Biology Reference
In-Depth Information
Sexual conflict and polygamy
The assumptions of the polygyny threshold model are like those of the 'ideal free
distribution', which were discussed in Chapter 5. The different resource patches available
are male territories of varying quality and females are assumed to be 'free' to settle
where they choose. Under 'ideal' conditions, they are expected to settle where their
reproductive success is greatest. However, we saw that ideal free conditions often do not
hold in nature because dominant individuals attempt to grab more than their fair share
of resources, so the assumption that polygyny arises from ideal free female settlement
may often be unrealistic. For example, if the first female suffers from the arrival of a
second female then it will pay the first female to try to prevent her from settling. Males,
too, may attempt to change the mating system in ways which are detrimental to female
success. The following two case studies provide good examples of such sexual conflict.
The pied flycatcher (Ficedula hypoleuca)
Rauno Alatalo, Arne Lundberg and their colleagues have studied this bird in the
woodlands around Uppsala, southern Sweden. Males defend nest sites, holes in trees or
nest boxes, and sing to attract a female. Once a male has attracted one female and she
has laid her eggs, he then goes to another nest hole and tries to attract a second female
(Fig. 9.11). Males do not simply advertise from the next nearest nest site but go, on
average, 200 m away and even up to 3.5 km from their first site! About 10-15% of the
males succeed in gaining a second female. They then desert her and go back to help their
first female with chick-feeding. Compared to monogamous females the first female
suffers little, if at all, from polygyny because she usually gains the male's full-time help,
but the second female, who is left to raise her brood on her own, suffers reduced success,
raising on average only 60% of the number of young she would have gained in
monogamy (Alatalo
et al
., 1981).
Why, then, do females ever settle polygynously? Three hypotheses have been proposed.
In pied
flycatchers,
second females
suffer, so why do
they settle
polygynously?
Three hypotheses
(i)
The 'sexy son' hypothesis
. Weatherhead and Robertson (1979) have suggested that
although second females produce fewer offspring, this may be offset if they have
sons who inherit their father's ability to be polygynous. The female loses out in the
first generation, but then makes up for this in the second generation when her sexy
sons sire lots of grandchildren compared to the sons of monogamous females.
According to this hypothesis (which derives from Fisher's sexual selection argument,
Chapter 7), secondary females are still making the correct choice when they settle
polygynously, but offspring quality is another factor which must be taken into
account in the
y
axis of the polygyny threshold model (Fig. 9.9).
For the pied flycatcher, calculations show that the heritability of male mating status
would have to be 0.85 for 'sexiness' of sons to offset the loss in offspring numbers.
The heritability is not known because young birds disperse and breed away from
their natal area. However, the probability that an individual male is polygynous in
successive years is only 0.29, which must give an upper limit to the heritability
value. Therefore, we can reject this hypothesis.
