Lack of evidence for a fine-scale magnetic map sense for fall migratory Eastern North American monarch butterflies (Danaus plexippus)

Abstract

How first-time animal migrants find specific destinations remains an intriguing ecological question. Migratory marine species use geomagnetic map cues acquired as juveniles to aide long-distance migration, but less is known for long-distance migrants in other taxa. We test the hypothesis that naïve Eastern North American fall migratory monarch butterflies (Danaus plexippus), a species that possesses a magnetic sense, locate their overwintering sites in Central Mexico using inherited geomagnetic map cues. We examined whether overwintering locations and the abundance of monarchs changed with the natural shift of Earth's magnetic field from 2004 to 2018. We found that migratory monarchs continued to overwinter at established sites in similar abundance despite significant shifts in the geomagnetic field, which is inconsistent with monarchs using fine-scale geomagnetic map cues to find overwintering sites. It is more likely that monarchs use geomagnetic cues to assess migratory direction rather than location and use other cues to locate overwintering sites.

Keywords: flight orientation; geomagnetic signposts; magnetic declination angle; magnetic inclination angle; migration; navigation; overwintering.

FIGURE 1

Change in geomagnetic parameters over time for (a) declination angle, (b) total intensity, and (c) inclination angle from 1974 until 2018 during November at a single overwintering site (19.850°N, 100.789°W). From 2004 to 2018, all geomagnetic parameters have a negative relationship as a function of time, indicating that monarch abundance should be decreasing at the more southern and/or eastern sites. The expectation is that if the butterflies are using the geomagnetic field associated with the geographical location of overwintering sites as either magnetic map sense guideposts or as “homing beacon” cues, we should see the strongest decline in abundance for the three most southerly sites. We note that inclination angle is cyclic, but during the monitoring period from 2004 to 2018 it was consistently declining.

FIGURE 2

The location of monarch butterfly overwintering sites in Central Mexico with isoclinic lines in 2004 (left panel) and 2018 (middle panel) showing the shift in magnetic field. The red dots indicate the location of overwintering sites. For both the 2004 and 2018 maps, the red bounding box shows the overwintering site relative to total intensity (top row), inclination angle (middle row), and declination angle (bottom row) in 2004; the blue bounding box shows the same for 2018. For total intensity and declination angle, all overwintering sites fall outside of the displacement area due to the shift of the Earth's magnetic field. When considering inclination angle, the three most southern sites fall outside of the displacement area based on the shift in the Earth's magnetic field over this 14‐year period, yet monarchs still overwinter with similar abundances at these locations. The right panel represents overwintering sites on opposite ends of the natural displacement, either north–south (i.e., total intensity and inclination angle) or east–west (i.e., declination). For all three geomagnetic cues across all sites, there were no significant relationships between area occupied and total intensity, inclination angle, or declination. The black and orange dots correspond to the colony area (ha) as a function of the geomagnetic parameter at each overwintering site in the corresponding color and insert box of all overwintering sites.

FIGURE 3

Change in total intensity (nT, top row) and declination angle (°, bottom row) over a one‐year interval from 2004 (left side, red box) to 2005 (right side, blue box) for the two geomagnetic parameters that had the greatest change over the 14‐year monitoring period. The red dots indicate the location of overwintering sites. Given the northward and westward shift, the southernmost sites (black arrow) would not be within the detectable region based on the geomagnetic parameters.

FIGURE 4

The relationship between overwintering colony size of D. plexippus (ha) and the declination angle of the geomagnetic field for 12 overwintering sites with data from 2004 to 2018. Sites are ordered from south to north, with the most southern site first. There was no relationship between colony size and magnetic declination for 11 of these sites. The significant relationship for Sierra El Campanario (p = .035, r ² = .24; trend line in red with 95% confidence intervals) should be viewed with caution, as it violates the homoscedasticity assumption for linear regression and represents extreme observations where since 2007 monarchs were not found at this site. Note that the scales differ among plots for colony area (ha) and declination.

FIGURE 5

The relationship between overwintering colony size of D. plexippus (ha) and the total intensity (nT) of the geomagnetic field for 12 overwintering sites with data from 2004 to 2018. Sites are ordered from south to north, with the most southern site first. There was no relationship between colony size and total intensity for 11 of these sites. The significant relationship for Sierra El Campanario (p = .031, r ² = .26; trend line in red with 95% confidence intervals) should be viewed with caution, as it violates the homoscedasticity assumption for linear regression and represents extreme observations where since 2007 monarchs were not found at this site. Note that the scales differ among plots for colony area (ha).

FIGURE 6

The relationship between overwintering colony size of D. plexippus (ha) and the inclination angle of the geomagnetic field for 12 overwintering sites with data from 2004 to 2018. Sites are ordered from south to north, with the most southern site first. There was no relationship between colony size and magnetic inclination for 11 of these sites. The significant relationship for Sierra El Campanario (p = .022, r ² = .29; trend line in red with 95% confidence intervals) should be viewed with caution, as it violates the homoscedasticity assumption for linear regression and represents extreme observations where since 2007 monarchs were not found at this site. The three most southerly sites had no significant relationships with colony area as a function of inclination angle. Note that the scales differ among plots for colony area (ha) and inclination.

FIGURE 7

No relationship between the slope of colony size and geomagnetic (a) total intensity, (b) inclination angle, and (c) declination angle versus the latitude of the overwintering sites (black dot) was found. There was also no relationship between the slope of colony size and geomagnetic (d) declination versus the longitude of the overwintering sites. If monarchs were responding to the changing geomagnetic field, we would expect more southerly sites to have greater slopes (decreasing abundance) relative to more northerly sites. Thus, there should be a positive slope in the relationship shown here; however, the slope was not significantly different than zero for total intensity (β = 0.00055 ± 0.00067, t = 0.83, p = .42), inclination angle (β = 2.00 ± 2.16, t = 0.93, p = .38), or declination angle (β = 0.26 ± 0.23, t = 1.14, p = .27). In the case of declination, we would expect more westerly sites to have greater slopes. Thus, we would expect a negative slope in the relationship; however, the slope was not significantly different than zero for declination based on longitude (β = −0.18, t = −0.76, p = .46).