Monarch Butterflies Are Losing Their Navigational Abilities. You Can Guess Why

Gizmodo

18 hours ago

Each fall, millions of eastern North American monarch butterflies migrate up to 3,000 miles south to the mountains of central Mexico, then return to their northern range in the spring. These winged insects rely on a complex navigation system to go to and from their overwintering sites, and new research suggests climate change may be messing with it.
Monarch butterflies have two distinct biological 'compasses.' The primary one infers direction based on their circadian rhythm and the position of the Sun, but on overcast days, they switch to a backup compass that infers direction from Earth's magnetic field. Scientists already knew seasonal temperatures play a critical role in calibrating these navigational biomechanisms during the fall migration, but they weren't sure if the same rules apply to the spring migration. A new study published Wednesday, August 13, in the journal PLOS One confirmed that they do, suggesting that rising winter temperatures in Mexico could prevent monarchs from returning to their northern range.
'The question was, if they use the Sun compass or the magnetic compass to go southwards, do they use the same kind of compass mechanisms to fly back north? And we weren't sure what triggered that,' co-author Patrick Guerra, an independent neuroecology researcher based in North Carolina, told Gizmodo. He participated in this research while working as an assistant professor at the University of Cincinnati.
When temperatures drop in the fall, this shift calibrates both of the butterflies' biological compasses to point south, guiding them toward their overwintering sites in Mexico. 'Once they get there, they're basically overwintering in a fridge,' Guerra explained. His previous research showed that this long period of exposure to cool temperatures recalibrates the Sun compass to point north so that it can guide monarchs back to their summer range in the spring. In this new study, he and his colleagues investigated whether this is also true for the magnetic compass.
To that end, the researchers conducted a series of righting response trials with migratory and non-migratory monarchs. During these trials, the butterflies were exposed to artificial magnetic fields that mimic geographic conditions south of their overwintering sites, then positioned so they were upside down. Upon righting themselves, non-migrating monarchs faced magnetic north, whereas migrating monarchs faced magnetic south. This indicated that monarchs exhibit something akin to migratory restlessness—a compulsion to go toward the direction their migratory instincts are telling them to follow even if they can't actually fly there.
With this established, Guerra and his colleagues chilled the monarchs to trick their bodies into thinking they had gone through an overwintering period. When they performed the righting response trials again, the migrating monarchs pointed toward magnetic north. The non-migrating monarchs, however, still pointed toward magnetic south.
'That told us that, just like the Sun compass, the magnetic compass is recalibrated by the cold conditions that are consistent with being in Mexico while they're overwintering,' Guerra explained. 'So that kind of hammers home that their whole ability to navigate is really tuned to the environmental conditions that they're experiencing.'
Greenhouse gas emissions are increasing average temperatures around the world, including in the overwintering range of monarch butterflies. Guerra and his colleagues already knew that this threatened their primary navigation mechanism, but their new study indicates that their backup mechanism is at risk too. If temperatures get too warm, monarchs may lose the ability to return to their northern range in the spring. Determining the minimum temperature monarchs need to recalibrate their biological compasses for spring migration will require further research, but these findings underscore the threat that climate change poses to this species.