Fantastic Feats of the Majestic Monarch
Photo Above: (Monarch Butterflies – Watercolor Painting; Colored Pencil Drawing)
The monarch butterfly or simply monarch is a milkweed butterfly. Other common names depending on region include milkweed, common tiger, wanderer, and black veined brown. It may be the most familiar North American butterfly, and is considered an iconic pollinator species. Its wings feature an easily recognizable black, orange, and white pattern, with a wingspan of 8.9–10.2 cm (3
1⁄2–4 in) A Müllerian mimic, the viceroy butterfly, is similar in color and pattern, but is markedly smaller and has an extra black stripe across each hindwing.
The eastern North American monarch population is notable for its annual southward late-summer/autumn migration from the northern and central United States and southern Canada to Florida and Mexico. During the fall migration, monarchs cover thousands of miles, with a corresponding multi-generational return north. The western North American population of monarchs west of the Rocky Mountains often migrates to sites in southern California but has been found in overwintering Mexican sites as well. Monarchs have been bred on the International Space Station.
The name “monarch” is believed to be given in honor of King William III of England, whose secondary title Prince of Orange makes a reference to the butterfly’s main color.
Description
Commonly and easily mistaken for the similar viceroy butterfly – the two species are Müllerian mimics, the monarch’s wingspan ranges from 8.9 to 10.2 centimetres (3.5–4.0 in). The uppersides of the wings are tawny orange, the veins and margins are black, and there are two series of small white spots in the margins. Monarch forewings also have a few orange spots near their tips. Wing undersides are similar, but the tips of forewings and hindwings are yellow brown instead of tawny orange and the white spots are larger. The shape and color of the wings change at the beginning of the migration and appear redder and more elongated than later migrants. Wings size and shape differ between migratory and non-migratory monarchs. Monarchs from eastern North America have larger and more angular forewings than those in the western population.
Monarch flight has been described as “slow and sailing”. Monarch flight speed has been estimated by a number of researchers. One scientist examined all prior estimates and concluded their flight speed is approximately 9 km/h or 5.5 mph. For comparison, the average human jogs at a rate of 9.7–12.9 km/h (6–8 mph).
Adults are sexually dimorphic. Males are slightly larger than females and have a black patch or spot of androconial scales on each hindwing (in some butterflies, these patches disperse pheromones, but are not known to do so in monarchs). The male’s black wing veins are lighter and narrower than those of females.
One variation, the “white monarch”, observed in Australia, New Zealand, Indonesia and the United States, is called nivosus by lepidopterists. It is grayish white in all areas of its wings that are normally orange and is only about 1% or less of all monarchs, but populations as high as 10% exist on Oahu in Hawaii.
The monarch has six legs like all insects, but uses only its middle legs and hindlegs as the forelegs are vestigial, as in all Nymphalidae, and held against its body.
Detailed measurements
A study in 2015 examined a preserved collection of male and female monarch specimens from eastern North America to evaluate the sex-based differences in fine-scale wing and body structure. The study found significant differences in overall wing size and in the physical dimensions of wings. Males tended to have larger wings than females, and were heavier than females, on average. Both males and females had similar thorax dimensions (wing muscles are contained in the thorax). Female monarchs tended to have thicker wings, which is thought to convey greater tensile strength. This would make female wings less likely to be damaged during migration. Also, females had lower wing loading than males (wing loading is a value derived from the ratio of wing size to body mass), which would mean females require less energy to fly.
Distribution and habitat
The range of the western and eastern populations expands and contracts depending upon the season. The range differs between breeding areas, migration routes, and winter roosts. However, no genetic differences between the western and eastern monarch populations exist; reproductive isolation has not led to subspeciation of these populations, as it has elsewhere within the species’ range.
In the Americas, the monarch ranges from southern Canada through northern South America. It has also been found in Bermuda, Cook Islands,Hawaii, Cuba, and other Caribbean islands: the Solomons, New Caledonia, New Zealand, Papua New Guinea, Australia, the Azores, the Canary Islands, Madeira, Gibraltar, the Philippines, and North Africa. It appears in the UK in some years as an accidental migrant.
