Butterflies of
Mexico, USA & Canada
The Monarch
Danaus plexippus
LINNAEUS, 1758
Family - NYMPHALIDAE
subfamily - DANAINAE
Tribe - DANAINI
subtribe - DANAINA
Danaus plexippus, USA
© Ingo Arndt
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.
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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
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Distant view of Monarch overwintering site
Monarchs basking on mulefat bushes
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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.
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A
circadian clock employs rhythms
of biochemical, physiological or behavioural processes that
control time based functions. These include daily activities
e.g. the mate location / rest / feeding cycle of males, and the
oviposition / rest / feeding cycle of females. The clock also
influences or controls one-off activities such as emergence and
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 © Ingo
Arndt
Habitats
Because of its
migratory nature the Monarch can be found in virtually any habitat
including prairies, grasslands, deciduous temperate forest, montane
coniferous forest, subtropical 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.
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Danaus
plexippus
© Ingo Arndt |
Limenitis
archippus
©
Benny Mazur |
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