Biology Reference
In-Depth Information
between mated and unmated males. In this situation females could clearly sample
both males (some were seen to do so) and the songs of both could be heard from either
nest site. In 20 such paired choices, nine females settled with the unmated male and
11 with the mated male - clearly no difference. Furthermore, the females who chose
the mated males raised significantly fewer young than those who later chose the
unmated males they had rejected. This result supports the deception hypothesis;
females did not discriminate between mated and unmated males even when they had
a simultaneous choice between them, and even though it would have paid them to
make a choice.
The dunnock (Prunella modularis)
Conflicts of interest have led to a very variable mating system in another European
songbird, the dunnock (Fig. 9.12), including simple pairs (monogamy), a male with two
females (polygyny) and a female with two (unrelated) males (polyandry).
A female has least success in polygyny, where she has to share a male's help with
parental care. She has greater success with monogamy, where she gains a male's
full-time help with chick feeding. And she has greatest success of all with polyandry
because if she copulates with two (unrelated) males she then gains both their help
and three adults provisioning a brood increases the number of young that survive
(Fig. 9.12a). In polyandry, shared copulations often lead to mixed paternity in the
brood. Observations and experiments (temporary removal of a male for parts of the
female's fertile period) revealed that a male will help to feed the brood only if he
gained a share of the matings. Furthermore, he increased his help in proportion to
his mating share, which predicts his paternity share. A female maximized the total
care she gained from her two males if she gave equal mating shares to each male
(Davies
et al
., 1992).
From a male's point of view, however, reproductive success is least in polyandry (the
system where a female does best) because although more young are raised through
extra male help, the increased production of a trio-fed brood does not compensate a
male for shared paternity. In fact, a male does best in polygyny (the system where a
female does least well) because despite the cost that each female suffers, the total output
of two females with part-time help exceeds that from monogamy (Fig. 9.12a).
These conflicts of reproductive interests make good sense of male and female conflicts
in behaviour. In polygyny, the dominant female attempts to drive the other female away
to claim the male to herself, while the male tries to keep between his squabbling females
so that both remain with him. On the other hand, females encourage copulations from
subordinate males in the hope that they will remain and provide parental care, while
dominant males attempt to guard the female and drive subordinate males away to claim
full paternity for themselves. The variable mating system can, therefore, be viewed as
the different outcomes of these conflicts of interest. Sometimes the conflict reaches a
'stalemate', in which two males end up sharing two females (polygynandry; Fig. 9.12b).
Here the dominant male is unable to drive the other male off to claim both females for
himself (polygyny) and the dominant female is unable to drive the other female off to
claim both males for herself (polyandry).
So the key question to ask is this: under what circumstances can particular
individuals gain their best mating option despite the conflicting interests of others?
In dunnocks,
males prefer
polygyny, females
prefer polyandry
Polygynandry as a
'stalemate'
outcome to the
conflict
A variable mating
system reflects
the different
outcomes of
sexual conflict
