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Monarch butterfly

Danaus plexippus

The monarchs that reach Mexico are the great-grandchildren of the ones that left it. Nothing in that chain can be remembering the route.

Monarchs fly from Canada and the northern United States to a small area of Mexican forest each autumn. No individual makes the round trip, so none of them can be remembering the way: the butterflies that arrive are the great-grandchildren of the ones that left.

The monarch migration is the clearest case in biology of a journey completed by a relay of animals none of whom knows the plan. One long-lived autumn generation flies south — up to several thousand kilometres — and overwinters clustered in a few high-altitude fir forests. In spring the same individuals fly part of the way north and lay eggs, and then two to four short-lived summer generations, each living a matter of weeks, complete the recolonisation of the northern range. The butterfly that arrives in Mexico next autumn has never been there, and neither had its parents or grandparents. That rules out memory as an explanation and forces the question of what is actually being carried forward. The answer is a set of mechanisms rather than a route. The autumn generation orients using a sun compass, time-compensated by a circadian clock that — unexpectedly — sits in the antennae rather than in the brain: black paint on the antennae abolishes the correction, clear paint does not. And the switch that turns the survivors around in spring is not a calendar but a condition: butterflies given a laboratory winter cold enough to match the overwintering sites reversed their heading, while those kept warm carried on flying south.

Developed coverage · 59% complete · reviewed 2026-09-03

What this page covers

The eastern North American population, which is the one that makes the long migration. Western North American, Caribbean and other populations behave differently, and some do not migrate at all.

Often confused with: The viceroy butterfly, which resembles it and does none of this; Other populations of the same species that are resident rather than migratory

Quick facts

Generations per cycle
One long-lived migratory generation, then two to four short summer ones
Compass
The sun, corrected for time of day
Where the clock is
The antennae — painting them black stops the correction
What turns them round
Having been cold, not a date

A journey no individual completes

The butterflies that arrive in Mexico are the great-grandchildren of the ones that left it. No monarch makes the round trip, so none of them can be remembering the way.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

The eastern North American monarch migration is completed by successive generations: a long-lived autumn generation flies to the overwintering sites and part of the way back, and two to four short-lived summer generations complete the northward recolonisation, so no individual experiences the full annual cycle.

Who this applies to
The eastern North American population. Other monarch populations, including non-migratory ones, differ.
Studied in
Danaus plexippus
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

Generational turnover is directly observable from lifespan and breeding records, and the migratory generation is physiologically distinct from the summer ones.

How far it can be extended

Established for this population by tagging, isotope analysis and rearing; western and tropical populations behave differently.

Caveats

  • The number of summer generations varies with latitude and season.
  • Some autumn individuals do survive to fly a substantial part of the return, so "no individual completes it" is about the full cycle rather than about a single leg.

Still unanswered

  • How the overwintering sites remain stable across generations that have never seen them — the inherited component is assumed rather than characterised.

Last reviewed 2026-09-03

The evidence (2 studies)

Diagram

Four generations, one circuit

Schematic. Generation counts vary with latitude and season; geography is indicative.

One circuit, four generations, no individual completing itSchematic. Generation counts vary with latitude and season.northoverwintering regionautumn generationflies the whole way southand lives for monthssame butterflies, in springpart of the way north, then breedtwo to four summergenerations, weeks eachThe butterfly that starts south has never been there. Nor had its parents.
The same explanation in words

A vertical circuit with the northern breeding range at the top and the Mexican overwintering region at the bottom. An arrow running the full distance southward is labelled as the autumn migratory generation, marked as long-lived — months rather than weeks. From the overwintering region, a shorter arrow runs part of the way north, labelled as the same individuals in spring, which then lay eggs and die. Above that, two to four short arrows in sequence, each labelled as a summer generation living a few weeks, together completing the return to the northern range. A note states that the butterfly beginning the southward flight has never made it before and is several generations removed from any butterfly that has.

How we know

Giving a butterfly a Mexican winter, indoors

If no monarch survives to make the whole round trip, what tells the spring generation to fly the other way?

Monarchs caught on the autumn migration were flown in a flight simulator to confirm they were heading south. They were then held for several weeks at temperatures matching those of the Mexican overwintering sites, while a comparison group was held warm. Both groups were then flown in the simulator again.

What happened

Butterflies that had been cold reversed their heading and flew north. Those kept warm carried on flying south.

What it shows

That the direction is re-set by a condition the animal encounters rather than being remembered or counted off a calendar. That is exactly what a relay migration needs: no individual has to know the plan, because the place itself flips the switch.

What it does not show

It does not show that cold is the only trigger, or that this is what happens in Mexico — a chilled cabinet is not an overwintering site, which also involves particular light, humidity and a million clustered butterflies. And a heading in a simulator is not a journey.

The controls — what makes this evidence rather than a story
  • The same individuals tested before and after, so the reversal is within-animal.
  • A warm-held group experiencing the same captivity, handling and time.
  • Simulator conditions kept identical between the two tests.

From Coldness Triggers Northward Flight in Remigrant Monarch Butterflies

What turns a monarch round in spring is having been cold. Butterflies given a Mexican winter in the laboratory flew north afterwards; those kept warm carried on flying south.

