Not all female animals look after their own young and the attention
devoted to their own offspring varies enormously among species. But
even within species with well-developed female parental care, certain
females might choose an alternative breeding tactic and leave their
offspring in the care of others. Such brood parasitism is particularly
common in ducks, where some females (‘hosts’) often receive eggs laid
by other females (‘parasites’) of the same species. But why would a
female accept someone else's responsibilities? After all, taking care
of another female's chicks can sap the caretaker's strength and
increase her exposure to predators. Several ideas have been proposed to
solve this puzzle, ranging from the simple notion that females might
not be able to discriminate between her own offspring and those of
others, to the hypothesis that a female might actually enhance the
survival of her own progeny by accepting offspring of others. An
intriguing idea is that parasites and hosts could be related. As
relatives share some of their genes, the costs paid by a host female
might be counterbalanced by the propagation of her own genes through
the offspring of her related parasite. Although the general importance
of ‘kin selection’ was proposed by W.D. Hamilton in the 1960s, and its
potential role in the evolution of brood parasitism was identified by
Malte Andersson in the early 1980s, this idea still remains a matter of
hot debate.
Malte Andersson and Matti Åhlund now present evidence [1 and 2] in favour of the kin selection hypothesis. As a part of a long-term field study on goldeneye ducks (Bucephala clangula),
they drew albumen samples from nearly a 1000 eggs, without harming the
developing chicks. As the albumen is secreted by specialized cells in
the oviduct of the egg-laying female, this method is equal to sampling
the female herself. Analysis of the protein contents of the albumen
revealed that host and parasite were indeed often related, their
average relatedness approximately that of first cousins. What is more,
this relatedness did not emerge as a passive by-product of related
individuals aggregating in the same area, but apparently through the
active recognition of kin. In their daily activities, female goldeneye
ducks were more likely to occur together with their old birth nest
mates than with other females, and these pairs lasted longer than did
pairs of other goldeneye ducks. Hence, old nest mates seemed to
recognize each other in the field, and females parasitized their birth
nest mates more often than would be expected by chance alone.
This
study not only suggests that the relatedness among individuals is a
crucial factor in the evolution of brood parasitism in ducks and other
species with related females breeding in the same area., but it also
shows brood parasitism in a new light: as an active strategy based on
social interactions and recognition of kin. Ultimately, it leads us to
ask whether the female dumping her eggs in the nest of another female
should even be labelled as a ‘parasite’–perhaps the foster female
actually benefits from looking after next-door's kids?
© Jan Uddén
References
1. M. Andersson and M. Åhlund, Host–parasite relatedness shown by protein fingerprinting in a brood parasitic bird. Proc. Natl. Acad. Sci. USA. 97 (2000), pp. 13188–13193. Abstract-EMBASE
| Abstract-Elsevier BIOBASE
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2.
Andersson, M. and Åhlund, M. Protein fingerprinting: A new technique
reveals extensive conspecific brood parasitism. Ecology (in press).
; Alternative breeding tactics
