Strategies for
Survival
Mate recognition
Butterflies occur in an
incredible variety of shapes, colours and patterns. Each design
serves a dual role. Firstly it acts as an identity badge,
advertising the butterfly to potential mates. Secondly and equally
importantly it functions to protect the butterfly from predators,
e.g. by using camouflage, disguise, aposematic or diematic
coloration, mimicry or transparency.
It is possible of course
to read too much into the "colour for survival" theme. There doesn't
have to be a survival-related reason for every nuance of pattern or
colour. After all what possible difference could it make if a
caterpillar has a brown head instead of a black one ? Having said
this however it is quite obvious that a butterfly that is
camouflaged as a dead leaf has a considerably higher chance of
avoiding predation than one that is not.
Butterflies have short lives -
typically less than 2 weeks as an adult, so they need to quickly
locate mates. Many species lose their attraction to the opposite sex
within just a day or two of emergence because their pheromones become
exhausted. Rapid mate recognition is therefore vital.
The first stage in the
recognition process consists of intercepting any flying object of
roughly the same size and colour as their own species - in
the neotropics for example it is easy to attract blue
Morpho
butterflies by waving a piece of blue foil in the air.
Purple Emperor
Apatura iris,
male �
Adrian Hoskins
In many genera e.g.
Apatura,
Morpho and
Doxocopa the wings of males have a highly
reflective blue sheen. During flight their wings glint brilliantly in
the sunshine and probably play a major role in mate location and
recognition.
Once initial contact has been made, a combination
of olfactory and visual factors hold the interest of both sexes as a
precursor to the courtship ritual. Visual stimuli include holding the
wings or body at particular angles, flicking the wings, or simply
displaying the patterns by opening the wings.
During the courtship process a
series of exchanges takes place during which various stimuli
trigger either negative or positive responses. A negative response
might indicate that the potential mate was not of the same species, or
was unwilling to mate - females of many Pierids for example signal an
unwillingness to mate by raising their abdomen and outspreading their
wings.
A positive response invariably
triggers another visual, olfactory and tactile stimulus, which leads
to another response. By going through these stimulus / response
sequences the butterflies are able to determine firstly that they
are of the same species, then that they of opposite sexes, and finally
that both are ready and willing to to mate. In some species this
process is very brief, but in others such as the Small Tortoiseshell
Aglais urticae, it is protracted and may
take several hours during which the male doggedly follows the female
from place to place, persuading her to copulate.
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Small
Tortoiseshell
Aglais urticae �
Adrian Hoskins |
Male butterflies will
usually intercept both sexes of their own species. The purpose of
male-male sorties is to challenge and oust an intruding male from
the territory of another, thereby increasing the territory owner's
chances of success with any female that passes by.
Males also "know" that they can often find food by following other
males :
In tropical regions
males of many species visit sandbanks and damp paths to indulge in
"mud-puddling". Their purpose is to imbibe mineralised water, from
which they obtain vital salts. These are passed to females during
copulation and are believed to be essential for the production of
fertile eggs.
Typically just one or
two males will chance upon a suitable feeding spot, but other
butterflies flying past seem able to recognise their brethren on the
ground, and swoop down to join them. The bright patch of colourful
butterflies quickly becomes a magnet to every passing male of the
same species.
Often
several different species may be present at these feeding places. In
these circumstances one might expect each species to be spread
randomly within a large group, but in fact each butterfly polarises
very strongly to it's own brethren, so that each species congregates
as a discrete group.
In calm weather the
butterflies in each group are positioned randomly, but on riverbanks
there is usually a constant gentle breeze, so all the butterflies in
each group tend to face into the breeze as this way they are less
likely to be blown about and lose their feeding spot to a
competitor.
Phoebis
argante and Rhabdodryas trite
aggregating at Rio Shima, Satipo, Peru �
Adrian Hoskins
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