Calculating wrens invest in their future
Monday, 21 March 2011 Genelle
Weule
ABC
They may look like they're doing a
favour, but babysitting siblings or unrelated nestlings is all part of a
purple-crowned fairy wren's cunning plan to inherit the nest.
In the latest issue of The American Naturalist, researchers challenge the theory
that collaborative behaviour in birds is driven by altruism.
Over four years, Dr Anne Peters of Monash University and colleagues from the Australian National University
and Max Planck Institute for
Ornithology followed a group of purple-crowned fairy wrens (Malurus
coronatus).
Like many other species of Australian birds, fairy wrens live as family
groups, with the non-breeding subordinate wrens helping to feed the young, not
only of their parents, but sometimes completely unrelated birds.
This behaviour flies in the face of evolutionary theories that dictate that
individuals need to be selfish in order to survive.
"Co-operation is harder to explain because, particularly in this case when
we're looking at cooperatively breeding birds, the [subordinate] birds don't
breed themselves, but they help others to raise their young," says Peters.
Unlike other species of wren, which are very promiscuous, purple-crowned
fairy wrens are monogamists. They also live in stable territories in northern
Australia all year round, which makes them ideal to study.
The researchers observed the nesting behaviour of each bird and tracked their
genetic lineage to work out each bird's likelihood of inheriting a breeding
position that matched its investment in the young.
They found a number of factors affected the inheritance pecking order.
Usually the oldest subordinate inherited the territory when the resident
same-sex breeder disappeared, and male subordinates were more likely to fill a
vacancy than females.
Future helpers
How much they fed their underlings depended on the subordinate wren's
relationship with the breeding pair, tending to feed related siblings more than
less-related nestlings.
By supporting their own kin the subordinate stands a better chance of
inheriting the group's territory and continuing the family line.
"Not only do these helpers feed the young but they actually increase the
production of the young. So they are making a difference and their mum and dad
survive better," says Peters.
"If you wait long enough you have a pretty good chance that you will get that
territory by inheritance."
But the researchers also discovered that unrelated subordinates fed the
younger nestlings, particularly if they had a chance to inherit the group's
territory.
"These helpers are actually playing a very calculating game. They're not just
randomly stuffing food in mouths just because there's a bit of a chance that
they might be related," says Peters.
"[They only feed the unrelated young] when they know they have a good chance
of inheriting the territory.
"The thinking is that they are producing their own future helpers."
Harsh, but fair
While commenting that the idea that altruism is only possible if there are
hidden benefits is a "harsh view of the world", Professor Gisela Kaplan, an
animal behaviourist at the University of New England, says the study is very well done and
examines all the evolutionary hypotheses.
"By looking at all the hypotheses they found there were direct and indirect
benefits," Kaplan says.
Kaplan, who studies magpies, another species of Australian bird that practise
cooperative behaviour, says the findings may not apply to all birds.
"It's interesting that there is a relatedness issue in that species, but it
doesn't necessarily hold for all species; and not all species of wren," says
Kaplan.
"They've examined all the different forms of helping behaviour in that
particular species to explain the evolution of helping behaviour. That is a
fascinating topic because [cooperative behaviour] is so much stronger in the
southern hemisphere.
"Australia has 22 species out of 36 in the whole world [that cooperate] so
cooperation is highly developed among Australian birds, but it is played out in
different ways."
http://www.abc.net.au/science/articles/2011/03/21/3169566.htm
