(fifty µT and a plan)
On clocks and cryptochrome, weak signals and steady habits, milkweed and the long road south.
Each August one of the greatest migrations on Earth begins. Monarch butterflies rise from fields and roadsides and, as a population, make their way to the oyamel fir forests of central Mexico. The destination is easy to mark on a map; the guidance is the interesting part.
Monarchs use a time‑compensated sun compass. Antennae contain circadian clocks that keep local time, which lets the animal correct for the sun’s motion and hold a steady heading through the day. Occluding or removing the antennae disrupts orientation. Painting the antennae to shift the light cycle shifts the course. When clouds erase the sun or night arrives, the heading persists. A second instrument contributes.
That instrument reads Earth’s magnetic field, roughly fifty microtesla (µT) at the surface. 50 µT. Vanishingly small, yet enough to steer a continent of wings. The signal is faint inside a cell full of thermal motion and charge. Monarchs extract it with cryptochrome, especially the CRY1 form in the antennae and eyes. Blue light excites a flavin cofactor (FAD) and passes energy along nearby tryptophans to form a radical pair. Two unpaired electrons then interconvert between spin states. The interconversion rate depends on the local magnetic field. Change the body’s orientation in the field and the downstream chemistry shifts; protein conformation nudges, binding partners rearrange, ionic currents adjust. Many molecules in many cells are oriented slightly differently, so the tissue produces a spatial pattern of responses that the nervous system can read as a bearing.
The compass is keyed to inclination, the angle at which field lines dip into the ground. Polarity reverses at the magnetic equator. Inclination keeps its sign. That small choice yields a global guide. The central complex of the insect brain integrates inputs from the sun compass, the magnetic sensor, and the clock into a stable direction of travel.
Quantum coherence at body temperature should blur in microseconds. Biology counters with ensembles and timing. Radical pairs do not need to last long if many are formed and sampled together. Cells average across moments; circuits average across cells. A population‑level migration uses the same idea at a larger scale. Spring and summer broods move north and reproduce. Late‑season monarchs enter a migratory state and live long enough to carry the route to Mexico. No individual remembers the round trip. The population preserves it by passing the path forward.
Milkweed closes the loop between breeding and migration. Its latex carries cardenolides, cardiotonic steroids that inhibit the sodium-potassium pump and can stop a vertebrate heart; most insects are similarly vulnerable. Monarch larvae (Danaus plexippus) are exceptions: mutations in their pump reduce sensitivity, and the larvae sequester cardenolides into tissues they will keep through metamorphosis. The result is chemistry that persists into the adult, advertised by orange-and-black warning colors and remembered by predators after a single bad taste. In late summer the same plants and neighboring asters supply nectar that loads fuel for the southbound flight. What looks like a ditch weed is, in practice, a chain of supply depots laid across a continent.
I am planting milkweed to keep that system intact. Stems in the ground now. Seeds in fall. Dead stalks left standing through winter. New growth in spring. Small actions at many sites become stepping‑stones and corridors that line up with timing and weather, which is what the animals require.
Reading small signals
The science points toward a simple practice. Weak signals can direct a journey if the system is built to preserve and average them. Monarchs combine a clock and a field sensor and give each one the conditions it needs: blue light for cryptochrome, stable cycles for timekeeping, room in the nervous system to integrate the results. Guidance is rarely a single clue. It is a set that aligns often enough to act on.
There is a human analogue here. We live among faint cues. Most arrive quietly. We can design our own ensemble: habits that stabilize attention, routines that keep timing honest, conditions that make the right signal visible. The work is to notice small consistencies, protect them long enough to accumulate, and move when they agree.
Design one habit that keeps your clock honest, one cue that makes the field visible. Then average.
Soon an adult will test its wing above a fence line and choose a line that many others are choosing at the same time. My part is clear and local. Plant the milkweed. Keep it going. Pay closer attention to guidance when it is quiet. When the instruments line up: clock, field, conditions, take the next step and let the larger tide do what it does.