Woodpeckers are classified into
the family Picidae, which contains approximately 220 species. They are small
birds growing from 9 – 57cm in length and their plumage comes in a range of colours,
often barred or spotted. Woodpeckers occur worldwide excluding Australia, New
Zealand and Madagascar (The Macmillan Encyclopedia, 2003). Most woodpecker
species are arboreal, drilling holes into trees to forage for insects and their
lava, as well as one group of specialized woodpeckers that use their brush-like
tongues to draw out sap from the tree (Gibson, 2005; The Macmillan Encyclopedia
2003). Although woodpeckers have evolved many adaptive structures, such as
their brush like tongues, only the woodpecker’s ability to withstand the
extreme forces their repeated pecking puts on their beaks, head and brain will
be focused on in this post.
Woodpeckers are capable of repeated pecking
at an incredible rate of 6 – 7.5m/s and decelerate on impact in 0.5 – 1.0
milliseconds (Gibson, 2005). These extreme rates of movement and the force of
the impact that follows both put immense pressure on the beak, brain and head
of the woodpecker. A number of suggestions have been made as to how these birds
avoid causing permanent damage over the course of their life, the earlier of
which included suggestions of the existence of shock absorbing mechanisms (Bock
1964; Spring 1965). It was found however, that such mechanisms would also
reduce the impact force of the woodpecker and therefore reducing its ability to
drill into trees (Bock 1999).
In his study, Gibson (2005) examined the
scaling effects in brain injury to explain how woodpeckers are able to
withstand such high forces impacting on them during deceleration while pecking.
Using comparisons between human brains and woodpecker brains, assuming that
brain densities are the same between humans and woodpeckers, Gibson (2005)
found that woodpeckers are able to withstand injury for acceleration forces 16
times those measured for humans. This value ranges from about 11 – 20 times
those measured for humans, for various species of woodpecker. In the same study
it was also found that the head of a woodpecker can tolerate accelerations of
4600 – 6000 g, with a variation of 69
– 125% for woodpecker species with smaller or larger brain sizes. It was concluded
that the woodpeckers ability to withstand the high acceleration forces acting
on them while pecking is due to three main factors: their small size, reducing
the stresses on the brain for a given acceleration, the short duration of the
impact and the orientation of the brain within the skull, increasing the
contact area between the brain and skull.
References
Bock, W., 1964, Kinetics of the
avian skull, Journal of Morphology,
vol. 114, pp. 1 - 42
Bock, W., 1999, Functional and
evolutionary morphology of woodpeckers, Journal
of
African Ornithology, vol. 70, pp. 23 – 31
Gibson, L., 2005, Woodpecker
pecking: how woodpeckers avoid brain injury, Journal
of Zoology, vol. 270, pp. 462 – 465
Spring, L., 1965, Climbing and
pecking adaptions in some north American
woodpeckers, Condor,
vol. 67, pp. 457 – 488
Bill Coulson, 2014, Red-bellied Woodpecker, Flickr, viewed 23 May 2014,
<https://www.flickr.com/search/?q=woodpecker>
2003 'Woodpecker' in The Macmillan Encyclopedia, Macmillan
Publishers Ltd,
Basingstoke, United
Kingdom. Accessed: 23 May 2014, from Credo Reference
Very interesting. I’m not sure I understand how increasing the contact area between the brain and the skull allows them to withstand such high forces? Please can you clarify. Are their beaks specifically modified morphologically to withstand the high pressure from pecking? Are their beautiful colours driven by sexual selection? Nice post!
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