Biology Reference
In-Depth Information
female drowns in the liquid dung as the males struggle for possession (Parker, 1970c).
Males are larger than females and in this case there seems to be little scope for mate choice.
Material (or direct) benefits from multiple mating
In some cases, multiple mating increases the number of young that a female can
produce. She may simply ensure fertility by acquiring more sperm, or she may gain
more resources from males, either food gifts or nutrients in spermatophores which
enable her to lay more eggs (Thornhill & Alcock, 1983). Where some males control the
best egg laying sites in the form of territories, a female may have to copulate each time
she comes to lay, simply to gain access to these sites (e.g. many dragonflies and
damselflies). In other cases, mating with multiple males may increase the amount of
care a female can gain for her offspring because several males will cooperate to help feed
a brood if they all have a chance of paternity; for example, Galapagos hawks
Buteo
galapagoensis
(Faaborg
et al
., 1995) and dunnocks
Prunella modularis
(Davies
et al
.,
1996b). A female may similarly reduce male harassment of herself and her offspring if
she mates multiply. In some primates where females live in multimale troups, it pays a
female to give each male a sufficient paternity chances that he will not commit
infanticide (Hrdy, 1999).
Genetic (or indirect) benefits
Here the female increases the genetic quality of her offspring by mating with more than
one male. Good evidence for this hypothesis comes from a surprising source, namely
studies of song birds, where social monogamy (one male breeding with one female) is
the most common mating system (Lack, 1968). Since 1985, when DNA markers first
became available for precise measures of paternity, studies have revealed that socially
monogamous song birds engage in frequent extra-pair mating. Typically, 10-40% of
the offspring are sired by males who are not the female's social mate (Griffith
et al
.,
2002; Westneat & Stewart, 2003). In song birds, males cannot easily force a mating
because the female can fly away to escape. Instead, extra-pair matings occur because
females sneak away from their home territories to solicit to these extra-pair males. A
comparative study of bird plumage suggests that extra-pair matings are an important
component of sexual selection. Owens and Hartley (1998) compared the plumage of
males and females across 73 bird species where there were data on rates of extra-pair
paternity. They ranked plumage dimorphism on a scale from zero (no difference between
males and females) to ten (males much more colourful than females). They found no
correlation between the degree of plumage dimorphism and the number of social mates
but a strong correlation with the rate of extra-pair paternity (Fig. 7.17). This suggests
that males have evolved colourful plumage not to attract their own females but rather
to attract other males' females for extra-pair matings!
Detailed studies of several species have now confirmed that females do indeed seek
extra-pair matings with males whose displays or plumage traits are more elaborate than
those of their social partners, for example: greater song repertoires, longer tails or brighter
plumage (Table 7.3). Females paired to attractive males (as measured by these traits) tend
to be more faithful; those paired to males with poorer developed traits are the ones who
Extra-pair matings
in birds: female
choice for male
plumage or
display traits