Overwintering populations of Monarchs are found in Mexico, California, along the Gulf Coast, year round in Florida, and in Arizona where the habitat has the specific conditions necessary for their survival. On the US East Coast, they have overwintered as far north as Lago Mar, Virginia Beach, Virginia. Their wintering habitat typically provides access to streams, plenty of sunlight (enabling body temperatures that allow flight), and appropriate roosting vegetation, and is relatively free of predators. Overwintering, roosting butterflies have been seen on basswoods, elms, sumacs, locusts, oaks, osage-oranges, mulberries, pecans, willows, cottonwoods, and mesquites. While breeding, monarch habitats can be found in agricultural fields, pasture land, prairie remnants, urban and suburban residential areas, gardens, trees, and roadsides – anywhere where there is access to larval host plants. Habitat restoration is a primary goal in monarch conservation efforts. Habitat requirements change during migration. During the fall migration, butterflies must have access to nectar-producing plants. During the spring migration, butterflies must have access to larval food plants and nectar plants.
Eggs
The eggs are derived from materials ingested as a larva and from the spermatophores received from males during mating. Eggs are laid singly on the underside of a young leaf of a milkweed plant during the spring and summer months. The eggs are cream colored or light green, ovate to conical in shape, and about 1.2×0.9 mm in size. The eggs weigh less than 0.5 mg each and have raised ridges that form longitudinally from the point to apex to the base. Though each egg is 1⁄1000 the mass of the female, she may lay up to her own mass in eggs. Females lay smaller eggs as they age. Larger females lay larger eggs. The number of eggs laid by a female, who may mate several times, ranges from 290 to 1180. Females lay their eggs on the underside of the milkweed leaves; the offspring’s consumption of the milkweed benefits health and helps defend them against predators. Eggs take 3 to 8 days to develop and hatch into larva or caterpillars.) Monarchs will lay eggs along the southern migration route.
Larvae
The caterpillar goes through five major, distinct stages of growth and after each one, it molts. Each caterpillar, or instar, that molts is larger than the previous as it eats and stores energy in the form of fat and nutrients to carry it through the nonfeeding pupal stage. Each instar usually lasts about 3 to 5 days, depending on various factors such as temperature and food availability.
Pupa
To prepare for the pupa or chrysalis stage, the caterpillar chooses a safe place for pupation, where it spins a silk pad on a downward-facing horizontal surface. At this point, it turns around and securely latches on with its last pair of hindlegs and hangs upside down, in the form of the letter J. After “J-hanging” for about 12–16 hours, it will suddenly straighten out its body and go into peristalsis some seconds before its skin splits behind its head. It then sheds its skin over a period of a few minutes, revealing a green chrysalis. At first, the chrysalis is long, soft, and somewhat amorphous, but over a few hours it compacts into its distinct shape – an opaque, pale-green chrysalis with small golden dots near the bottom, and a gold-and-black rim around the dorsal side near the top. At first, its exoskeleton is soft and fragile, but it hardens and becomes more durable within about a day. At this point, it is about 2.5 cm (1″) long and 10–12 mm (3/8–7/16″) wide, weighing about 1.2 grams. At normal summer temperatures, it matures in 8–15 days (usually 11–12 days). During this pupal stage, the adult butterfly forms inside. Within a day or so before emerging is due, the exoskeleton first becomes translucent and the chrysalis more bluish. Finally, within 12 hours or so, it becomes transparent, revealing the black and orange colors of the butterfly inside before it ecloses (emerges).
Adult
An adult butterfly emerges after about two weeks as a chrysalis, and hangs upside down for a few hours until its wings are dry. Fluids are pumped into the wings, and they expand, dry, and stiffen. The monarch expands and retracts its wings, and once conditions allow, it then flies and feeds on a variety of nectar plants. During the breeding season adults reach sexual maturity in four or five days. However, the migrating generation does not reach maturity until overwintering is complete. Monarchs typically live for two to five weeks during their breeding season. Larvae growing in high densities are smaller, have lower survival, and weigh less as adults compared with those growing in lower densities. Monarch metamorphosis from egg to adult occurs during the warm summer temperatures in as little as 25 days, extending to as many as seven weeks during cool spring conditions. During the development, both larvae and their milkweed hosts are vulnerable to weather extremes, predators, parasites and diseases; commonly fewer than 10% of monarch eggs and caterpillars survive. However, this is a natural attrition rate for most butterflies, since they are low on the food chain.
