Not so
lady-like?
At first she is shy and reserved, just
like a nice girl ought to be. Then she is transformed into a macho
chick.
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It is rare to find species in the
animal kingdom where both the male and the female are highly
colourful. But this is the case with the little two-spotted
goby. Here a lovely orange lady has found a gentleman with
elegant blue stripes.
Photo: E. Forsgren, NAS, USA
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The two-spotted goby is a tiny creature that
lives in shallow waters along the Norwegian coast. It is probably
Norway’s most common shallow water fish, and plays an important
role in the coastal ecosystem, eating tiny crustaceans and in turn
being eaten by young cod. The twospotted goby is just 4 to 5 cm
long, but both the male and female are beautifully coloured – a
feature that can be somewhat uncommon in the animal kingdom. The
male has bright blue stripes on its fins, while the female has an
equally bright orange belly.
For years, researcher Elisabet Forsgren has
kept her feet on the ground, studying the tiny gobies, while one
of her colleagues, Prof. Trond Amundsen, has had his head in the
clouds, studying birds’ mating behaviour. But when the two got together
on a collaborative project, they discovered some surprising things
about the two-spotted goby’s sex life.
REVERSED SEX
ROLES
The two-spotted goby lives for only one year, but can spawn
5 or 6 times during the mating season, from May to July. In the
beginning, everything pretty much follows the norm: The male shows
off, flirts and tries to look tough, fights off rivals and does
more or less whatever it takes to mate. The female watches demurely,
but can be persuaded to mate by an extra-attentive male.
But halfway through the season, everything
turns up side down. Suddenly, the female is the aggressive one.
She flirts and tries to look good – while the male becomes passive.
There are species where sex roles are constantly
reversed. But this kind of somersault in the middle of the breeding
season is extremely rare; in fact, this kind of role reversal midway
though the breeding season has never before been seen in vertebrates.
So why should it happen with two-spotted gobies? The short answer
is: nobody really knows for certain.
However, researchers are happy to speculate.
“We believe it has something to do with the gradual disappearance
of males during the course of the breeding season,” Forsgren and
Amundsen said. “Males might quite simply die off from all the hard
work. Not only do the males have to fight for females again and
again; they also have several broods of eggs to take care of – which
is a job that male two-spotted gobies assume. It’s possible that
all their fighting, flirting and parental duties simply kills them.
Whatever the cause, we have found that there are very few males
around by the mating season’s end. Those who are still around are
surrounded by great numbers of females – who in turn behave like
the most aggressive macho males in order to produce yet another
brood of eggs.We think the females are driven by pure desperation.”
ORANGE FEMALES
GET THE FIRST DANCE
It’s highly uncommon to find species where both the male
and the female are colourful. In the animal kingdom, colours and
finery are usually reserved for the male, while the female blends
in with the background. This makes sense when sex roles are based
on competitive males and choosy females.
However, there are a few species where the
female is bright and the male is dull. In those cases the sex roles
are usually reversed, and the females flirt and compete with each
other while the males pick and choose.
But it is rare that both males and females
are extravagantly – and differentially -- coloured. Researchers
have previously assumed that female finery was simply a side effect
of male evolution, and without any real significance.
Before Forsgren and Amundsen’s work on the
two-spotted goby, nobody had studied species where the female is
attractive in her own way and sex roles are not permanently reversed.
“The theory that female attractiveness is
simply a side effect of male evolution is simply not applicable
in these cases,” the research team points out.
The two biologists first believed that the
sex roles were traditional in the two-spotted goby, and they didn’t
understand why the female had such a lovely, orange belly. They
soon realized that some of the females were more orange than others
and wanted to find out whether males preferred more highly coloured
females.
That turned out to be the case. The males
went straight for the females with the brightest colours. As much
as 80 per cent of the flirtation was targeted at the most colourful
females. The same thing happened when the researchers picked out
two pale females and coloured the belly of one bright yellow while
the other was treated with uncoloured Indian ink. The yellow female
was immediately asked to dance, while the other remained a wallflower.
“The females’ glowing colours must have developed
for the benefit of the female, perhaps to attract males,” the two
researchers conclude.
NEW PERSPECTIVES
Åsa Borg and Jens Bjelvenmark are research fellows who have
also participated in the research on the tiny two-spotted goby.
Results from the study have attracted major international scientific
and media attention. The research team is continuing with new studies
of the two-spotted goby's peculiar sexual life.
”There are still many unanswered questions
related to the females.We would like to find out whether there is
a connection between colour and quality. It appears that the females
with the most orange colouring lay their eggs sooner than the others,
but we don't know if they produce better offspring," the researchers
explain.
They would also like to look more closely
at the dynamic sex roles in the fish, to understand the mechanisms
behind the sudden somersault. Why do the roles reverse at that precise
moment? Is it hormonally connected? And what happens to the males?
Do they die, and if so,why?
“The two-spotted goby turned out to be a
lot more complex and interesting than we could have possibly imagined.
Its situation might apply to other species as well, which opens
the door for new and exciting perspectives of sex roles in the animal
kingdom,” Forsgren and Amundsen conclude.
By Lisa Olstad
Contact: Elisabet Forsgren, Department of
Biology, NTNU
Tel: +47 73 59 53 65, email: elisabet.forsgren@bio.ntnu.no
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