Biology Reference
In-Depth Information
resources such as food or nest sites. Where these are patchily distributed, males able
to defend the best patches can gain the most mates (Fig. 9.2). It is useful to distinguish
various ways in which such 'resource defence polygyny' can arise, bearing in mind
that we need to consider the costs and benefits for each sex separately (Searcy &
Yasukawa, 1989).
Resource defence
polygyny in birds:
males controlling
the best resources
get more mates
No cost of polygyny to females
In some species the males contribute very little to parental care and so females suffer
little, if any, cost from mating polygynously. For example, female yellow-headed
blackbirds (
Xanthocephalus xanthocephalus
) build their nests in marshes and feed in fields
away from the breeding site. In one study there were apparently no costs or benefits
from settling near other females and they settled more or less at random in the marsh
(Lightbody & Weatherhead, 1988). In yellow-rumped caciques (
Cacicus cela
) females
also do not suffer from sharing a male but they benefit from nesting close together in
safe sites and by cooperative nest defence against avian predators (Robinson, 1986). In
both species females may be largely indifferent to the mating system that emerges,
which is determined simply by a male's ability to monopolize mates. If a small number
of males is able to control the area with the most nesting females then high degrees of
polygyny may occur.
In some species,
females may
experience no
polygyny costs
Cost of polygyny to females
In many species, however, females will suffer costs from polygyny through having to
share either the resources a male controls (food, nest sites) or his contribution to
parental care. Females may be forced to accept these costs if a fraction of the males
control all the suitable breeding habitat, their choice being 'accept polygyny' versus
'forego breeding'. For example, in Leonard and Picman's (1987) study of marsh wrens,
Cistothorus palustris,
females settled with mated males only after all the bachelor males
had paired. For these later settling females there was no choice but to accept the costs
of polygyny.
In other cases, however, most of the males may be able to gain breeding territories.
If there is variation in male territory quality then a female's choice may be 'settle
on a good territory with an already-mated male, that is choose polygyny' versus
'settle on a poor territory with an unmated male, that is choose monogamy'. Jared
Verner and Mary Willson (1966) suggested that females may choose the polygyny
option if the costs of sharing a male's help with parental care were outweighed by
the benefits of gaining access to good resources, such as food or nest sites. Gordon
Orians (1969) presented this idea in a graphical model, known as the 'polygyny
threshold model' (Fig. 9.9).
In many species males with the best territories are indeed the ones to attract the most
females, just as the model predicts. However, showing that females are making the best
choice among the breeding options available is difficult unless a great deal is known
about the costs of sharing and the choices available to them (Fig. 9.10). We consider
two species which have provided excellent systems for testing the model.
In other species,
females may be
forced to accept
polygyny costs
In others, females
may choose
polygyny because
the costs are
outweighed by
the benefits
