Biology Reference
In-Depth Information
juncos (
Junco hyemalis
) the figures were 66 and 38%, respectively (Smith
et al
.,
1982; Greenlaw & Post, 1985; Wolf
et al
., 1990). For some species, removals suggest
that male help is more important when food is scarce (Lyon
et al
., 1987; Bart &
Tornes, 1989). These experiments show that male help can clearly increase
reproductive success, but it is not essential. If male desertion reduces productivity to
a fraction 1/x of a pair-fed brood, then provided a male can gain more than
x
females, desertion will be the more profitable option from his point of view. Even if
success is reduced to a half or less, and a male can gain only two females, polygyny
will still pay provided the male helps to provision at least one of the broods. As
predicted, male songbirds readily desert to gain extra females if given the chance, for
example by removal of a neighbouring male, and they often help to provision either
one of their female's broods full-time, or several females' broods part-time.
Occasional polygyny has been reported in 39% of 122 well-studied European
songbirds.
These experiments suggest that the predominance of monogamy in many birds
arises not, as Lack proposed, because each sex has the greatest success with
monogamy but because of the limited opportunities for polygyny. The two most
obvious constraints are: (i) strong competition among males may make it difficult for
a male to gain a second female; and (ii) females are likely to suffer in polygyny through
the loss of male help and, as predicted, females are often aggressive to other females,
which may decrease the chance that their partners are able to gain a second mate.
This latter constraint is particularly well illustrated by burying beetles
Nicrophorus
where, just like birds, males help to feed offspring (Chapter 8). Females lay eggs on
vertebrate carcasses and both parents provision the larvae on regurgitated carrion.
On small carcasses, the male cooperates to feed the offspring of his one female.
However, on large carcasses the male adopts a 'headstand' posture and exposes his
last abdominal segment to emit pheromones in an attempt to attract a second female.
A second female increases the male's reproductive success from the carcass, but it is
costly to the first female because, with reduced male care, she produces fewer
offspring herself (Eggert & Müller, 1992). It is not surprising, therefore, that the first
female interferes with her male's attempts to gain polygyny; when he displays, she
mounts him, pushes him over or bites his abdomen with her mandibles (Eggert &
Sakaluk, 1995).
In the last chapter, we saw that social monogamy does not necessarily imply
genetic monogamy because extra-pair matings are rife in many species (Box 9.1). In
some species of birds males protect their paternity by following the female closely
during her fertile period ('mate guarding', e.g. magpies and swallows). In other
species this is not possible because one partner has to defend the nest site while the
other goes off to forage (many seabirds and birds of prey). Here males engage
in frequent copulations to swamp the sperm of rivals, sometimes copulating several
hundred times per clutch, clearly far more than necessary simply to make sure
that the eggs are fertilized (Birkhead & Møller, 1992). Even so, despite these paternity
guards the frequency of extra-pair paternity can be very high (25-35%) in
some species (Westneat & Stewart, 2003). For example, in the red-winged
blackbird (
Agelaius phoeniceus
) Lisle Gibbs and his colleagues (1990) found that
extra-pair fertilizations accounted for on average 21% of a male's reproductive
success (Fig. 9.8).
Monogamy in
birds because of
limited
opportunities for
polygamy
Burying beetles:
females oppose
their male's
attempts to gain
polygyny
Social monogamy
in birds does not
mean genetic
monogamy: extra-
pair matings are
often common
