Module 5: Monarch Butterfly Migration

The monarch butterfly is a member of the Nymphalidae family, one of ~300 species in the milkweed-butterfly subfamily Danainae. Adults weigh 0.4–0.75 g with a wingspan of 9–10 cm. The life cycle has four stages: egg, larva (five instars), pupa, adult. The larva feeds exclusively on milkweed (Asclepias) and sequesters cardiac glycosides (cardenolides) from the plant, which render both the caterpillar and the adult unpalatable to vertebrate predators—a textbook case of chemical defence (Brower 1969).

Eastern and western populations

North America holds two genetically and behaviourally distinct monarch populations separated by the Rocky Mountains. The eastern population breeds from Texas through the Midwest to southern Ontario and overwinters in 10–12 micro-colonies in the Trans-Mexican Volcanic Belt. The western population breeds west of the Rockies and overwinters in 100–400 sites along the California coast in Eucalyptus, Monterey pine, and cypress groves, typically without crossing the Sierra Nevada. The two populations mix very little; genetic divergence is low but behavioural separation is near-complete (Talla et al., 2020).

Other monarch populations

Smaller monarch populations occur in southern Florida (non-migratory), Cuba and the Caribbean (non-migratory), Hawai‘i (introduced ~1840), Australia and New Zealand (introduced), and the Iberian Peninsula (recent range expansion). Only the North American populations execute long-distance migrations, and only the eastern population performs a multi-generational relay that reaches its dramatic peak at the Mexican overwintering sites.

The annual eastern monarch cycle unfolds as five distinct generations:

• G0 (overwintering migrants): the super-generation arriving in Mexico in November, inactive until March.
• G1 (March–April): G0 survivors leave Mexico, fly to southern Texas, lay eggs on milkweed, die within 4–6 weeks of emergence.
• G2 (May–June): offspring of G1, breed across central US as the milkweed belt greens up.
• G3 (June–July): breeding reaches the northern US and southern Canada.
• G4 (July–August): breeding peaks across the northern range; adults die in 4–6 weeks.
• G5 = new super-generation (late August onward): eclose into reproductive diapause, fly 3 000–4 500 km to Mexico, arrive in November; live 6–8 months.

The super-generation is biologically distinct: its corpora allata suppress juvenile hormone, arresting reproductive development; its lipid reserves accumulate (~25% body mass as fat); its metabolic rate falls 80%; and its lifespan extends 4–6× that of the breeding generations. This diapause is induced by the shorter days, cooler nights, and ageing milkweed of late summer. Herman & Tatar (2001) characterised the molecular signature of the diapause transition, showing upregulation of diapause-hormone receptor and downregulation of vitellogenin.

How the super-generation finds Mexico

The challenge is that no individual G5 adult has ever made the journey; its parents (G4) lived and died without migrating; its ancestors four generations back did complete the same flight but their memory is not available. The heading south is therefore innate, relying on a sun-compass calibrated by the circadian clock in the antenna (Reppert et al. 2009, see Module 6), with magnetic-compass back-up. The selection of specific overwintering sites on oyamel fir is fine-tuned: monarchs arrive en masse at the same dozen mountainside groves every year, with near-perfect spatial fidelity.

For most of the twentieth century the winter destination of the eastern monarch was unknown. Frederick Urquhart (University of Toronto) and his wife Norah ran a citizen- science wing-tagging project for decades from 1937, fitting 10–20 mg wing tags to tens of thousands of butterflies across North America and accumulating a handful of southward recoveries through Texas and northern Mexico. In January 1975, Kenneth Brugger and Catalina Aguado, volunteers trained by the Urquharts, located the first known overwintering site at Cerro Pelón in the Trans-Mexican Volcanic Belt, later reported in National Geographic (Urquhart, 1976). The site was covered in monarchs at densities of thousands per square metre on the oyamel branches.

Subsequent surveys located a total of about a dozen overwintering colonies within a small area bounded by the volcanoes Nevado de Toluca, Cerro Pelón, and El Rosario in the states of Michoacán and the State of Mexico. All colonies lie between 2 900 and 3 300 m elevation on north- and north-west-facing slopes of oyamel fir, spanning only ~800 km² total area. The restriction to a handful of microsites is one of the most remarkable features of the monarch migration—and one of the most concerning from a conservation standpoint.

Monarch Biosphere Reserve

The Mexican government created the Monarch Butterfly Biosphere Reserve (Reserva de la Biosfera Mariposa Monarca) in 1980 and expanded it in 2000 to ~56 000 ha, and UNESCO declared it a World Heritage Site in 2008. Enforcement against illegal logging has varied over the decades; the reserve authority Profepa reports 20–80 ha of illegal clearing per year inside the reserve as of 2020.

The monarch colony on a single oyamel tree can contain \(10^4\text{--}10^6\) butterflies, densely packed on branches and trunks at densities of 5 000–15 000 individuals per square metre of surface. Adjacent trees support similar aggregations; a whole colony spans 1–10 ha, and the aggregate population at the peak of the season can exceed \(10^9\) butterflies in the whole reserve.

Thermal buffering

The function of clustering is thermal. Ambient temperatures at 3 000 m drop to -3 °C at night and rise to +18 °C midday; monarchs are in cold torpor at below +4 °C but suffer chill mortality below 0 °C when wet. The cluster acts as a composite thermal mass: metabolic heat from the 5 000+ individuals per m² is released slowly through the loosely packed wings and bodies, and the outer shell of the cluster shields the interior from convective and radiative cooling. Alonso-Mejía et al. (1997) measured interior temperatures of +2 to +8 °C on nights when ambient air fell to -3 °C, a buffering gain of 5–11 °C.

Canopy shelter

The oyamel canopy itself plays a critical role. Dense foliage reduces radiative cooling to the night sky by a factor of 2–3 relative to an open clearing; the canopy traps a warm-air pocket below; and the trunks provide wind shelter. The fir stand is not interchangeable with any other tree species; monarchs tried transplanting to lower- elevation pine forests arrive but experience sharply elevated mortality (Brower et al. 2004).

Milkweeds (genus Asclepias, ~130 North American species) are the obligate larval host plants of monarchs. The caterpillar sequesters cardiac glycosides (cardenolides) from the milkweed latex, making it toxic to most vertebrate predators. The adult retains enough cardenolide load to be aposematic, warning predators with bright orange-and-black wing colouration. Brower & Glazier (1975) showed that adult monarchs can induce emesis in naive blue jays, and surviving birds subsequently avoid orange-and-black insects for weeks.

Milkweed distribution

The common milkweed (Asclepias syriaca) is the primary host in the central and eastern US, found along roadsides, field edges, and in Conservation Reserve Program fields. In the Midwest specifically, milkweed densities were historically highest within agricultural landscapes because of abundant weedy-edge habitat.

Pleasants & Oberhauser 2013: the Roundup link

John Pleasants and Karen Oberhauser (Iowa State, University of Minnesota) published a landmark analysis in Insect Conservation and Diversity (2013) linking the introduction of glyphosate-tolerant (Roundup Ready) corn and soybean to a 58% decline in milkweed density in Midwest agricultural fields from 1999 to 2012. Before herbicide-tolerant GMO crops, farmers could not spray row crops without killing them, so milkweed persisted in field edges and inter-row spaces. After 2000, glyphosate spraying over in-crop Roundup Ready corn and soy allowed farmers to eliminate virtually all milkweed from row-crop fields. This milkweed loss closely tracks the measured monarch population decline over the same period.

Lincoln Brower and colleagues (Sweet Briar College) developed the overwintering-hectares census, an annual measurement of the total area occupied by monarch colonies in the Mexican reserves, conducted by the Mexican NGO Alianza WWF-Telcel. Brower et al. (Insect Conservation and Diversity, 2011) reviewed the 1994–2010 time-series and documented an 80% decline from peak values of ~18 ha (1996-97) to ~3 ha (2009-10).

Subsequent censuses reached an all-time low of 0.67 ha in the 2013-14 season, corresponding to perhaps 30 million butterflies, down from an historical peak of around 1 billion. Partial recoveries to 2–6 ha occurred in 2015–2020, but the long-term trend remains strongly negative. In December 2022 the IUCN Red List classified the migratory monarch (D. plexippus plexippus) as Endangered.

Abies religiosa is a Neogene-relict conifer restricted today to a narrow elevational band (2 800–3 500 m) in central Mexico. It requires cool summers, moist winters, and specific soil drainage; the canopy-density and microclimate conditions of the old-growth oyamel forest are what makes them habitable for overwintering monarchs. Sáenz-Romero et al. (2012, Forest Ecology and Management) used species-distribution models driven by climate projections to forecast that by 2090 the climatic envelope of A. religiosa will retreat upslope by 600–900 m, and across much of its current range will disappear entirely.

Assisted upslope migration trials are underway. The results so far are mixed: seedlings transplanted 300 m above current range show good survival at decadal scales, but the mature-forest conditions that monarchs require take a century to develop. Even in optimistic scenarios the overwintering habitat is likely to shrink sharply through the 21st century.

Storm-related mass mortalities

The overwintering clusters are also vulnerable to catastrophic cold-and-wet storms. The January 2002 storm killed an estimated 270 million monarchs (75% of that year’s cohort); the March 2016 storm killed 45–100 million. Brower et al. (2004) analysed the microclimatic determinants: wet butterflies experiencing even brief exposure below -4 °C show near-100% mortality. A warmer, wetter Mexico raises the frequency of such events even as the mean winter climate becomes more favourable.

Ophryocystis elektroscirrha (OE) is a protozoan parasite that infects monarchs, forming spores on the adult’s scales and transmitted vertically (female to eggs) and horizontally (scales dusted onto milkweed during oviposition). Infected adults show reduced flight performance, shorter lifespan, and smaller body size. Sonia Altizer and colleagues (University of Georgia) have run a long-term citizen-science monitoring project (Monarch Health) documenting OE prevalence across North America.

Altizer et al. (2011) summarised two decades of data showing that OE prevalence in the non-migratory Florida and Caribbean monarch populations is 50–80%, whereas in the migratory eastern population it is 5–15%. The difference is consistent with a migratory-escape hypothesis: long-distance migration culls heavily infected individuals, keeping OE prevalence low. The recent appearance of year-round breeding monarchs on exotic tropical milkweed (Asclepias curassavica) in the southern US raises concerns that OE prevalence may rise if migration is partially abandoned.

How does an individual super-generation monarch, with no memory of the route and no parents to follow, find its way to Mexico? Steven Reppert and colleagues at the University of Massachusetts Medical School showed in a series of papers (Sauman et al., 2005; Heinze & Reppert, 2011; Reppert et al., 2016) that the monarch uses a time-compensated sun compass, with the circadian clock located in the antennae (not the brain, in contrast to most insects). Painting the antennae with opaque black paint abolishes the directed southward flight, confirming the antennal-clock role.

Module 6 will treat the compass biochemistry in full detail, including the antennal cryptochrome (type-II CRY, absent from mammals but present in monarchs), the photoreceptor-based magnetic compass, and the central-complex integrator in the monarch brain that combines sun-compass, magnetic, and visual-cue information. For Module 5 the key point is that the multi-generational migration is supported by an integrated compass system capable of navigating 4 000 km with first-generation precision.

The figure below sketches the five-generation eastern monarch annual cycle, from the Mexican overwintering clusters in January to the super-generation departure from the northern breeding range in late August.

The western monarch population, separated from the eastern by the Rocky Mountains, overwinters in 100–400 coastal California sites from Santa Barbara to Mendocino. Roosting sites include Eucalyptus globulus groves (introduced from Australia in the 1850s), Monterey pine (Pinus radiata), and Monterey cypress (Cupressus macrocarpa). The overwintering period is December–February, shorter than the Mexican overwinter because the California climate is milder.

The western population has collapsed even more dramatically than the eastern. Xerces Society counts from 1997 showed ~1.2 million overwintering monarchs; by 2020 the count was 1 900 individuals—a 99.9% decline over two decades. A partial rebound to 250 000+ occurred in 2021–2023, but the long-term trajectory remains deeply negative. The western collapse is probably driven by the same milkweed loss and pesticide factors as the eastern population, compounded by California’s persistent drought and fire regime.

