Mating Systems - An Introduction to Behavioural Ecology - page 277

Polyandry threshold
In dunnocks, polyandry is advantageous to the female but not to the alpha male. Indeed,
it occurs despite the best attempts of the alpha male to prevent it! In theory, however,
there may be conditions when the increased production of offspring from cooperation
by a team of males does offset the costs to an individual male of paternity sharing.
Therefore, there would be a 'polyandry threshold' (analogous to the 'polygyny threshold'
discussed earlier), at which males would do better by agreeing to share a female rather
than attempting to go it alone in monogamy (Gowaty, 1981). Two situations in which
the benefits of cooperation among males are likely to outweigh the cost of sharing
paternity are: a scarcity of food (so more than two individuals are needed to raise
offspring effectively) and intense competition for territories or females (so a team of
males is more effective in gaining reproductive opportunities).
A good example of this last scenario is provided by lions Panthera leo , where larger
coalitions of males are both more likely to gain control of a pride of females, and to
maintain longer tenure of a pride, with the result that individual male lifetime fitness
increases with male coalition size. In lions, male teams often consist of relatives
(brothers, half brothers), so there is kin selection for cooperation (Chapter 11). But it
pays even unrelated males to cooperate in small teams rather than to attempt to take-
over a pride on their own (Packer et al ., 1991).
In some populations of Galapagos hawks, Buteo galapagoensis , groups of unrelated
males (usually two to three, but up to eight) cooperate to defend a breeding territory.
They share matings with the single breeding female and share paternity of the offspring
(Faaborg et al ., 1995). In remarkable contrast to the dunnocks, males do not squabble
over matings and they share paternity roughly equally (DeLay et al ., 1996). Two likely
factors promoting male cooperation are the larger size of the female (which may enable
her to control copulation shares) and the increased number of years of tenure of a
breeding territory when it is defended by a larger male team. This means that a male's
short-term paternity sharing costs each season could be compensated by breeding over
many more years, leading to increased lifetime reproductive success. Such compensation
cannot occur in shorter-lived dunnocks, where males breed for just one or two seasons.

Male lions
benefit from
polyandry …

… and so might
male Galapagos
hawks

Female desertion and sex role reversal
In most birds, if one sex deserts it is usually the male, because he has the opportunity to
desert first. Usually, he also has more to gain from increasing his number of mates
because he can potentially fertilize eggs at a faster rate than a female can lay them
(Chapter 7). Thus, polygyny is far commoner than polyandry. Some studies, however,
suggest sexual conflict over opportunities to desert first (Chapter 8). In the Florida snail
kite, Rostrhamus sociabilis, either sex may desert leaving the other to care for the brood.
Which sex deserts depends on who has the greatest chance of gaining another mate,
which varies depending on the operational sex ratio. Desertion is also more frequent
when food is abundant, so that the remaining partner is better able to raise the young
unaided (Beissinger & Snyder, 1987).
In shorebirds (Charadrii), most species are monogamous with biparental care.
However, in some species the normal sex roles are reversed, with the females being