Reproduction
Healthy males are more likely to mate than unhealthy ones. Females and males typically mate more than once. Females that mate several times lay more eggs. Mating for the overwintering populations occurs in the spring, prior to dispersion. Mating is less dependent on pheromones than other species in its genus. Male search and capture strategies may influence copulatory success, and human-induced changes to the habitat can influence monarch mating activity at overwintering sites.
Courtship occurs in two phases. During the aerial phase, a male pursues and often forces a female to the ground. During the ground phase, the butterflies copulate and remain attached for about 30 to 60 minutes. Only 30% of mating attempts end in copulation, suggesting that females may be able to avoid mating, though some have more success than others. During copulation, a male transfers his spermatophore to a female. Along with sperm, the spermatophore provides a female with nutrition, which aids her in egg laying. An increase in spermatophore size increases the fecundity of female monarchs. Males that produce larger spermatophores also fertilize more females’ eggs.
Migration
In North America, monarchs migrate both north and south on an annual basis, in a long-distance journey that is fraught with risks. The population east of the Rocky Mountains attempts to migrate to the sanctuaries of the Mariposa Monarca Biosphere Reserve in Mexico and parts of Florida. The western population tries to reach overwintering destinations in various coastal sites in central and southern California. The overwintered population of those east of the Rockies may reach as far north as Texas and Oklahoma during the spring migration. The second, third and fourth generations return to their northern locations in the United States and Canada in the spring. Captive-raised monarchs appear capable of migrating to overwintering sites in Mexico, though they have a much lower migratory success rate than wild monarchs do. See section on captive-rearing below. Recent discoveries have located monarch overwintering sites in Arizona.
Vision
Physiological experiments suggest that monarch butterflies view the world through a tetrachromatic system. Like humans, their retina contain three types of opsin proteins, expressed in distinct photoreceptor cells, each of which absorbs light at a different wavelength. Unlike humans, one of those types of photoreceptor cells corresponds to a wavelength in the ultraviolet range; the other two correspond to blue and green. In addition to these three photoreceptors cells in the main retina, monarch butterfly eyes contain orange filtering pigments that filter the light reaching some but not all green-absorbing opsins, thereby making a fourth photoreceptor cell sensitive to longer wavelength light. The combination of filtered and unfiltered green opsins permits the butterflies to distinguish yellow from orange colors. The ultraviolet opsin protein has also been detected in the dorsal rim region of monarch eyes. One study suggests that this allows the butterflies the ability to detect ultraviolet polarized skylight in order to orient themselves with the sun for their long migratory flight.
These butterflies are capable of distinguishing colors based on their wavelength only, and not based on intensity; this phenomenon is termed “true color vision.” This is important for many butterfly behaviors, including seeking nectar for nourishment, choosing a mate, and finding milkweed to lay eggs on. One study found that floral color is more easily recognized at a distance by butterflies searching for nectar than floral shape. This is may be because flowers have highly contrasting colors to the green background of a vegetative landscape.[80] On the other hand, leaf shape is important for oviposition so that the butterflies can ensure their eggs are being laid on milkweed.
Beyond the perception of color, the ability to remember certain colors is essential in the life of monarch butterflies. Researchers have found that these insects can easily learn to associate color and, to a lesser extent shape, with sugary food rewards. When searching for nectar, color is the first cue that draws the insect’s attention toward a potential food source, and shape is a secondary characteristic that promotes the process. When searching for a place to lay one’s eggs, the roles of color and shape are switched. There may also be a difference between male and female butterflies from other species in terms of the ability to learn certain colors; however, there is no differences between the sexes for monarch butterflies.
Defense against predators
In both caterpillar and butterfly form, monarchs are aposematic—warding off predators with a bright display of contrasting colors to warn potential predators of their undesirable taste and poisonous characteristics.
Large larvae are able to avoid wasp predation by dropping from the plant or by jerking their bodies.
Loss of overwintering habitat
The area of forest occupied has been declining and reached its lowest level in two decades in 2013. The decline is continuing but is expected to increase during the 2013–2014 season. Mexican environmental authorities continue to monitor illegal logging of the oyamel trees. The oyamel is a major species of evergreen on which the overwintering butterflies spend a significant time during their winter diapause, or suspended development.
A 2014 study acknowledged that while “the protection of overwintering habitat has no doubt gone a long way towards conserving monarchs that breed throughout eastern North America”, their research indicates that habitat loss on breeding grounds in the United States is the main cause of both recent and projected population declines.
