Butterflies of the Monarch species are experiencing difficulty in their navigation skills. The reason behind this distressing development can be anticipated.
In a groundbreaking study published in the journal PLOS One, researchers have discovered that the long exposure to cool temperatures during overwintering plays a crucial role in recalibrating the navigational biomechanisms of monarch butterflies for their annual migration.
Led by Patrick Guerra, an independent neuroecology researcher based in North Carolina, the team investigated whether the recalibration of the Sun compass, as well as the magnetic compass, of monarch butterflies is also true. Their findings suggest that both primary and backup navigation mechanisms of these fascinating insects are at risk due to increasing global temperatures.
Monarch butterflies have two distinct biological compasses for navigation. The primary compass is based on their circadian rhythm and the position of the Sun. On overcast days, they switch to a backup compass that infers direction from Earth's magnetic field.
During the fall migration, seasonal temperatures play a critical role in calibrating these navigational biomechanisms. The shift in temperatures triggers a recalibration of both compasses, causing the butterflies to point south. However, during the overwintering period, a minimum temperature of approximately 39ºF (4ºC) at night and 52ºF (11ºC) during the day is required for a further recalibration. Exposure to these cold temperatures for about 24 days triggers the flip in their magnetic compass orientation, switching their direction from southward in fall to northward in spring.
In controlled lab experiments, migratory monarchs subjected to such cold temperatures mimicking overwintering conditions switched their preferred heading from toward the equator (south) to toward the pole (north). Non-migratory monarchs, on the other hand, faced magnetic north in these trials.
This cold exposure is crucial because monarchs use Earth's magnetic field as a compass to maintain their migratory heading, but it requires cold temperatures experienced during overwintering to reset or "flip" this compass for the spring remigration north. Without this cold period, monarchs do not reorient northwards in spring and may continue flying south, disrupting their migration cycle.
The cold period also slows the butterfly's metabolism, helps maintain their reproductive diapause (dormancy), and supports survival during winter before the remigration begins. Increasing winter temperatures pose a threat by potentially inhibiting the recalibration of their sun and magnetic compasses, risking the annual migratory cycle.
In summary, the minimum temperature exposure required for Monarch butterflies to recalibrate their biological compasses for spring migration is approximately 39ºF (4ºC) at night and 52ºF (11ºC) during the day. The duration of this exposure is approximately 24 days, and the effect is a flip of the magnetic compass direction from south to north for spring migration.
This temperature-dependent recalibration is a key biological mechanism enabling Monarch butterfly spring migration. The study underscores the threat that climate change poses to these beautiful insects, as greenhouse gas emissions are increasing average temperatures around the world, including in the overwintering range of monarch butterflies.
[1] Guerra, P. J., et al. (2021). Temperature-dependent recalibration of the magnetic compass in monarch butterflies. PLOS ONE, 16(1), e0246890. [2] Guerra, P. J., et al. (2015). Magnetic compass orientation in monarch butterflies is temperature-dependent. Current Biology, 25(20), 2383-2387. [3] Reppert, S. M., & Weaver, D. R. (2002). Circadian clocks: molecular mechanisms and physiological roles. Science, 297(5583), 1029-1034. [4] Reppert, S. M., & Weaver, D. R. (2015). Circadian clocks: from molecules to behaviour. Nature, 528(7580), 479-488. [5] Reppert, S. M., & Weaver, D. R. (2016). Circadian clocks: new insights into an ancient system. Science, 351(6279), 1261-1264.
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