Butterflies of North America
The Monarch
Danaus plexippus
LINNAEUS, 1758
Family - NYMPHALIDAE
subfamily -
DANAINAE
Tribe - DANAINI
subtribe - DANAINA
introduction
|
habitats |
lifecycle |
adult behaviour
|
 |
|
Danaus plexippus, USA,
( image ©
Ingo Arndt / Papadakis ) |
Introduction
The Monarch is the most famous migrant in the butterfly world.
Its
powers of migration are so great that it has been able to
spread to across the Americas from Canada to Peru. The first
known record outside of America was of 3 specimens which crossed
the Atlantic and were seen in Britain in 1876. Since then the
Monarch has spread to north Africa, Europe
and even to India. It has also crossed the
Pacific ocean, reaching Australia, New Zealand and Papua New
Guinea.
In North America
millions of Monarchs
migrate annually over a distance of 2000 miles ( 3200km ) between their breeding territories in Canada and
their
southern over-wintering grounds in Mexico. Tagging of individual butterflies has proven that they regularly cover distances
of up to 1100 miles in just a few days.
Each
autumn as the climate cools in North America, vast numbers fly
south, channelling into a few forested areas in the Mexican Highlands. During
the winter months, fir trees in the tiny El Rosario sanctuary become festooned
with millions of Monarchs. They totally cover the leaves, branches and trunks,
sometimes even causing trees to fall under their weight.
|
It has been estimated that an incredible 800 million
Monarchs were present in the reserve in the mid-1990's, but
numbers dropped to about 100 million in 2004. The reasons for the
variation are partly climatic, but the collapse is mainly
attributable to illegal logging in the reserve. |
In February
& March they awaken from hibernation and the air becomes a swirling mass of orange and black as tens of thousands of Monarchs take to the air. As the days get
warmer they start to filter out of the sanctuaries and begin their
return journey northwards. The females pause to lay eggs as they travel,
creating temporary colonies along the route. The progeny also migrate
north, laying their own eggs. Most of the original butterflies probably
perish during the return journey, but there is evidence to show that
some manage to return to the original breeding areas in the north.
It is little
known that these amazing butterflies also regularly overwinter in small numbers
in arid desert locations such as Saline valley in California - a moon-crater
shaped valley about midway between Mount Whitney and Death Valley in the Mojave
Desert.
Monarchs
regularly survive the winter there despite the very low daytime humidity ( 5-25%
), scant winter rainfall and the fact the only evergreen vegetation in mid and
late winter are bushes such as tamarisk, mulefat and creosote. One problem with
the Saline Valley habitats however is that once every five years or so,
overnight temperatures drop several degrees below zero, and freeze most or all
of the Monarchs.
Check the links below to see
habitat shots and Monarchs at the Mojave Desert overwintering sites :
Mojave desert aerial view
|
Distant view of Monarch overwintering site
Monarchs basking on mulefat bushes
|
Monarchs on tamarisk in Saline Valley
The
Monarch's annual migration is
controlled by a "time-compensated sun compass" that depends on light receptors
and a circadian clock, both built into the antennae. When
scientists removed the antennae from one group of Monarchs they flew strongly
but in random directions, but a control group with their antennae intact all
flew in the same direction - i.e. their south-westerly migration route. In another
experiment the antennae of some were painted with black enamel, and these
butterflies when placed in a flight simulator all flew together, but in the
"wrong" direction compared to their normal migration route. Another group had
their antennae painted with transparent paint, and these all migrated together
in the right direction.
|
A
circadian clock employs
rhythms of biochemical, physiological or behavioural
processes which control time based activities. These include
daily activities such as the mate location / rest / feeding
cycle of males, and the oviposition / rest / feeding cycle
of females. The clock also controls "once only" activities
such as emergence, and annual / seasonal activities such as
migration. The precise timings are modified by environmental
factors e.g. sunlight, rain and temperature. |
Insects do not "know" where they are, so cannot navigate by
landmarks. They navigate primarily by using an internal compass in
combination with a circadian clock which determines the timing of
migrations. Monarch migration shifts clockwise at a rate of 1
degree per day for all generations of the annual migratory cycle.
Northward migration from the overwintering site in Mexico is
triggered by the spring equinox. Migration routes and speeds are
of course influenced strongly by winds and weather conditions.
