CAN BE SUPERB EXAMPLES OF EVOLUTION
by Whit Gibbons
September 30, 2007
I noted that evolution can be an easily understandable concept in its
simplest form but that some biological mysteries can be difficult to unravel
even by evolutionary biologists. A true life example with an Australian
bird, the superb fairy-wren or blue wren, shows how complex some evolutionary
processes can be for scientists to decipher.
I once saw
a flock of these pixie-like birds flitting around in Royal National Park
near Sydney. They reminded me of blue-colored Tinkerbells. But aside from
being pretty little birds, these wrens have an interesting trait in common
with many other birds, including red-cockaded woodpeckers. That is, the
parents have helpers during the period when young are in the nest. A helper
for a pair of birds is typically a young, non-breeding male that brings
food to nestlings.
get a break from the chore of finding food for themselves as well as for
their young. From an ecological viewpoint, the benefit to the parents
and young seems obvious: the presence of helpers assures that the offspring
are well fed, which would mean they grow faster and get larger than babies
in a comparable situation in which only the parents are providing food.
The parents benefit by producing bigger babies that have a better chance
evolutionary perspective, a benefit to a helper male participating in
this cooperative breeding system is through what is known as kin selection,
whereby the individual is assisting close relatives. Hence, the helper
is assuring its own reproductive success indirectly by increasing the
survival probabilities of individuals that carry many of the same genes
as it does. Another way in which such cooperative behavior might benefit
the helpers themselves is by increasing their own efficiency at gathering
resources. By being part of a group engaged in gathering food they learn
from being in the company of others.
A study by
scientists at the University of Sheffield in England addressed a previously
unexplained situation in which the offspring of fairy-wrens did not grow
faster or get bigger when helper males were present. Instead, they usually
grew at the same rate and reached the same size before leaving the nest
as nestlings that were being fed solely by the parents. The mystery: why
wouldn't baby birds that are having their diets supplemented by helper
males get bigger faster?
biologists discovered a subtle and simple but very important distinction
between helper-fed baby wrens, which got almost 20 percent more food,
compared to those being fed only by their parents. The scientists conducted
their study on two types of fairy-wren groups, one with helpers present
and the other with only breeding pairs of the birds. They observed that
when helper males were absent in a breeding group at egg-laying time,
female fairy-wrens laid larger eggs with a higher nutritional content.
If helpers were present, they laid smaller eggs. Both the larger and smaller
eggs of fairy-wrens took the same amount of time to incubate, and the
babies stayed in the nest for the same amount of time before fledging.
So why would females lay smaller eggs with less nutrition when they clearly
are capable of laying larger, more nutritional eggs?
did experiments in which, unbeknownst to the birds, they exchanged eggs
between nests with and without helpers. When smaller eggs were substituted
for the larger eggs of parents without helpers, the size and ultimately
the survival of the young birds were reduced. The experiments confirmed
that the mother wrens were anticipating whether they could expect supplemental
feedings from helpers and adjusting the quality of the eggs accordingly.
If helpers are present, the female lays small eggs, because helper birds
provide the extra food needed for babies to reach the preferred size.
Darwin presented his ideas more than a century ago, but his principles
of natural selection and survival of the fittest continue to explain how
many intricacies of life work. And what is the advantage to the female
to lay small eggs when opportunity arises? Females that invest in smaller
eggs with lower nutritional value are more likely to survive longer and
produce more eggs in the future.
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