Female Butterflies Go for Sparkle -- Not Size -- When Choosing to Mate
Release date: Wednesday, June 29, 2005
BUFFALO, N.Y. -- Size doesn't matter, at least not the size of the
eyespots on a male butterfly's wings when female butterflies consider
Contact: Ellen Goldbaum, firstname.lastname@example.org
Phone: 716-645-5000 ext 1415
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 female butterflies.
purpose of the research was to explore some of the evolutionary reasons
behind butterfly wing patterns in the African satyrid butterfly, Bicyclus anynana.
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 sciences.
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.
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.
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
"Once we found a trait that appeared to be important, we then would exaggerate it or reduce it to pin it down," said Monteiro.
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
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
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 ultraviolet reflectivity.
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.
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.