Biology Reference
In-Depth Information
(b)
(a)
Monogamous
A
C
2nd polygyny
PT
B
B
A
Territory quality or
quality of breeding situation
Fig. 9.9
The polygyny threshold model. (a) A female has the choice of settling with an
unmated male on a poor quality territory B, or with an already-mated male on a good
quality territory A. (b) Female reproductive success increases with territory quality. There
is a cost C of sharing with another female, so the curve for the second female in
polygyny lies below that for a monogamous female. Provided the difference in territory
quality exceeds PT (the polygyny threshold), a female does better by choosing to settle
with an already-mated male on territory A rather than with an unmated male on
territory B. Modified from Orians (1969).
Great reed warblers (Acrocephalus arundinaceus)
This species breeds in reed beds on lake edges in Europe and Asia, weaving its nest
around the reed stems. In a study in Sweden, Bensch and Hasselquist (1992) captured
some newly arrived females in spring and fitted them with radio transmitters. They then
released them onto a study area where male territories had been mapped to see how
they sampled territories before pairing. Most females paired up within 24 hours having
visited the territories of from three to eleven different males, sometimes going back to
pair with a male they had previously sampled. Some females selected already-mated
males despite the conspicuous presence of another female on the territory, and even
though they had previously sampled the territories of unmated males. These
observations show that females sample and choose male territories in exactly the way
envisaged in the polygyny threshold model.
Do females make the best choice available? This is a more difficult question to answer.
In another study of the same species in Lake Biwa, Japan, Ezaki (1990) found that from
30 to 80% of the males were polygynous each year, some attracting up to four females
to their territories, while other males were monogamous or remained unpaired. The
polygynous males were those who claimed territories containing the best nest sites,
namely dense reeds where predation was lowest. Females who settled polygynously as
second females did not seem to suffer from their choice because they did at least as well
as simultaneously nesting monogamous females on poor territories. Thus the
Evidence for the
polygyny
threshold model
in great reed
warblers
