Sparkle matters as far as butterflies are concerned, not size29 Jun 2005
Size doesn't matter, at least not the size of the eyespots on a male
butterfly's wings when female butterflies consider potential mates.
Instead, females are attracted to the "sparkle" created by the
ultraviolet reflectivity of the pupils, the white circles at the center
of eyespots, according to new research from University at Buffalo
The research, to be published online June 29 in Proceedings of the
Royal Society B: Biological Sciences, overturns previous work
indicating that larger eyespots might be considered more desirable by
The purpose of the research was to explore some of the evolutionary
reasons behind butterfly wing patterns in the African satyrid
butterfly, Bicyclus anynana.
The findings were surprising in the context of the natural world, where
dramatic colors and physical features often win the sexual-selection
game, according to the UB researchers.
"This is one of the first studies to show that such a small pattern
element really matters in female choice," said Antonia Monteiro, Ph.D.
a co-author on the paper and UB assistant professor of biological
"We always think of something huge or ornamental as determining sexual
choice," noted Kendra Robertson, co-author, who recently received her
master's degree from the Department of Biological Sciences in the UB
College of Arts and Sciences.
In a series of carefully controlled tests on both the dorsal and
ventral sides of wings, Robertson induced a dozen subtle variations in
the eyespot size and pattern of males and then studied how they
influenced female's mating decisions.
"It's very easy to change the size, color composition and shape of
these patterns, using artificial selection," said Monteiro. "The
question then becomes, 'Why do these populations remain unchanged?'
What are the selective forces that maintain these patterns constant
through time in any one species in nature?"
In this butterfly species, females make the ultimate decision about whether to mate.
The UB researchers altered wing-pattern elements through carefully
painting the wings or by pairing males displaying traits of different
size and color. They then tested female preference for wing size,
eyespot size, quantity of eyespots on the wing, eyespot and pupil
color, and pupil reflectivity.
"Once we found a trait that appeared to be important, we then would exaggerate it or reduce it to pin it down," said Monteiro.
None of the variations induced on the ventral side appeared to have any
affect on the females' mating decisions, leading the researchers to
conclude that the ventral side of the wing does not play a role in the
But when the researchers painted the white pupil on the dorsal side
with black paint, thereby eliminating the pupil, these males were much
less desirable to females by a ratio of two to one, demonstrating
clearly that females preferred the presence of the white pupil.
However, a large white pupil, about twice the diameter of a natural
pupil, also was not found desirable by females, indicating strong
sensitivity to a set of rather narrowly defined features, such as
eyespot pupils that measure approximately half of one millimeter.
The most conclusive finding resulted when the researchers painted the
white pupils in male eyespots on the dorsal side with a plant extract,
rutin, which maintained the pupils' whiteness, but eliminated their
"When there was no UV reflectivity, which butterflies can see, females
registered a strong distaste," said Monteiro. "Selection against the
absence of UV reflectivity was as strong as selection against the
absence of a pupil altogether."
The reasons for this phenomenon are complex, but Robertson noted that
the UV reflectivity may be important in what is known as photic
stimulation -- a flashing light effect -- during the series of events
that lead up to mating.
"When the male approaches the female, he opens and closes his wings in
rapid succession so she can observe his pupils," she explained. "We
believe the purpose of the fluttering of his wings is two-fold: to
spread pheromones to her antennae and to stimulate her visually. The
female appears to be very sensitive to this rapid flickering, which
probably looks to her like a strobe-light effect."
Robertson added that while these conclusions are applicable only to
this particular species of butterfly, other species of related
butterflies feature much broader UV reflective patterns on their wings
in the form of blue or violet bands of coloration.
"Our assumption is that they are there for sexual selection purposes as
well," she said, "but we still don't know what causes the change from a
female's preference for relatively small pupils in this species to the
likely female preference for much larger UV-reflective patterns in
these other species."
Monteiro says her next step is to study the role of eyespots in male
mate choice, since females also display them and it is not clear who
actually has a chance to observe them, since the female butterfly
usually hides them at rest.
The research was funded by the National Science Foundation.
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