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Can being a
beautiful baby increase one's chance for
survival?
Red can be claimed as
the favorite color of birds. At least in many species it is
important in provoking certain critical behaviors
(aggressive displays and courtship displays being most
common). Where bright coloration contributes to reproductive
success by affecting mate selection it is said to result
from what Darwin called "sexual selection." Does color have
other social value?
In the American
Coot--a water bird of the family Rallidae--the adults have a
white bill and uniformly greyish-black feathers covering
most of their bodies.
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The newly
hatched coots are not drably colored like most
nestlings, but are quite striking in their
coloration. They have long orange-tipped slender
feathers, brilliant red papillae around the eyes, a
bright red bill and bald red head. They lose their
colorful appeal at about three weeks of age. But in
the
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meantime,
does it help them survive?
One team of Canadian
biologists thought that the selective advantage arises in
the eye of the (parental) beholder. They speculated that the
plumage may make some chicks more attractive to their
parents. That is, the bright coloration might in some way
affect the quality of care given (mostly in the form of food
provided) by the parents. The more attractive chicks might
be more successful in begging for food, thus having a higher
rate of survival and a greater likelihood of passing on
their genes (including those for bright coloration). As many
as half of all chicks do die of starvation, so this might
actually confer a significant advantage.
- How might that
notion be tested?
- What is the
specific question being asked?
- What is the
hypothesis being suggested?
- What predictions
(deductions) can you make if the hypothesis is
correct?
- How can your
predictions be tested? (i.e. What exactly might we do if
we were the field biologists who had studied coots for
several years.
Remove the color
advantage?
Here are some
additional pieces of information:
- Large numbers of
floating reed-basket nests were accessible to the
biologists.
- The long
orange-tipped feathers could be clipped from selected
hatchlings.
- Coots lay an egg a
day for 8 to 12 days.
- Eggs hatch in the
order laid.
Designing the
experiment
- How should
clipping be done to test the hypothesis?
- Clip all the
hatchlings in a given nest or just some of
them?
- If just some, then
which ones?…and in what order?
Predicting the
results
- If observations
and measurements of normal, unclipped "Orange Chicks"
produce the results indicated in the graphs a, c, and e,
what would you expect in a brood of all clipped "Black
Chicks?" (In the graph the boxes show the results of most
of the chicks, while the lines extending above and below
indicate some extreme cases.) Make your own prediction in
the form of boxes positioned level with those already
drawn if you think there would be no change. Draw boxes
either somewhat higher or lower to show that you think
the clipped Black Chicks would fare better or
worse.
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This graph show how
frequently chicks in a nest of all Orange Chicks
were fed during feeding sessions.
Where do you think a
horizontal line should be drawn to show how
frequently chicks would be fed in a nest of all
Black Chicks?
After taking a guess click on
the graph to see the actual results.
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This graph
show the relative rate of growth of the chicks in a
nest of all Orange Chicks.
Where do you think a line or
box should be drawn to show the relative growth in
chick in a nest of all Black Chicks.
After taking a guess click on
the graph to see the actual results.
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This graph show the percent
of of chicks surviving in a nest of all Orange
Chicks.
Where do you think a line or
box should be drawn to show the percent of of
chicks surviving in a nest of all Black
Chicks.
After taking a guess click on
the graph to see the actual results.
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- If you would like
to see the result of all three data sets, click
here
for graphs of the actual field results .
The "Control Broods"
were clutches in which all chicks were either clipped and
made "black" or simply handled as long as it would take to
clip them but returned to the nest still
orange.
- Why do you suppose
the biologists handled the orange chicks when they had no
intention of clipping them?
The biologists
reasoned that if parent birds would show a preference, it
would only be demonstrated in broods in which some chicks
were clipped (Black) and some unclipped (Orange).
- How should they go
about deciding which birds in a given nest should be
clipped and which merely handled briefly?
More
predictions
- What would you
predict would be the results in the experimental broods,
if the hypothesis of an "orange advantage" is
correct?
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Make a prediction or guess
about where a box or a horizontal line should be
made in the graph to indicate how often orange
chicks and black chick would be fed when they are
members of the same brood.
After taking a guess click on
the graph to see the actual results.
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Make a
prediction or guess about where a box or a
horizontal line should be made in the graph to
indicate how fast orange chicks and black chick
would grow when they are members of the same
brood.
After taking a guess click on
the graph to see the actual results.
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Make a prediction or guess
about where a box or a horizontal line should be
made in the graph to indicate the percent of orange
chicks and black chick that would survive when they
are members of the same brood.
After taking a guess click on
the graph to see the actual results.
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- If you would like
to learn how the experimenters decided which chicks to
clip and look at all three data sets together, click
here
to see the
actual results from the field study.
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