Climate
Climate variations during the fall and summer affect butterfly reproduction. Rainfall, and freezing temperatures affect milkweed growth. Omar Vidal, director general of WWF-Mexico, said “The monarch’s lifecycle depends on the climatic conditions in the places where they breed. Eggs, larvae and pupae develop more quickly in milder conditions. Temperatures above 95°F can be lethal for larvae, and eggs dry out in hot, arid conditions, causing a drastic decrease in hatch rate.”[160] If a monarch’s body temperatures is below 86 °F a monarch cannot fly. To warm up they will sit in the sun or rapidly shiver their wings to warm themselves.
There is concern that climate change will dramatically affect the monarch migration. A study from 2015 examined the impact of warming temperatures on the breeding range of the monarch, and showed that in the next 50 years the monarch host plant will expand its range further north into Canada, and that the monarchs will follow this. While this will expand the breeding locations of the monarch, this will also have the effect of increasing the distance that monarchs must travel to reach their overwintering destination in Mexico, and this could result in greater mortality during the migration.
Milkweeds grown at increased temperatures have been shown to contain higher cardenolide concentrations making the leaves too toxic for the monarch caterpillars, but these increased concentrations are likely in response to increased insect herbivory which is also caused by the increased temperatures, so it is unknown whether increased temperatures in isolation will make milkweed too toxic for monarch caterpillars. Additionally, milkweed grown at carbon dioxide levels of 760 parts per million (ppm) plants were found to produce a different mix of the toxic cardenolides, one that was less effective against monarch parasites.
Conservation efforts
Although numbers of breeding monarchs in eastern North America have apparently not decreased, reports of declining numbers of overwintering butterflies have inspired efforts to conserve the species. Because of concerns over the overwintering numbers, the Center for Biological Diversity, the Center for Food Safety, the Xerces Society and Lincoln Brower have filed a petition to the United States Department of the Interior to protect the monarch by having it federally protected.
On 20 June 2014, President Barack Obama issued a presidential memorandum entitled “Creating a Federal Strategy to Promote the Health of Honey Bees and Other Pollinators”. The memorandum established a Pollinator Health Task Force, to be co-chaired by the Secretary of Agriculture and the Administrator of the Environmental Protection Agency, and stated:
The number of migrating Monarch butterflies sank to the lowest recorded population level in 2013–14, and there is an imminent risk of failed migration.
In May 2015, the Pollinator Health Task Force issued a “National Strategy to Promote the Health of Honey Bees and Other Pollinators”. The strategy lays out current and planned federal actions to achieve three goals, two of which are:
• Monarch Butterflies: Increase the Eastern population of the monarch butterfly to 225 million butterflies occupying an area of approximately 15 acres (6 hectares) in the overwintering grounds in Mexico, through domestic/international actions and public-private partnerships, by 2020.
• Pollinator Habitat Acreage: Restore or enhance 7 million acres of land for pollinators over the next 5 years through Federal actions and public/private partnerships.
se the Eastern population of the monarch butterfly to 225 million butterflies occupying an area of approximately 15 acres (6 hectares) in the overwintering grounds in Mexico, through domestic/international actions and public-private partnerships, by 2020.
• Pollinator Habitat Acreage: Restore or enhance 7 million acres of land for pollinators over the next 5 years through Federal actions and public/private partnerships.
Many of the priority projects that the national strategy identifies will focus on the I-35 corridor extending for 1,500 miles (2,400 km) from Texas to Minnesota that provides spring and summer breeding habitats in the monarch’s key migration corridor.
There have been a number of national and local efforts underway to establish pollinator habitat along highways and roadways, although this effort is controversial. Conservationists are lobbying transportation departments and utilities to reduce their use of herbicides and specifically encourage milkweed to grow along roadways and power lines. Reducing roadside mowing and application of herbicides during the butterfly breeding season will encourage milkweed growth. Conservationists lobby agriculture companies to set aside areas that remain unsprayed to allow the butterflies to breed. This practice is controversial because of the high risk of butterfly mortality near roads, as several studies have shown that millions of monarchs and other butterflies are killed by cars every year. There is also evidence that monarch larvae living near roads experience physiological stress conditions, as evidenced by elevations in their heart rate.
A 2020 resource from the Cooperative Research Programs of the Transportation Research Board developed products for roadway corridors to provide habitat for monarch butterflies and developed tools for roadside managers to optimize potential habitat for monarch butterflies in their road right-of-ways.
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