The
mating of one female with more than one male while each male
mates with only one female is known as polyandry (literally,
"many males"). It is a rare mating system, occurring in less
than one percent of all bird species, and is found mostly in
shorebirds. Polyandry is often accompanied by a reversal of
sexual roles in which males perform all or most parental
duties and females compete for mates. The common pattern of
sexual dimorphism is often reversed in polyandrous birds:
the female is often larger and more colorful than the male.
This reversal confused early biologists and led Audubon to
mislabel males and females in all of his phalarope
plates. Two types of polyandry have
been documented: simultaneous polyandry and sequential
polyandry. In simultaneous polyandry, each female holds a
large territory containing the smaller nesting territories
of two or more males who care for the eggs and tend the
young. In our region, only Northern jacanas
characteristically practice this form of polyandry. Females
may mate with all of their consorts in one day and provide
each male with help in defending his territory. A female
will not copulate with a mate while their eggs are being
incubated or during the first six weeks of the fife of the
chicks. If a clutch is lost, she will quickly copulate with
the broodless male and lay a new batch of eggs within a few
days. A very rare variation on the
preceding theme is "cooperative simultaneous polyandry," in
which more than one male mates with a single female and the
single clutch of mixed parentage is reared cooperatively by
the female and her several mates. This arrangement occurs in
some populations of Harris' Hawks and occasionally in Acorn
Woodpecker groups. In sequential polyandry (the
most typical form of this mating system), a female mates
with a male, lays eggs, and then terminates the relationship
with that male, leaving him to incubate the eggs while she
goes off to repeat this sequence with another male. Spotted
Sandpipers, Red-necked and Red Phalaropes are examples of
sequentially polyandrous species that breed in North
America. A possible evolutionary precursor of sequential
polyandry is found in Temminck's Stint, Little Stint,
Mountain Plover, and Sanderling. In these species, each
female lays a clutch of eggs that is incubated by the male,
followed by a second clutch that she incubates herself.
These two-clutch systems can be envisioned as a step toward
the sort of sequential polyandry seen in the Spotted
Sandpiper, but females of that species never incubate a
clutch alone unless their mate is killed -- even when
resources are abundant. There is an interesting
sidelight to the story of polyandry in birds. In polygynous
mammals (one male mating with several females) such as lions
and gorillas, infanticide can occur when a new male takes
over a harem. By killing the young of the previous harem
ruler, the new male presumably brings females back into
heat. This gives him a chance to increase his own
reproductive contributions and, perhaps, to reduce use of
resources by unrelated offspring. In Northern jacanas it has
been reported that females taking over the territories of
other females occasionally practice infanticide, destroying
the offspring of previous females. The males attempt to
defend their broods (which represent their genes, but not
those of the new female), just as lionesses attempt to
defend their cubs from infanticidal male lions taking over a
pride. However, the actual killing of young has not been
observed -- only empty nests. If substantiated, this
behavior in jacanas is the first known example of
infanticide being used as a reproductive strategy by
females. SEE: Polyandry
in the Spotted Sandpiper;
Monogamy;
Polygyny;
Cooperative
Breeding;
Natural
Selection. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.