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Australia?s separation from the other major land
masses of the world has led to the evolution many extraordinary species of
plants and animals. One such animal is the purple-crowned fairy wren
(Malurus coronatus) which is only found in certain regions of Northern
Australia. One peculiar aspect of the behaviour of this tiny bird is that
adult birds will frequently help in the raising of offspring that are not
their own.
In a world dictated by the passage of favourable genes
from one generation to the next, this seemingly selfless behaviour has
puzzled biologists for some time. However, recent research at the
Australian National University may be set to shed new light on this
phenomenon.
Dr Michelle Hall, a visiting fellow in the ANU Research
School of Biology, was recently part of a long-term study on the
cooperative breeding habits of purple-crowned fairy wrens. The study was
led by Sjouke Kingma from the Max Planck Institute for Ornithology with Dr
Anne Peters from Monash University as senior author.
Dr Hall says
?Fairy wrens are habitual cooperative breeders, with breeding pairs
receiving help in the nest from non-breeding helpers. But what our study
has shown for the first time is that essentially this helping in the nest
boils down to both family and inheritance.?
With individual birds
looking very much like each other (to a researcher anyway!), the only way
to keep track of who?s who was to capture each bird using mist nets then
tag their legs with coloured bands. ?The great thing about the banding
technique is that you can read them from quite some distance? Dr Hall
explains, ?So unlike number tags, it wasn?t necessary to keep recapturing
them to identify each bird, which would of course, disturb their natural
behaviour.?
The study was quite extensive, tagging and following every member of
the Fairy-Wren population along a 10km stretch of river. ?We spent many
hours watching nests, monitoring who was feeding the nestlings, keeping
tabs on changes in social groups, and recording when and where youngsters
obtained breeding territories.? Dr Hall says.
?By looking at the
dynamics of these social groups we realised that when it comes to helping
raise the young, it is a matter of relatedness. For example in
purple-crowned fairy-wrens over 60 per cent of helpers live with both
parents in the territory they hatched on and over 90 per cent live with at
least one parent. So the young they help raise are usually siblings or
half-siblings.?
?We also found that in general helpers that are
more closely related to the nestlings provide more help and their efforts
thus see more of their shared genes entering the gene pool. In other
words, helpers adjust their behaviour to maximise these indirect benefits
by helping kin.?
Such apparently selfless cooperative breeding
behaviour has intrigued biologists for decades. However, this new research
suggests that the birds performing the helping in this case, have very
strong genetic ties to the offspring being raised which makes perfect
sense from an evolutionary perspective. Cooperative insects such as bees
relinquish their personal ability to breed entirely in favour of offspring
from a single queen, though of course all the bees that are produced carry
genes closely related to those of the workers who raise them.
While
the findings present ?an elegant theoretical solution? to the problem of
unexpected animal altruism, Dr Hall said that they are by no means
universally observed.
In a twist, some helpers in purple-crowned
fairy-wrens, and other cooperative breeders, also provide help for
unrelated young in the nest. But it turns out these helpers are also
playing a selfish game and maximising direct benefits by producing future
helpers of their own. They help when their chances of inheriting the
current breeding territory are greater, and thus are helping to raise
their own future assistants.
It would seem that even in the world
of the purple fairy wren, there?s no such thing as a free
lunch!
The study?s findings have been published in the latest
edition of The American Naturalist.
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