The spectacular biology and migration of monarch butterflies

Butterflies are one of the most recognizable and beautiful insects in the world. The planet is home to between 17,000 and 19,000 different species; from the tiny Western pygmy blue butterfly to the gargantuan Queen Alexandra's birdwing. Each species is unique, reflecting the diversity of the ecosystems they inhabit. What’s even more fascinating is that scientists are continuously discovering new, previously unknown species all over the world.

Butterflies are fascinating creatures — but one species in particular is unique to any other and that’s the common monarch butterfly. While not the rarest species in the world, what makes monarch butterflies so special is their incredible migration across North America — a navigational phenomenon that has fascinated scientists for generations.

How do monarch butterflies travel so far?

Most butterflies spend their entire lives in one area or ecosystem. Preferring to stick closer to home, butterflies feed off flower nectar and distribute pollen as they go, providing a crucial link in local food chains by supporting the growth of countless flower species.

But the monarch butterfly (Danaus plexippus) is different. They are the only butterflies on the planet known to make a two-way migration, just like birds. Every fall, thousands of butterflies migrate from as far north as Canada to overwinter* in the forests of central Mexico — a journey of approximately 3,000 miles. Monarchs do this because they cannot survive cold winters in the northern hemisphere. Unlike other butterflies, who are able to overwinter early in their development, monarchs need temperatures between 70 and 80°F (21-27°C) to thrive, so they fly south. 

Migration: the circadian clock

Every fall, an internal circadian clock, combined with external factors like cooling temperatures and less daylight, tells millions of monarch butterflies that it’s time to travel south. But how do they know where to go? For a long time this was a mystery, but scientists think they have finally figured it out. The butterfly’s circadian clock is located in the antenna, which helps them use the sun to navigate. This time-compensated sun compass relies on cues like the sun and polarized light.

Their internal clock is where specific proteins are found, they are called PERIOD (PER), TIMELESS (TIM), and CRY1. Monarchs also have a second important protein called CRY2 which is similar to one found in vertebrates.

In monarchs, CRY1 is linked to a pathway in the brain that uses light-dependent magnetic sensing for sun navigation, while CRY2 is part of another pathway that sends timing information to the brain’s navigation center, called the central complex, which also acts as a regulator. PER and TIM help the butterflies orient and are part of a molecular feedback loop that helps the butterfly “know” the time of day. Cues from light and sun are processed through both eyes, adjusting the compass based on the time of day. This incredible and unique biology lets the butterfly know when it’s time to travel south. 

What is “the super generation?”

The “super generation” is what makes the migration of the monarch butterfly even more incredible. Decreasing day length and cooling temperatures, along with aging milkweed (their primary food source) and other nectar sources trigger the birth of the super generation.

This genetic wonder allows some generations of monarch butterflies to live far longer than their predecessors. While typically monarchs live for only 2 to 6 weeks, the super generation can live for up to 8 months. This allows them to quickly make the incredible journey south to the forests of Mexico. They then spend 4-5 months overwintering before their internal circadian clock tells them it’s time to move once again.

Millions and millions of butterflies gather in just a few hectares of forest canopy to wait out the winter. For thousands of years, they have chosen this spot because these forests provide a unique microclimate that has cool temperatures and high humidity, essential for the butterflies' survival during winter. The dense canopy of the oyamel fir trees offer safety from the elements, where they cluster together to conserve heat and energy in their millions. This is called a diapause* — a period of energy conservation and reproductive dormancy. Some other species of butterflies enter into a diapause during the pupal stage of their lifecycle, which means that their metamorphosis slows down during cold weather.

 When spring finally arrives, the monarch super generation begin their epic journey back north on a migration that needs to be split into multiple generations.

The life cycle of monarch butterflies

What happens once the butterflies leave the protection of Mexico’s oyamel fir forests? While the super generation is fascinating, the “normal” lifecycle of the common monarch butterfly is just as awesome. In spring, the super generation leaves Mexico and flies around 600 miles (950 kilometers) north to the US where they lay their eggs on milkweed plants, marking the end of their eight-month life cycle. 

Like most butterflies, the monarch goes through four distinct stages: egg, larva (caterpillar), pupa (chrysalis), and adult butterfly. Monarch butterflies only lay their eggs on milkweed, and after 3 to 5 days, these tiny, pin-sized eggs hatch into tiny caterpillars. The caterpillars feed exclusively on the milkweed, which makes these plants crucial for their survival. During the larva stage, the caterpillars must eat as much as possible, growing to around 2000 times larger than when they hatched. After around 10 to 14 days, the caterpillar stops eating and hunts down a secure spot to pupate and for a chrysalis.

During this incredible stage, the monarch caterpillar’s body undergoes extensive reorganization. Within the safety of its cocoon, tissues break down and are rebuilt into adult structures, including wings, legs, eyes, and the proboscis* (feeding tube). This process is fueled by nutrients stored during the larval stage. The length of time this  transformation takes can depend on a few factors. Warmer temperatures tend to speed up their development, but it typically lasts 10 to 15 days. 

When the transformation is complete, the cocoon splits open and the adult monarch emerges. To begin with, their wings are soft and folded, but the butterfly will then pump fluid into the wings to expand and harden them, a process which can take a few hours. During this process the butterfly is most vulnerable to environmental changes and predators. 

Why do we need butterflies?

Butterflies, like bees, are key pollinators. They feed on nectar and transfer pollen between flowers, playing an important role in plant reproduction. Butterflies also serve as a primary food source for predators, especially during the caterpillar stage of their life cycle, which is the top food for many birds. They are also a key indicator species. The diversity of butterfly species and their population numbers are a sign of a healthy, thriving ecosystem. Monitoring butterfly species can help scientists assess the impacts of threats to their habitat.

Threats to monarch butterflies

And sadly, there are many threats. The increased use of pesticides, in particular neonicotinoids*, are toxic to butterfly larvae. For example, a study found that monarch larvae feeding on their all-important milkweed treated with imidacloprid, a common neonicotinoid, died within seven days. Climate change is also a big impacting factor. Monarch butterflies rely on specific cues in their environment to trigger their migration and reproduction cycles. Rising temperature and unpredictable weather disrupt these cues, lead to mistimed migrations and reduced reproductive success. 

Studies indicate that widespread butterfly populations are in decline. In the western US, over 450 species have declined at an average rate of nearly 2% per year since 1972, with warmer falls identified as a key reason why. In Europe, many butterfly species are moving northward due to warming climates, or because they cannot adapt quickly enough.

Monarch butterflies are incredible insects, but even these resilient creatures are facing an uncertain future in the face of our changing planet. Conservation efforts, like that of WWF Mexico, are crucial in mitigating these impacts and supporting the monarch’s survival. Thankfully, it looks like these efforts are working. A recent survey reported that the eastern monarch butterfly population nearly doubled in 2025, an encouraging development giving us hope for the future of this incredible species.  

Glossary of terms

Overwinter - The term used for when an insect, plant, or other organism survives the winter. 

Diapause - A period of suspended development in an insect, other invertebrate, or mammal embryo, especially during unfavourable environmental conditions.

Pupate - A stage of life where an insect undergoes a total metamorphosis, occurring between the larval and adult stages.

Proboscis - Part of an insect. An elongated sucking mouthpart that is typically tubular and flexible.

Neonicotinoids - A class of synthetic insecticides, chemically similar to nicotine, used for controlling insects in crops. They are systemic, meaning they are absorbed by the plant and distributed throughout its tissues, including pollen and nectar. 

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