Zoopedia: Monarch Butterfly

People across North America will be familiar with the annual mass migration of the beautiful looking monarch butterflies. These brightly coloured butterflies can block out the sky as they migrate across North America. Today, we’ll be looking at these butterflies, and what their lives look like.

Taxonomy and Evolution

The scientific name for monarch butterflies is Danaus plexippus, meaning that it belongs to the genus Danaus. Their relatives are naturally found in every continent bar Antarctica and Europe, although migratory populations do exist in Europe. Danaus butterflies belong to a tribe called milkweed butterflies which first appeared in the Oligocene/Miocene around 23 million years ago, and butterflies themselves first appeared in the early Jurassic, around 200 million years ago. As monarch butterflies are orange it is believed that they were named after British monarch William III, aka William of Orange. To the best of my knowledge monarch butterfly society isn’t based around a feudal hierarchy whose sovereign inherits their role, so for now we have to accept the William of Orange explanation. There are six subspecies of monarch butterfly with D.p. plexippus being the most common having two colour morphs as well – the most notable being the white morph found on Oahu, Hawai’i.

Biology

Monarch butterflies live symbiotically with certain species of plants, specifically milkweeds, where females lay between 300 and 500 eggs on the milkweeds. These millimetre sized eggs hatch after a week, and the caterpillars grow up eating the milkweeds. Milkweeds are toxic to most animals, but not to monarch butterflies, so the caterpillars ingest the milkweed toxin which allows them to become toxic themselves. Over a period of a month the caterpillars become larger and molt, doing this five times until they reach around 4.5 centimetres in length. They can also become territorial as they get bigger. If food is not readily available monarch caterpillars will attack each other over the dwindling resources. When reaching max size the caterpillar will find a safe place to form a chrysalis where it will develop into its final, imago, stage after two weeks. When hatched the monarch imago will take several hours to pump blood into its wings and dry out before it can start flying. The time of year will then determine how long the imago lives for. If they hatch in spring or early summer they have five weeks to live as this is their breeding season, however, late summer or autumn hatchings will live longer with them not breeding during the winter. Adults have a wingspan up to 10.2 centimetres, or 4 inches.

Adult monarchs are known for their orange wings covered in black veins and white spots on the head and edges of the wings. This is a special visual defense for them. Animals often adopt a red or orange and black colour scheme as a way to highlight to potential predators that they are venomous or poisonous. As a caterpillar they ingest milkweed toxins which stay in their body throughout the rest of their life, and this not only makes them toxic, but also makes them taste disgusting. Other butterflies have evolved similar defense mechanisms, like viceroys, which mutually aid each other.

Monarch butterflies also possess a unique eyesight. Instead of seeing colours as we do, they detect wavelengths, which allows them to pinpoint objects more efficiently than if they were basing their eyesight on shape alone. As a result, monarchs find it very easy to find the best plants for them to get nectar from, or where to lay their eggs. Monarch eyesight can also pick up ultraviolet light, something that is essential during migration as it allows them to pinpoint where the sun is.

Migration

When monarch butterflies migrate they do so in the thousands. Not all monarchs migrate though – those born in the spring or early summer and the subspecies D.p. megalippe do not migrate. Monarchs butterflies cannot function as well in the cold, so this is the impetus which drives migration. Eastern populations will migrate from Canada and the northernmost U.S. states south over 3,000 miles to central Mexico where they hide out in oyamel fir trees. These fir trees are only found in twelve mountainous regions in Mexico where it is cool and moist. Western populations migrate to spots in California across the coast until the spring comes. As a result, the skies across the Americas are often full of butterflies. However, their short lifespan means that butterflies do not make the full migratory route themselves. Instead, they overwinter in certain areas, such as north Mexico and the southern U.S., so its their offspring that continues the journey. Then they will reproduce, and then their offspring will etc. In all, it can take up to five generations for monarch butterflies to take the full route down to Mexico and back north to Canada.

Habitat and Distribution

Monarch butterflies are found in forests and grasslands, with these environments providing enough flowering plants for them to derive nectar from. Normally, they roost in forests (favouring trees) and breed in grasslands where there is plenty of vegetation for their caterpillars. Monarch butterflies are very sensitive to the temperature and climate, and prefer warm and moist environments. Too warm, cold, wet, or dry, and monarchs struggle to survive. Normally, the best places to find monarch butterflies is North America, especially Mexico, Canada, the USA, and Caribbean. However, they are also found in Oceania, western North Africa, and Spain, and there have been sightings in the UK after adults have accidentally migrated there. It is especially popular in the USA where several states have adopted it as the state insect in seven states, and has twice been put forward as the national insect. Monarch butterflies have also been to space! In 2009 several were reared on the ISS.

