BIO 203

Form and Function

This bar-headed goose ascends using its poweful chest muscles    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).
    A flock of bar-headed geese begin their ascent.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.


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