's two-sample test). Data points outside circles indicate nightly mean vector bearings of individual birds relative to magnetic north. Vectors originating at centers of circles are mean vector bearings for each of the birds, calculated by vector addition from each bird's nightly mean vector bearings (Batschelet 1981). Radii of circles correspond to a mean vector length of 1. Ellipses indicate 95% confidence intervals for second order mean vectors.

To determine whether the change in orientation in the SimN condition was due to an effect on the map or compass, more recent experiments were carried out to compare the effects of somewhat larger magnetic field changes on the fall migratory orientation of young, inexperienced birds captured in Tasmania shortly after fledging ("Juveniles") with the effects on experienced adult birds that had completed at least one round trip migration ("Adults"). Birds on their first migration are believed to utilize only compass information combined with a temporal program that specifies distance, while experienced adults exhibit true navigation that utilizes both map and compass information. Consistent with the magnetic map hypothesis, the SimS treatment did not affect either group of birds, while the SimN treatment affected only the older birds. While consistent with a "map effect", the failure of experienced adult birds in the SimN condition to reorient back to the south remains to be explained (Deutschlander et al. 2003).

Evidence for a Specialized "Map Detector"

Behavioral studies of newts (Phillips 1986b, Phillips & Borland 1994, Phillips et al. submitted), and silvereyes (Munro et al. 1997a, Munro et al. 1997b) have provided evidence for the involvement of a second magnetoreception mechanism in deriving map information. In contrast to the light-dependent magnetic compass, the putative "map detector" appears to be non-light-dependent, and to involve permanent magnetic material, possibly magnetite (e.g., Brassart et al. 1999, Phillips et al. 2002).

This material is based upon work supported by the National Science Foundation under Grant Nos. IBN95-07826 and IBN98-08420

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