Biology Reference
In-Depth Information
Female choice
The second process suggested by Darwin, namely female choice for the most 'charming
or agreeable' males took longer to gain acceptance simply because Darwin had little, if
any, firm evidence that females did choose. In fact, we had to wait for 90 years since
Darwin first proposed his theory before anyone thought to test it experimentally. Malte
Andersson's (1982) classic experiments on the long-tailed widowbird (
Euplectes progne
)
in Kenya, took advantage of the invention of Superglue. The males of this sparrow-sized
bird have a remarkable tail which is often more than half a metre long (females have
normal, short tails). Males defend territories in grassland to which they may attract
several females for nesting. Their tails are not displayed in contests with other males.
However, whenever a female flies past the male performs a slow, cruising flight in which
his tail is displayed and expanded into a deep keel. Could this extraordinary tail have
evolved by female choice?
Early in the breeding season, Andersson mapped the territories of 36 males and
scored the number of females each had attracted. He then divided them at random
into four groups. In one group, he cut the tails to about 14 cm; he then used these
severed pieces to increase the tails of another group, which became elongated by an
average of 25 cm. The remaining two groups were controls: one was left untouched
while the other had their tails cut and then re-glued without any change in length.
By counting the number of nests in each territory (which measured the number of
females attracted), Andersson showed that before the experimental manipulations
there was no difference in mating success of the different groups, while afterwards
the long-tailed males did significantly better than the controls or the shortened-tail
group (Fig. 7.5). Shortened-tail males did not become less active in courtship
displays, nor were they more likely to give up their territories. Therefore, the increased
success of the elongated-tail group reflected female choice. Andersson's key finding
was that females preferred tails that were even longer than normal-sized tails. This
suggests there must be a balance between the forces of sexual selection (favouring
longer tails) and natural selection (limiting tail length, perhaps because longer tails
hinder survival).
The costs of sexual ornaments have been demonstrated in similar tail manipulation
experiments with barn swallows,
Hirundo rustica
. Males with experimentally elongated
tails paired up more quickly and were also preferred by females seeking extra-pair
matings (Møller, 1988). However, these males were handicapped in their foraging; they
caught smaller prey and grew poorer quality feathers and shorter tails at the next
moult. As a result, they were slower to attract a mate the following year and suffered
reduced reproductive success (Møller, 1989).
Another early elegant experimental study demonstrating female choice is that of
Clive Catchpole (1980; Catchpole
et al
., 1984) on the song of the European sedge
warbler,
Acrocephalus schoenobaenus
. The song consists of a long stream of trills,
whistles and buzzes and is sung by the male after arriving back on its breeding
territory from the winter quarters in Africa. As soon as the male has paired, it stops
singing (this species is monogamous). Catchpole's measurements showed that
males with the most elaborate songs were the first to acquire mates (Fig. 7.6a).
Furthermore, when females were brought into the laboratory and treated with
Malte Andersson's
classic experiment
with long-tailed
widowbirds
Females prefer
elaborate
ornaments …
… but elaborate
ornaments may
be costly to male
survival
