Every year, gray whales undertake a mammoth 10,000-mile (16,100-kilometer) round-trip from their winter spot in Baja California, Mexico, to their summer feeding grounds in the Arctic. Whilst carrying out one of the longest mammalian migrations, the whales swim close to the shoreline of western North America, where any small deviation in their route could have disastrous consequences. Sadly, some of the marine mammals on this journey end up stranded.
In a new study, published in Current Biology, gray whale strandings were found to be more likely on days where there are more sunspots, an indicator of solar activity. Subsequent effects on the Earth’s magnetosphere from this solar activity suggest gray whales may depend on a magnetic sense to navigate, like several other animals do.
Lead author Jesse Granger of Duke University, along with colleagues, first compiled a database of gray whale strandings over a period of 31 years. After eliminating all the whales that were sick, malnourished, injured, or entangled, the remaining 186 strandings were then compared to records of sunspots. On days with a high sunspot count, whales were more than twice as likely to be stranded.
Sunspots, the magnetically active regions on the Sun’s surface, are strongly correlated with solar storms. Two ways in which solar storms can alter our planet’s magnetosphere were studied by the researchers, in order to better understand the correlation between increased sunspots and increased gray whale strandings.
Firstly, information on the AP-index, a measure of the displacements in Earth’s magnetic field was gathered. Secondly, data of solar radio flux (a globally averaged measure of radio frequency (RF) noise) that increases with solar storms were also analyzed. RF noise has previously been shown to affect magnetic orientation in other species, such as European robins. The broadband radio-frequency noise can temporarily prevent them from using their geomagnetic sense, inhibiting their ability to know where they are and where they're going. Measures of both AP-index and RF noise would help to explain the correlation.
"Is it that the solar storms are pushing the magnetic field around and giving the whales incorrect information,” Jesse Granger, a graduate student from Duke University said in a statement. “Or is it that the solar storms are messing up the receptor itself… [and the whale] has just gone blind?”
After sifting out several other possible factors like seasons, weather, ocean temperatures, and food abundance, the researchers determined that the increase in strandings under high solar activity is best explained by an effect on the sensor (i.e. the whales’ navigation sense), not on the magnetic field itself (i.e. the “map” the whale was using).
Compared to randomly selected days, when there was a high solar radio flux index, the likelihood of strandings was 4.3 times greater; there was no significant increase in strandings on days with large deviations in the magnetic field.
"I really thought that the cause of the strandings was going to be inaccurate information," Granger said. "When those results came up negative, I was flummoxed. It wasn't until one of my co-authors mentioned that solar storms also produce high amounts of radio-frequency noise, and I remembered that radio-frequency noise can disrupt magnetic orientation, that things finally started to click together."
Granger stresses that this is only one possible cause of whale stranding, citing mid-frequency naval sonar as another. However, the correlation found in this study may help predict live whale strandings. Further study on other species of whales will help to establish whether this pattern is observed on a global scale, and enhance our understanding of whales’ magnetic sense.