Diet and Threats

As expected with butterflies their diet changes over their lifespan. As a caterpillar they eat the milkweed that they’ve been born on, although they are capable of eating other plant matter. This is not simply a one-way street as the milkweed also benefits. As the monarch butterflies lay their eggs on the milkweed they also spread the plant’s pollen and seeds which helps reproduction. Adults survive on nectar and will take it from any available flowering plant. They also do take minerals from damp soil and gravel, as this helps them get extra nutrients and proteins for reproduction or migration (when the body needs more energy).

As they do contain toxins in their bodies monarch butterflies have a small variety of natural predators. The most common being birds with the black-headed oriole and black-headed grosbeak evolving an ability to withstand monarch toxins. However, these birds can only eat a certain amount of the butterflies, and grosbeaks will hold off on eating monarchs to allow their body to expel the increase of ingested toxins. Jays, robins, cardinals, and sparrows have also hunted butterflies, and black-eared mice are actually very resistant to monarch toxin. While overwintering the monarchs become less toxic, so this is when birds and mice target them. Monarchs also do suffer from protozoic parasites, although their toxins do help fight off some parasites.

Conservation

As a species, monarch butterflies are not endangered, but the migratory subspecies has been classified as endangered by the IUCN for various reasons. Since the 1990s monarch populations have dropped between 50% to 90%, depending on the area, and a 2016 paper estimated that there was between an 11 to 57% chance that the eastern monarchs would be extinct within the next twenty years. A big reason for this is the disappearance of milkweed populations as a mixture of pesticides, deforestation, and GM crops have wiped out 120-150 million acres of milkweed habitat. Further devastating their numbers has been the loss of overwintering habitats to deforestation and climate change. The remaining milkweeds are being driven further and further north by raising temperatures which not only forces the butterflies to travel further, but it also makes the plants more toxic. This means that they slowly become too toxic for monarch caterpillars to eat. The changing climate impacts milkweed growth and also the development of the butterflies themselves, as they cannot survive in temperatures over 35 degrees Celsius.

Due to how popular they are, and how noticeable their drop in numbers has been, has led to a considerable drive to conserve their populations. People have been encouraged to grow milkweeds in their gardens as a ‘waystation’ for migrating butterflies. There have also been an increase in captive breeding, but this does have issues as captive bred monarchs are not as strong as their wild relatives. Canada and Mexico have worked together to set up monarch reservations and to protect the forests which they live, and several U.S. federal agencies have been encouraging farmers and highway agents to reserve land for butterflies and to avoid using certain herbicides. However, U.S. federal agencies and environmentalism normally do not go well together – one environmental rights group has taken the EPA to court for not prohibiting the use of a serious herbicide.

Bibliography:

• Paul Ehrlich and Peter Raven, ‘Butterflies and Plants: A Study in Coevolution’, Evolution, 18:4, (1964), 586-608
• T. Reichstein, J. von Euw, J.A. Parsons, and Miriam Rothschild, ‘Heart Poisons in the Monarch Butterfly’, Science, 161:3844, (1968), 861-866
• ‘Migratory Monarch Butterfly’, IUCN, (30/12/2021), [Accessed 12/08/2022]
• ‘Monarch Butterfly’, IUCN, (19/08/2021), [Accessed 12/08/2022]
• Brice X. Semmens, Darius J. Semmens, Wayne E. Thogmartin, Ruscena Wiederholt, Laura López-Hoffman, Jay E. Diffendorfer, John M. Pleasants, Karen S. Oberhauser, and Orley R. Taylor, ‘Quasi-extinction risk and population targets for the Eastern, migratory population of monarch butterflies (Danaus plexippus)’, Scientific Reports, 6:23265, (2016)
• ‘Monarch butterfly’, National Geographic, [Accessed 12/08/2022]
• Douglas Blackiston, Adriana Briscoe, and Martha Weiss, ‘Color vision and learning in the monarch butterfly, Danaus plexippus (Nymphalidae)’, Journal of Experimental Biology, 214:3, (2011), 509-520