Biology Reference
In-Depth Information
Females increase
the genetic
quality of their
offspring by
extra-pair matings
genes for their offspring by selecting those males with the best developed traits? The key
test is to compare the success of the extra-pair sired young with that of their within-pair
sired half-siblings, raised in the same nest. This comparison has been made for blue tits
and collared flycatchers; in both cases the extra-pair sired young survived better, suggesting
that females did indeed gain a genetic benefit from extra-pair matings.
One of the problems of showing male 'good gene' effects is that females themselves
may invest more if they mate with an attractive male. In mallard ducks, for example,
females lay larger eggs (which increases offspring survival) after copulating with more
attractive males (Cunningham & Russell, 2000). Therefore, the increased success of
young sired by more attractive males may partly (or entirely) reflect increased female
investment. A study of the brown antechinus (
Antechinus stuartii
), a marsupial mouse-
like mammal from Australia, has eliminated this confounding factor (Fisher
et al
.,
2006). The antechinus is semelparous, which means that the female has just one litter
in her lifetime, after which she dies. Males, too, are semelparous and play no part in
offspring care. In this case, the female should invest maximally in her litter (there are no
future litters to save resources for). Field observations showed that females usually
mated with several males during their two-week mating period. Why? In a carefully-
controlled experiment, females were caught and mated in the laboratory with a pool of
41 'stud' males. Each female was allowed to mate three times. One group of females
(monandrous) mated three times with the same male. A second group (polyandrous)
mates three times, but with three different males (once each). When the young were
born, they were marked individually with little microchips under the skin. The mothers
and their litters were then released back into the wild in nesting boxes. Two months
later, the number of surviving young was scored from each litter.
The results were striking. There was no difference between the two female groups in
their initial litter size, but polyandrous females produced three times the number of
surviving young to the post-weaning stage. A closer look at the success of the
monandrous group was revealing; a monandrous female did better if the male she had
been paired with was a good competitor in polyandry, in other words had gained high
paternity in competition with other females. This suggests that the advantage of
polyandry to a female was she was more likely to gain a good genetic sire for her offspring
(Fisher
et al.,
2006).
The brown
antechinus:
multiple mating
improves a
female's
reproductive
success
Constraints on mate choice
and extra-pair matings
These results leave us with two puzzles. Firstly, why do females not choose to mate with
an ideal male in the first place? In cases where competition for the best males is intense,
females may be forced to settle for less than the best social mate and then rely on extra-
pair matings to increase the genetic quality of their offspring.
Secondly, what limits extra-pair mating? Two hypotheses have been suggested. One is
that in cases where males provide help with parental care, the social mate would reduce
his care in response to cuckoldry. There is good evidence for this in collared flycatchers:
when a female was removed experimentally for one hour during her fertile period (to
simulate the female going off to seek extra-pair matings), then her social mate reduced
Cuckolded males
may reduce
parental care
