Monarch butterflies have a built-in compass that flips when it's cold, disrupting migration

North American monarch butterflies do not use Earth’s magnetic field as a map to find Mexico. They use it as a compass to hold a heading, and cold temperatures later flip that compass for the return trip north.

In lab tests that mimicked different locations relative to their overwintering sites, fall migrants kept pointing toward the equator under every magnetic setting.

After a 24 day chill, roughly 52ºF days and 39ºF nights, the same butterflies switched to a poleward heading, while controls kept aiming south.

A group of scientists in the Department of Biological Sciences at the University of Cincinnati (UC) collaborated on the research

Monarch compass navigation

The team used a righting response setup inside a Helmholtz coil system, which generates controlled Earth-strength magnetic fields.

The butterflies oriented on a vertical mesh after righting themselves, giving clear headings without relying on visual landmarks.

The team tested three field conditions: locations about three days’ flight south of the overwintering sites, the overwintering sites themselves, and locations three days’ flight north. They then recorded the butterflies’ preferred direction.

The magnetic parameter that acts like latitude is inclination – the angle at which magnetic field lines enter the ground. It becomes steeper toward the poles.

Cold triggers migration change

“Our discovery that coldness triggers the northward flight direction in spring remigrants solves one of the long-standing mysteries of the monarch migration,” wrote Patrick A. Guerra, who led the study.

A decade ago, researchers showed that winter cold flips the monarch’s time-compensated sun compass, turning a fall southbound program into a spring northbound one. 

The new study extends that logic to the magnetic system. Fall migrants chilled for 24 days under overwintering-like temperatures reversed their magnetic headings to poleward, while monarchs held at fall temperatures did not.

This points to temperature as a shared recalibration signal across compass systems. It also raises a practical question: how warm can the overwintering microclimate get before this switch fails to flip on schedule.

Implications for monarch conservation

Rising winter temperatures in central Mexico could interfere with the recalibration process by preventing monarchs from experiencing the consistent cold needed to reset their orientation systems.

Without this signal, they might depart on their spring migration still heading equatorward, wasting critical time and energy reserves.

Conservation strategies may need to account not only for preserving habitat but also for maintaining the microclimate stability of overwintering sites.

Protecting forest canopy cover, limiting deforestation, and monitoring local weather trends could help ensure that monarchs continue to receive the environmental cues they depend on for successful migration cycles.

A compass, not a map

Several animals use geomagnetic information in two ways, as a compass for direction and as a map for position.

Tests of fall monarchs have found no evidence that they possess a fine-scale magnetic map that could guide them to a specific Mexican mountain forest.

By contrast, some species do form magnetic maps that help them correct course or home to precise destinations.

Evidence spans hatchling and adult sea turtles and at least some birds, where animals learn or inherit positional signatures in the field.

Monarchs likely lean on other cues to decide when to stop, including the cool, moist microclimate of oyamel fir forests at roughly 8,000 to 12,000 feet and consistent humidity in those groves.

Those conditions bracket temperatures close to freezing up to about 59ºF, which help insects conserve energy until spring.

Monarch compass works without sun

“We now show that migratory monarch butterflies have an inclination magnetic compass that allows them to orient in the proper southern direction during the fall migration,” reported Guerra and colleagues.

When the sun’s position is clear, monarchs use their sun compass. On overcast days, they can still stay on course with a light-dependent magnetic compass. 

Recent genetics and neurobiology work has shown that the cryptochrome 1, a UV-A/blue-light sensitive protein, is essential to this magnetosensory response. Both antennae and eyes appear to serve as magnetosensory organs in this context.

Directional and timing cues are integrated in the brain’s central complex, a midline hub that combines skylight and clock information for orientation. Reviews of monarch neurobiology place this circuit at the center of migratory control.

Travel speed and timing

Monarchs do not sprint to Mexico. Typical progress averages about 25 to 30 miles per day in reasonable weather, and it can stall or surge with changing winds.

Telemetry studies capture faster days under strong assistance, with a radio-tracked monarch covering about 89 miles in a single day.

Temperature and wind together pushed that pace, demonstrating how sensitive the journey is to weather.

That sensitivity extends to winter. Federal assessments warn that changes to the oyamel fir belt, through logging and climate warming, threaten the microclimate that monarchs rely on to survive and to trigger their spring reorientation.

The study is published in the journal PLOS ONE.

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