The cardenolide (cardiac-glycoside) load of adult monarchs varies across the geographic range because different milkweed species produce different cardenolide cocktails and at different concentrations. Northern-tier milkweeds such as Asclepias syriacaproduce relatively low cardenolide concentrations (~400 μg/g leaf dry mass), whereas southern species such as A. curassavica and A. humistrataproduce 2 000–5 000 μg/g. The adult monarch sequesters typically 30–80% of the larval cardenolide load, and this sequestration is reflected in adult wing cardenolide content measurable by HPLC.

Brower, Seiber, Nelson, Tuskes & Lynch (1982) showed that mice, lizards, and passerines respond with vomiting or rejection after experimental ingestion of cardenolide-rich monarchs. Some species—notably black-headed grosbeaks (Pheucticus melanocephalus) and black-backed orioles (Icterus abeillei)—can tolerate moderate cardenolide doses by excising the cardenolide-rich wing muscles and abdomen, and these species are the principal mammalian predators at the Mexican overwintering sites. Arellano-Guillermo et al. (1993) estimated that these two bird species consume up to 10% of the overwintering monarch cohort in some years, a significant demographic pressure.

Na+/K+-ATPase resistance

Cardenolides work by inhibiting the Na⁺/K⁺-ATPase, a central ion pump in vertebrate cells. Monarch caterpillars and adults carry a series of amino-acid substitutions in their own Na⁺/K⁺-ATPase that render them insensitive to the toxin. Karageorgi et al. (2019, Nature) used CRISPR to transplant these substitutions into Drosophila and produced the first cardenolide-tolerant fruit fly—a remarkable validation of the evolutionary trajectory. The monarch thus exemplifies a classic case of convergent adaptation to a chemically defended host plant.

The spring northward expansion and fall southward migration of monarchs can be tracked at continental scale through citizen-science reporting. The Journey North programme (Annenberg Foundation; later University of Wisconsin-Madison) collects first-sighting reports from thousands of volunteers across North America. The resulting phenological maps show the milkweed-following northward expansion of successive generations in spring (G1 through G4), and the reverse-direction sweep of the super-generation in fall, concentrated into a 6–8 week window from late August through mid-October depending on latitude.

Peak fall passage at the Texas Gulf Coast bottleneck in early October sees millions of monarchs funnel through a narrow corridor along the coastline, a spectacle analogous to the raptor migration spectacle at Hawk Mountain or Veracruz. The Texas funnel is the point of greatest density for the entire eastern population; protection of staging habitat here is a high conservation priority.

Flight mechanics of the super-generation

Super-generation monarchs maintain a sustained flight speed of 3–5 m/s airspeed, climbing to 500–1 500 m AGL on warm days and gliding on thermals where available. Gibo & Pallett (1979) showed that monarchs exploit thermal soaring much like hawks, climbing on rising air columns and then gliding down-sun—a marked energy saving. On a typical migration day the super-gen monarch covers 50–100 km; over the 60–80 day migration window this totals the required 3 000–4 500 km.

The eastern monarch migration is biologically unique—a multi-generational, innate relay navigated by antennal-clock sun compass to a handful of oyamel-fir groves at 3 000 m in central Mexico. It is simultaneously one of the most threatened migrations on Earth: milkweed loss has reduced the breeding-ground carrying capacity by half, oyamel-fir climate-zone shifts threaten the overwintering habitat, climate whiplash events kill millions of clustered individuals in single storms, and the possible abandonment of migration in tropical-milkweed-hosted populations may usher in a non-migratory, heavily parasitised future. The species has been IUCN-listed as Endangered since 2022.

Conservation responses have focused on milkweed restoration programmes in Midwest agricultural landscapes, assisted migration of oyamel-fir seedlings to higher elevations, enforcement of Mexican Biosphere Reserve protections, and citizen-science monitoring programmes such as Journey North and Monarch Watch. Module 6 next examines the compass biochemistry that makes the super-generation’s 4 000 km navigation possible, connecting the integrative biology of this module with the quantum-spin and sun-compass mechanisms already introduced in Module 2.