New Chapter 9 · 2002. 11. 11. · 3rd lecture: Migration of the Monarch butterfly. The monarch butterfly is an example of an insect that exhibits a fascinating ... Why don’t they

Chapter 9Choosing where to live

3rd lecture: Migration of the Monarch butterfly

The monarch butterfly is an example of an insect that exhibits a fascinating migration phenomenon

This insect originally evolved in the tropics, but secondarily evolved the

ability to exploit a superabundant but largely non-utilized food resource:

milkweed plants. The capacity to eat these poisonous plants may stem

from a genetic mutation.Each year it migrates to

North America to feed on milkweed plants. However, owing to its tropical origins, it cannot tolerate the cold

winters, and must return to a subtropical area.

Each Fall, the monarch migrate to one of two overwintering areas, depending on whether they are west or east of the

Rockies

Over 100 million monarchs migrate to a handful of high-altitude overwintering sites in central Mexico

This aggregation contains millions butterflies, which are taking flight above the oyamel fir forest, in which they overwinter

Within each aggregation, the butterflies festoon the trees, and on sunny days, fill the air with a magica

In the early spring (or summer) the monarchs migrate north, with successive generations taking advantage of seasonally abundant milkweed plants at progressively higher latitudes

Why does the late summer population of monarchs initiate a perilous and energetically costly migration to the high-altitude overwintering sites of central Mexico each year?

The most likely answer to these questions is that the Monarch originally evolved in the tropics, and thus lacks the cold hardiness necessary to

tolerate frigid winter conditions

Why don’t they just hibernate in tree crevices or under logs in the US and Canada, like lots of other butterflies?

What is so special about the climatic conditions at the overwintering sites?

On average, the climatic conditions are

cold enough to keep metabolic demands at a

minimum, but not so cold that they kill the

monarchs

Unfortunately, logging in the overwintering

sites is threatening this delicate balance

In addition to the dangers of the long migration, the monarch must also contend with large numbers of hungry predators at the overwintering sites. This is because the large aggregations constitute a food bonanza for potential

predators

What is the response of the local bird and mammal fauna? Do they decimate the colonies? If not, why?

There are 6 different species of rodent, and at least 36 species of

insectivorous bird in the overwintering areas

However, only one species of rodent (Peromyscus melanotis) and 2 species of bird eat large numbers

of monarchs

The P. melanotis not only recruit to the colonies in large numbers, but

they also initiate high levels of reproduction (at a time when

conspecifics are not breeding)

In contrast to the other species of rodent, P. melanotis is less

sensitive to the taste and toxicity of the monrachs defensive chemicals

Thus, the monarch’s defensive chemicals deter some but not all

of the potential predators

So, how did these spectacular migration phenomena evolve?

Competition for resources (e.g., food, nesting areas) created a situation where many individuals experienced high mortality

An individual with a mutation or unusual genetic variant appeared in the population that

(a) was able to exploit a new food resource (e.g., milkweed plants)(b) happened make have a tendency to fly (or swim) long distances

Accordingly, these aberrant individuals would have experienced higher reproductive success than conspecifics lacking these behavioral attributes

Scenario 1

Animals may have evolved a relatively simple migratory pattern in distant geological past, when, for instance, the continental land masses were closer together, or connected by land bridges (owingto lower sea levels. However, as the continental land masses broke up, or the sea level rose, the animals had to increase the length of their migration

Scenario 2 (not mutually exclusive with scenario 1)

The Cretaceous Period144-65 million years ago

Jurassic Period206-144 million years ago

Triassic Period248 - 206 million years ago

To which migratory phenomenon would thisscenario be most relevant? Why?