Why some female birds havebright plumage

HOW the peacock got his tail is one of the “Just So”
stories of natural history which biologists like to think
they have cracked. His tail is for showing off to the
ladies (and also to rival males) just how fit he and his
genes are. A less than perfect tail means no offspring.
Genes for spectacular male tails are thus preserved and
promoted over the generations in a process that is
called sexual selection.

There is, though, a problem with this story. Peahens,
though not as showy as cocks, are by no means dowdy.
Their heads have fetching feathered crests, and their
necks are a beautiful iridescent blue. Evolutionary logic
suggests this is foolish. Such flummery is
physiologically costly to grow and is likely to attract
predators. If you do not have to strut your stuff to get a
mate, why not dispose of it completely?
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Even more confusingly, there are many species where
both sexes are showy—the Gouldian finches in the
picture above, for example (the female is on the right).
So, though no one thinks the theory is wrong, as far as
it goes, it clearly does not go far enough. To
understand things better James Dale of Massey
University, in Auckland, New Zealand, and his
colleagues have therefore examined the plumage of
both sexes of all 5,983 species of passerine bird
(peafowl, not being passerines, are not among them),
and compared them in exquisite detail.
First, Dr Dale and his colleagues had to devise a way to
deal with the 11,966 types of plumage they had set out
to examine. Using as their reference the “Handbook of
the Birds of the World”, regarded by ornithologists as
the definitive work in the field, they picked six points
on a bird’s body (nape, crown, forehead, throat, upper
breast and lower breast) and performed a spectral
analysis of each to measure how red, green and blue it
was. The average of these values let them plot each
plumage type as a dot on a graph with three axes—
namely red, green and blue.
To translating these data into a “showiness” score, the
researchers started from the fact that, despite the
exceptions, showiness is still more a male than a
female phenomenon. They therefore calculated, as a
proxy for showiness, a “maleness” score for each dot,
regardless of which sex it represented, by counting the
sexes of its nearest 120 neighbours in the graph (ie, the
nearest 1% of dots). It was then a question of running
these scores against other characteristics, such as the
size of a species, its habitat and its pattern of family
life.
The researchers’ first observation, as they report this
week in Nature, was that in polygynous species (ie,
those where a few males monopolise all the females)
and a consequent lack of male involvement in parental
care, males were more colourful than females. This is
precisely what the theory of sexual selection would
predict. What it would not predict in its most simple
form, though, was a second finding—that females in co-
operatively breeding species (those in which, for lack of
other opportunities, several females collaborate to raise
the young of only one of them) are more ornamented
than those in which all adult females have a chance of
breeding. In this case it is females who are competing
for the right to reproduce, thus putting themselves in a
more male-like position.
Another widespread belief Dr Dale and his colleagues
confirmed is that tropical species are more colourful
than those from temperate climes. But again, there was
a twist—the effect was much more marked in females
than in males. Something about the tropics favours
colourful females. It may be that tropical birds, which
face more intense competition for food and nesting
sites than temperate ones do, because the tropics have
more species, form more stable and collaborative pair-
bonds than do temperate birds. In these circumstances
males also need to be choosy, and females competitive.
Selection for gaudy plumage therefore works in both
directions.
The final effect the researchers found was that big
species are more colourful than small ones. That is true
of both sexes, and probably reflects the fact that bigger
birds are more difficult prey and thus have less need to
hide. When released from the threat of predation, then,
females tend to be gaudier. That suggests gaudiness is
always good when it can be got away with (for even in
a promiscuous species, pretty females are likely to be
at an advantage to ugly ones)—and probably explains
the decorated necks and heads of peahens, which are
among the biggest of birds.
Putting these results together, then, suggests that what
is happening in the arena of sexual selection is as
much to do with females as with males. Just as females
are half the world, so the conventional explanation of
the peacock’s tail, though not wrong, is only half the
story.