Press Release - 10 October 2006
Childcare tug-of-war influences shorebird breeding systems
The battle over who cares for the kids has played a key evolutionary
role in deciding whether different species of shorebird are monogamous
or polygamous, according to new research in the journal BioScience.
A demanding youngster means that parents are more likely to stay
together to help rear their young, yet those with more hardy offspring
are likely to battle it out to see who gets to leave the nest.
Played out over evolutionary time, it is this childcare tug-of-war
which has shaped the varied breeding systems found amongst the world’s
shorebirds, say researchers.
Scientists from the universities of Bath, Bristol and Imperial College
London (all UK) focused on the Kentish plover ( Charadrius alexandrinus
) to help investigate the influences on the shorebirds’ breeding
The Kentish plover is particularly interesting because whilst some
pairs are monogamous, in other pairs either the male or female can be
sequentially polygamous, mating several different times throughout the
breeding season and leaving the abandoned partner to raise the chicks.
The scientists also found that whilst both males and females are
equally adept at raising the offspring, the female parent is more
likely to leave childcare responsibilities to her partner if there is a
high ratio of unpaired males around. Mathematical modelling showed that
if the sex ratio was reversed, so that there were more females than
males, it would be likely that males would fly the nest in search of a
The scientists also discovered that unpaired females would find a new
mate faster (on average less than two days) after deserting the nest
than males (average 12 days).
“Having systems of independent self-feeding young, compared to those
that require feeding by the parents, opened the possibility for the
evolutionary divergence of breeding systems to those where either
females or males had more than one mate,” said Dr Tamas Szekely from
the Department of Biology & Biochemistry at the University of Bath.
“In those species which have demanding young, the parents are more
likely to share responsibilities, suggesting that the burden of
childcare has shaped breeding systems.
“Care is costly to parents because it takes time and energy, and
incubating eggs and feeding young may put a parent at risk of predation.
“Unless they are likely to breed again in the future, each parent has
only a short-term interest in its mate’s welfare.
“These short-term interests may be at odds with long-term interests in
securing its own reproductive potential.
“An outcome of this is that a parent may gain by shunting parental care
duties to its mate, so that it is free to mate with a new partner.
“We have much to discover about how and why population sex ratios are
maintained and regulated by nature.
“We also need to grasp better how breeding systems function in nature,
for example – if a youngster is reared in a father-only family, will
this influence how it will behave with its own family.
“Although we assume the deserted parent loses out, we also need to find
out if the deserted parent gets any benefit – for example by being able
to demonstrate that it is a competent parent.
“We noted in our fieldwork that female Kentish plovers were unusually
receptive to the courtship of males caring for nearly fledged and
apparently healthy offspring.”
The research was funded by the Natural Environment Research Council,
the Biotechnology & Biological Sciences Research Council and the
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