Biology Reference
In-Depth Information
end of the experiment. Polygynous females suffered higher predation and their
young were fed less frequently due to reduced male assistance, so they had
significantly lower reproductive success than monogamous females. So there was
indeed a polygyny cost to females (Pribil, 2000).
(ii)
Is female settlement influenced by male mating status?
In another experiment,
female removals were again made to give further arriving females a choice
between two male territories of similar quality, one with one female and one
with no females. The next female to arrive therefore had a choice between
polygyny and monogamy. In all 16 cases the first female to settle chose the
monogamous option (Pribil & Picman, 1996). Therefore, male mating status
influenced female choice.
(iii)
Can increased male territory quality induce females to choose polygyny?
In a third
experiment, 16 pairs of male territories were chosen, each pair again matched for
vegetation quality and numbers of already-settled females. One territory of each
pair was allowed to retain one female (any further females were removed) and the
quality of this territory was improved by adding wooden nesting platforms of
cattail shoots, supported underneath by chicken wire and placed over open water.
Females preferred to build nests over water because this reduced predation by
raccoons and weasels, so these territories were given additional good nesting sites.
The other territory of each pair had all the females removed and was also provided
with the same additional cattails, but this time the platforms were placed over dry
land, less suitable for nesting. On these territories, cattails over water were also
trimmed.
Thus, newly-arriving females now had a choice between joining an already-paired
male on an improved quality territory versus an unpaired male on a reduced quality
territory. There were 16 pairs of territories in this experiment. In two pairs new
females settled on both territories on the same day. In the remaining 14 pairs, only one
new female settled; in 12 cases she chose polygyny on the good territory and in only
two did she choose monogamy on the poor territory. Therefore, females can be induced
to choose polygyny, provided the male's territory is of sufficient quality (Pribil &
Searcy, 2001).
(iv)
Is female choice for polygyny on a good territory adaptive?
Pribil and Searcy
(2001) then calculated that the benefit of nesting over water was 1.02
more young raised, compared to nesting over land. The cost of polygyny to
a female was 0.62 fewer young raised, compared to monogamy. Therefore,
female choice for polygyny on a good territory was adaptive; the benefits
exceeded the costs.
In red-winged
blackbirds,
females benefit
by choosing
polygyny on a
good territory
rather than
monogamy on a
poor territory
These results provide strong support for the polygyny threshold model, but the model
may not apply to all populations of red-winged blackbirds. In another study in
Pennsylvania, females did not suffer costs from polygyny (Searcy, 1988). Thus, the costs
and benefits of female choice are likely to vary geographically depending on local
predation pressures and food supply. Further studies are needed to examine how females
make their choices in the face of varying selective pressures.
