Form and Function
The bar-headed goose is famous for its functional adaptations to
living in a region which forces high-flying migration about the
Himalayas. Many questions arise about how this amazing aviator
ascends to such astonishing altitudes up to 9,000 meters.
First, it is remarkable to note that the bar-headed goose
seems ordinary for its impressive feat. Like many birds, the
bar-headed goose relies heavily on bulky pectoralis muscles in
its chest to thrust its wings downward in its ascent. Opposite
of these are the supracoracoideus muscles, which account for the
elevation of the wings (Tobalske, 2007). Together, these massive
muscles produce much of the vigorous flapping in the bar-headed
goose’s incredible incline. Against expectations, the bar-headed
goose has shown to rely on its own strength rather than upward
wind drafts, as it departs for its high-flying migration at
night or in early mornings, when the wind is typically less
strong (Hawkes et al., 2011).
Although the bar-headed
goose displays a large wingspan (nearly 1.5 meters) within its
genus, Anser, the variation is not great enough to distinguish
it from related low-flying geese (Lee et al., 2008). In fact,
when compared to a large phylogeny of related species, including
red-breasted geese, brent geese, greylag geese, and pink-footed
geese, it is clear that the wingspan of the bar-headed goose is
proportional to its body size. The familiar
canadian goose is a great comparison, as it has some similar
morophology but does not fly so high. Ultimately, no significant
difference in wing shape, size, or beating frequency can fully
account for this bird’s lofty levels (Lee et al. 2008).
What
appears to really set the bar-headed goose apart involves its
blood, better circulation regardless of conditions, and
behavioral adaptations. Hemoglobin within the blood attracts a
greater amount of oxygen to be carried efficiently throughout
the circulatory system. Even among other high-flying birds, the
hemoglobin of bar-headed geese is able to become amply saturated
with oxygen in atmospheric conditions that have approximately
26% less oxygen pressure (Butler, 2010). Unlike birds of lower
altitudes, bar-headed goslings’ development of high capillary
density in its flight and leg muscles is virtually unaffected by
hypoxia during egg incubation. Therefore, bar-headed geese are
well-adapted to a low oxygen environment from before they even
hatch (Butler, 2010). Recent studies also indicate that
declining temperatures, which are also associated with high
altitude, can stimulate an increase in the already high oxygen
affinity of hemoglobin in bar-headed geese (Meir and Milsom,
2013). Finally, when exposed to environmental hypoxia, the
bar-headed goose initiates a hyperventilation mechanism to
increase oxygen uptake. For many organisms, such as mammals,
hyperventilation results in a condition called hypocapnia, which
is low carbon dioxide pressure in the blood. Hypocapnia can
cause drastic deficiencies in the transport of oxygen to the
brain; however, bar-headed geese and other birds do not
encounter this issue (Butler, 2010). Of all the bar-headed
goose’s functional advantages, this may be one of the largest
contributors to the longevity of its lofty flights, as it can
maintain vital brain function and focus.
Continue to
Reproduction to see where these high-flying flocks come
from.
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