Well supported

Good evidence backs this, though some details remain open.

Autumn migrant monarchs exposed to temperatures matching overwintering-site conditions for several weeks reversed their flight-simulator orientation from southward to northward, while individuals held at warmer temperatures maintained southward orientation.

Who this applies to
Eastern North American migrants, tested in a flight simulator.
Studied in
Danaus plexippus
Why we rate it this way, and what the caveats are
Well supportedHigh confidence

A clean experimental contrast with a large behavioural effect, and a mechanism that explains how a relay migration stays coordinated without memory.

How far it can be extended

Tested in this population only.

Caveats

  • Flight-simulator headings rather than tracked journeys.
  • Laboratory cold exposure is not a Mexican winter, which also involves particular light, humidity and clustering.

Still unanswered

  • What a warming overwintering range does to a switch that requires being cold enough for long enough.

Last reviewed 2026-09-03

The evidence (1 study)

The journey

Monarchs of the eastern North American population

From Breeding range across the northern United States and southern Canada to A small area of high-altitude fir forest in central Mexico

When
Southward from late summer; the return north begins in spring and is triggered by having been cold rather than by a date
How far
Up to roughly 4,000 km on the southward leg, from tagging recoveries and isotope work rather than from tracking individuals

What it steers by, in the order they take over

  1. Sun compass, corrected by an antennal clock
  2. Inherited direction
  3. Cold exposure, which reverses it in spring

How anybody knows

Tagged butterflies recovered at the overwintering sites, isotope signatures identifying where individuals grew up, and flight simulators that record the direction a tethered butterfly tries to fly under manipulated conditions.

The biggest thing still unknown

How they find the specific groves. A sun compass gets a butterfly to a region; nothing established explains the final approach to a handful of particular forests, and nothing explains what specifies the inherited bearing in the first place.

Words used here
Remigrant
A monarch that overwintered and is flying north again in spring — the same individual that flew south, not a descendant.

A compass in the eyes and a clock in the antennae

How we know

Painting a butterfly’s antennae black

Where is the clock that lets a monarch correct for the moving sun?

Migratory monarchs were flown tethered in a flight simulator that records the direction a butterfly tries to go. Some had their antennae removed. Others kept their antennae but had them painted with black enamel, blocking light while leaving the structures attached and intact — and a further group was painted with clear enamel, which does everything the black paint does except block light.

What happened

Butterflies without antennae still flew, and flew without a consistent direction. Black-painted antennae produced the same loss of orientation; clear-painted antennae did not.

What it shows

That the circadian clock time-compensating the monarch’s sun compass sits in the antennae and is entrained by light reaching them. The paint pair is the whole argument: amputation removes touch and smell along with the clock, and the black-versus-clear comparison removes everything except the light.

What it does not show

It does not show how the antennal signal reaches the compass in the brain, and it does not show anything about the route: a tethered butterfly in a simulator is expressing an intended heading, in a room, and has not travelled anywhere.

The controls — what makes this evidence rather than a story
  • The clear-enamel group, which separates the effect of light from the effect of being painted.
  • Intact butterflies flown in the same simulator.
  • Flight performance itself checked, so that a butterfly failing to orient is not simply a butterfly failing to fly.

From Antennal Circadian Clocks Coordinate Sun Compass Orientation in Migratory Monarch Butterflies

The clock a monarch uses to correct for the moving sun is not in its brain. It is in its antennae, and painting them black stops the compass working.

Well supported

Good evidence backs this, though some details remain open.

Antennectomy abolishes time-compensated sun-compass orientation in migratory monarchs while leaving flight intact; painting the antennae with opaque enamel produces the same deficit and clear enamel does not, indicating light-entrained circadian clocks in the antennae drive the time compensation.

Who this applies to
Autumn migratory monarchs of the eastern North American population.
Studied in
Danaus plexippus
Why we rate it this way, and what the caveats are
Well supportedHigh confidence

The opaque-versus-clear paint control converts an amputation result into a specific claim about light reaching the antennae, which is what makes the finding load-bearing.

How far it can be extended

Antennal clocks have since been reported in other insects, but the sun-compass link was established in this species.

Caveats

  • Tethered flight in a simulator measures intended heading, not a journey.
  • Removing antennae removes olfaction and mechanoreception too, which is why the paint control matters more than the amputation.

Still unanswered

  • How the antennal clock signal reaches and is combined with the visual compass pathway in the brain.

Last reviewed 2026-09-03

The evidence (2 studies)

How we know

Moving a butterfly’s clock and watching it turn

Migrating monarchs hold a consistent southwesterly heading. Is that a sun compass, and if so, is it corrected for the time of day?

A sun compass makes a quantitative, falsifiable prediction that almost nothing else does: if an animal reads direction from the sun while correcting for the sun’s movement using an internal clock, then shifting that clock should rotate its chosen heading by a predictable angle. Monarchs were tethered in a flight simulator that let them fly continuously while their heading was recorded. Some were clock-shifted beforehand by several hours on an artificial light cycle; others were not.

What happened

Untreated monarchs held a consistent autumn southwesterly heading. Clock-shifted monarchs shifted their heading by approximately the predicted angle, in the predicted direction.

What it shows

That migrating monarchs use a time-compensated sun compass. The prediction is precise enough that passing it is strong evidence, and it establishes the mechanism as a compass rather than the inherited map often assumed in popular accounts of the monarch journey.

What it does not show

A compass gives direction, and the striking thing about the monarch migration is the destination: specific groves the arriving generation has never seen. Nothing here addresses that. A tethered butterfly is also not flying anywhere, so motivation and persistence cannot be assessed, and clock-shifting is a substantial physiological manipulation with effects beyond the compass.

The controls — what makes this evidence rather than a story
  • Unshifted monarchs flown in the same apparatus, establishing the natural heading and that tethering does not itself determine direction.
  • The shift magnitude predicts a specific angular displacement, so the result is quantitative rather than a change in the right general direction.
  • Shift direction varied, predicting rotations both ways — a general disorientation would not follow the sign.
  • Only monarchs in migratory condition tested, since a breeding butterfly has no migratory heading to measure.

From Virtual migration in tethered flying monarch butterflies reveals their orientation mechanisms

A sun compass with a clock explains how a monarch holds a bearing through a day. It does not explain how the bearing gets set to the right value in the first place, and that remains genuinely open: the direction appears to be inherited, since butterflies that have never migrated orient correctly, but what specifies it is unknown. A magnetic backup has been reported and is not as securely established as the sun compass; NatureHQ treats it as a live proposal rather than a settled second mechanism.

The mechanisms in general

How it knows where to go

A migrating monarch

  • The sun

    Demonstrated

    A compass bearing, held through the day

    Tethered butterflies in a flight simulator orient consistently, and shift predictably when their internal clock is shifted.

  • A circadian clock in the antennae

    Demonstrated

    The correction that keeps the sun compass honest as the sun moves

    Removing the antennae abolishes time compensation; painting them black does the same and clear paint does not.

  • Cold at the overwintering site

    Demonstrated

    The switch that reverses the direction for the spring return

    Autumn migrants held at overwintering temperatures for weeks flew north afterwards; those kept warm continued south.

  • The magnetic field

    Proposed

    A possible backup compass when the sun is obscured

    Reported in flight-simulator work, and not established as securely as the sun compass.

What is still unsettled

  • What sets the inherited direction?

    Why it matters: A butterfly that has never migrated orients correctly, so something specifies the bearing. What it is, and how it survives four generations, is unknown.

    What would settle it: Genetic and neural work linking the compass output to a specifiable heading, which is beginning but is not there.

  • How do they find a forest they have never seen?

    Why it matters: A compass gets a butterfly to a region. The overwintering sites are a handful of specific groves, and how the last stage of the journey works is not established.

    What would settle it: Tracking individuals through the final approach, which is at the edge of what tags light enough for a butterfly can do.

  • What happens to a cold-triggered switch in a warming climate?

    Why it matters: If the spring reversal depends on having been cold enough for long enough, warmer overwintering conditions could break the return leg without affecting the outward one.

    What would settle it: Long-term monitoring of overwintering temperatures against spring departure timing and direction.

Claims about this, checked

Things people have heard, and what the evidence actually supports.

The research behind this page

5 studies, newest first. Each one has a page explaining what it found and what it could not show.

2013Current Biology

Coldness Triggers Northward Flight in Remigrant Monarch Butterflies

Butterflies that had experienced the cold reversed their preferred heading from southward to northward; those held warm continued to orient south.

2013Nature Reviews Genetics

The genetic causes of convergent evolution

The same genes are implicated in independent origins of similar traits far more often than chance would suggest.

2012Science

Parallel molecular evolution in an herbivore community

Insects from unrelated orders feeding on cardiac-glycoside plants repeatedly acquired substitutions at the same small set of sites in the same gene, with several identical changes arising independently.

2010Trends in Neurosciences

Navigational mechanisms of migrating monarch butterflies

Monarchs use a time-compensated sun compass whose clock sits in the antennae rather than the brain; removing or painting the antennae disrupts orientation while leaving flight intact.

2009Science

Antennal Circadian Clocks Coordinate Sun Compass Orientation in Migratory Monarch Butterflies

Removing the antennae abolished time-compensated orientation while leaving flight intact; painting them black — blocking light to them but leaving them attached — did the same, and clear paint did not.

Where to go from here

Each of these follows from something on this page — a relationship in the evidence, a claim people ask about, or the next mechanism along.

How complete this page is, and what it is still missing

NatureHQ publishes its own gaps. This page is at 59% completeness against what we would call a finished subject, and was last reviewed on 2026-09-03. It carries 4 claims and answers 9 mapped search questions.

  • 5 high-priority search intent(s) not yet covered
  • no research from the last few years is attached — check for newer work
  • The western North American population, which overwinters on the Californian coast and has declined sharply, is named and not treated.
  • Milkweed, toxicity and the mimicry story are absent; this page is about the journey.
  • Population decline and its causes are outside the page, which is a deliberate scope choice rather than a judgement about importance.