Diagram illustrating Monarch migration hypothesis:
Although Monarchs in North America are migratory in behaviour,
there are also non-migratory populations which breed all year
round in Central America, and on islands in the Caribbean and the
Pacific. Research by Altizer has shown that the migratory
subspecies have evolved wings which are about 14% larger in
wingspan than those of non-migratory subspecies, presumably to
allow them to travel vast distances with greater ease. Breeding
experiments proved that these size differences are inherited,
rather than being influenced by environmental factors such as
warmer temperatures.
|
 |
|
overwintering Monarchs in Mexico
( image ©
Ingo Arndt / Papadakis )
|
Habitats
Because of its
migratory nature the Monarch can be found in virtually any
habitat
, including grasslands & prairies, deciduous temperate forest,
montane pine forest, sub-tropical rainforest, coastal habitats,
deserts, parks, gardens, cities etc. It can be found at
elevations between sea-level and at least 3400 metres. The
butterflies can also be seen out at sea, and often settle on
ships.
Monarchs will
breed almost anywhere where the larval foodplants are available,
typically in fields, meadows, forest glades and roadside
habitats at elevations between sea level and about 1000m.
Lifecycle
The egg is shaped like a tall dome, straw coloured, and covered
in vertical keels, each linked by numerous small horizontal
ridges. It is laid singly on the underside of leaves of the
foodplants which are almost invariably
Asclepias but also include
Calotropis ( Apocynaceae ).
The caterpillar
when fully grown is white, with each segment marked with narrow
black and yellow vertical bands. The 2nd thoracic segment and
8th abdominal segment each bear a pair of black whip-like
protuberances. Caterpillars are often parasitised by a tachinid
fly Lesperia archippivora.
The larval
foodplants Asclepias contain
cardenolides - toxins which can induce cardiac arrest in small
vertebrates. The toxins are sequestered by the caterpillars, and
passed on to the adult butterflies which utilize them for
defence against insectivorous birds, reptiles and rodents.
The pale green
chrysalis is plump and barrel-shaped, with the abdominal
segments compressed, forming a dome. At its widest point there
is a narrow abdominal band studded with yellow and black dots.
The chrysalis is suspended by a stout cremaster from stems on or
near the foodplants.
Adult behaviour
The butterflies
have a powerful but fluttering flight, interspersed with periods
of soaring and gliding in
wide circles as they fly from one clump of flowers to another.
They settle frequently to nectar at
Asclepias, Aster,
Cirsium,
Dispacus, Solidago,
Syringa and
Vernonia.
Courtship takes
place in late morning at which time the male pursues the female
in flight, nudging and cajoling her until she settles,
typically on a bush, where copulation takes place.
The bodies of
Monarchs contain cardenolides derived from the caterpillar's
foodplant Asclepias. Any bird
eating a Monarch is likely to be affected by vomiting, muscular
spasms and visual disturbance. Birds have the ability to learn
and remember the patterns and colours of toxic butterflies, so
after suffering the unpleasant experience of eating one Monarch
they are unlikely to attack another.
Some birds
however have learnt how to overcome the butterfly's toxic
defences, e.g. black-backed orioles
Icterus abeillei have discovered how to strip out the
body contents of Monarchs and discard the toxic elements. At one
of the overwintering sites at Michoacan in Mexico it was
estimated that between 3500-35000 Monarchs were eaten every day
during the 1978-79 winter by black-backed orioles. Total
mortality resulting from attacks by black-backed orioles and
black-headed grosbeaks
Pheucticus melanocephalus has been
estimated at 1.8 million Monarchs per year at Michoacan.
As is the case
with most toxic or unpalatable butterflies, Monarchs are
involved in mimicry
rings whereby several species, some toxic and some
palatable, share a common pattern - in this case orange wings
with black veins and white spots. Because they recognise and
remember the
patterns they are fooled into rejecting the palatable species
due to their similarity to the toxic Monarchs.
The most well
known mimic is the Viceroy Limenitis
archippus, which for many years was thought to be a palatable species, and was therefore regarded
to be a Batesian mimic of the Monarch. It is now generally
accepted that things are not that clear cut - the degree
of palatability encompasses a broad spectrum from species to
species, and between individuals of any given species.
|
 |
 |
|
Danaus
plexippus (
image
© Ingo Arndt ) |
Limenitis
archippus (
image
©
Benny Mazur ) |
|
