First published online December 10, 2003
Journal of Experimental Biology 207, 285-293 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00746
Are melanized feather barbs stronger?
Michael Butler and
Amy S. Johnson*
Biology Department, 6500 College Station, Bowdoin College, Brunswick,
ME 04011, USA
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Fig. 1. An SEM photograph of the cross-section of one feather barb included in this
study (fractional distance=0.63). Barb sections are typically rectangular to
oval, almost always with the dorso-ventral axis (up-down in this photograph)
longer and thicker walled than the lateral axis (right-left in this
photograph). Scale bar, 30 µm.
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Fig. 2. An example of a force-extension curve of a feather barb extended at 8 mm
min-1 until breakage. Data points were initially sampled at 100 Hz,
but data points shown were resampled at 10 Hz for presentation. Mean values
for mechanical variables are given in Table
1.
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Fig. 3. Mean breaking force (A) and mean cross-sectional area (B) of unmelanized
(open circles) and melanized (closed circles) bands of barbs as a function of
mean fractional distance along the feather, where `0' represents the proximal
end and `1' represents the distal end of the feather. Breaking force and
cross-sectional area differed significantly between bands (ANOVA,
P<0.001). Mean values for bands sharing the same letter were not
significantly different (a posteriori; P>0.05); mean
values for all other bands were significantly different.
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Fig. 6. Toughness (T) as a function of fractional distance (d)
along the feather, where `0' represents the proximal end and `1' represents
the distal end of the feather. There were no significant differences in slope
(P2,153=0.76) or intercept
(P2,153=0.22) between barb colors (overall equation:
T=-10d+15, r2=0.19,
P1,154<0.001 that the overall slope is zero).
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Fig. 8. The relative curvature of a bent beam at failure as a function of the
slenderness of the beam. The relative curvature is given by the radius of
curvature at failure ( evaluated as in Equation 4) divided by the length
of the beam (L) plotted against the length divided by the height of
the beam (L/h). Averages (dots) and ranges of values
(horizontal lines) shown are for the osprey feather tested in the current
experiment. Barbs deform much more relative to their length before failing by
buckling than does the rachis of a feather. Thus, barbs avoid failure by
bending, whereas the rachis avoids failure by depending more on structural and
material strength.
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© The Company of Biologists Ltd 2004
brk) as a function of fractional
distance (d) along the feather, where `0' represents the proximal end
and `1' represents the distal end of the feather. There were no significant
differences in slope (ANCOVA; P2,153=0.17) or intercept
(P2,153=0.09) between barb colors (overall equation:
brk) of the unmelanized (open circles) and
melanized (closed circles) barbs. There were no significant differences in
slope (ANCOVA; P2,153=0.42) or intercept
(P2,153=0.74) between barb colors for the log-transformed
data (overall equation:
