|
| |
|
|
|
|
||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | ||||
First published online December 16, 2008
Journal of Experimental Biology 212, 152-153 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.024661
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Correspondence |
The nearly columnar limbs of elephants are very different from the more flexed, spring action limbs of running mammals and birds
3109 N. Calvert Street, Baltimore, MD 21218, USA
e-mail: gsp1954{at}aol.com
A review of recent research on limb posture and action in elephants indicates that their appendages are highly divergent in form and function from those of running mammals and birds.
Ren and colleagues' conclusion in a recent paper
(Ren et al., 2008
) that
elephant limbs are significantly less columnar and much more similar to those
of other animals than previously thought is an over-interpretation that
discounts the highly unusual form, action and performance of elephantine
limbs. Their study also incorporates under-appreciated methodological problems
that inherently limit researchers' ability to understand limb function in big
animals.
No recent technical study states that elephants possess perfectly straight
and rigid jointed limbs even during the load-bearing, propulsive phase of the
limb stroke (Gambaryan, 1974;
Alexander et al., 1979;
Hildebrand and Hurley, 1985;
Paul, 1998;
Paul and Christiansen, 2000;
Christiansen and Paul, 2001),
Bakker's narrative (Bakker,
1986) was a simplified analogy for a popular audience, and all
illustrations in these publications show some degree of joint rotation and/or
flexion.
In many mammals the forelimb is strongly flexed
(Muybridge, 1957;
Gambaryan, 1974;
Hildebrand and Hurley, 1985;
Paul, 1998;
Paul and Christiansen, 2000).
This posture applies to ungulates in which the humerus is short in order to
prevent the forefoot from being placed too far posteriorly relative to the
elbow joint with the humerus sloped strongly posterior–ventrally from
the shoulder joint. The system is characteristic of modern galloping rhinos,
appears to be present in hippos, which can trot, and was apparently
characteristic of gigantic indricotheres, which retained a proportionally
short, ungulate-like humerus and long distal segments
(Granger and Gregory, 1936;
Kingdon, 1979;
Alexander and Pond, 1992;
Paul, 1998;
Paul and Christiansen, 2000).
Ren and collegues confirm that all the joints of the elephant forelimb are
highly extended during the entire load-bearing phase even at a fast pace, so
the elephant forelimb is columnar and the humerus is elongated compared with
those of ungulates (Ren et al.,
2008). Even though Paul and Christiansen [see their figure 5D
(Paul and Christiansen, 2000)]
illustrate even more elbow flexion than do Ren and colleagues [see their
figure 10 (Ren et al., 2008)],
and the former also diagram significant knee flexion and rotation in an
elephant, the latter stereotype Paul and Christiansen's work as consistently
characterizing elephant limbs as hypervertical.
In the study by Ren and colleagues [see their figure 10
(Ren et al., 2008)] the
elephant foot appears so plantigrade that it seems more flexed than the
digitigrade and unguligrade feet of ungulates. The recording of foot posture
and action via a line from the ankle to the tip of the toe in their
paper makes the foot look flatter than it really is. In most cases foot
posture and rotation should be measured along the long axis of the metatarsus.
Even this exaggerates hindfoot flexion when a massive footpad helps support
the pes along the entire length of the metatarsus to the ankle, which is held
well above ground level, rather than in contact with the ground as in truly
plantigrade feet [Fig. 1; see
also figure 1193 in Osborn (Osborn,
1942), who notes that the elephant pes is effectively
unguligrade]. In elephants the main axis of the main body of the foot is
nearly vertical at the middle of the propulsive stroke when ambling
[Fig. 1; see also
figure 1 in Hildebrand and
Hurley (Hildebrand and Hurley,
1985)], so the classic view of the foot as functionally columnar
is correct.
|
)], who do not include the toes, observed much less foot
rotation in an ambling elephant. Because the elephant foot is immersed in
pliable padding it is difficult to quantify the exact angle of flexion between
the foot and shank. In any case it is clear that the elephant foot is too
short and insufficiently flexible to produce the strong spring action that
many extant mammals and birds use to achieve a fully suspended phase running
gait (Paul, 1998;
Paul and Christiansen, 2000;
Christiansen and Paul, 2001).
Reduced rotation of the elephant hindfoot results from the
astragalus–tibia articulation being flatter [see figure 1193 in Osborn
(Osborn, 1942)] than in other
mammals with a roller-type joint.
Problems in marking actual points of joint rotation in large animals
chronically hinder understanding of their limb function. True limb action can
be measured only with motion x-rays, which are not practical above a modest
body size. Placing markers on the skin is potentially misleading because the
marker may not be accurately placed, and because it may float relative to the
joint's center of rotation as the skin slides over the musculature during limb
action. A casual examination of humans shows that the latter is the case. So
external markers are not necessarily precise measurements of locations of
internal joint rotation, they are estimates that may in part be measuring skin
rather than joint movement. Whether Ren and colleagues have established that
elephant knees are about as flexed and flexible as those of horses using
external markers is therefore open to question. Likewise, the motion diagrams
in the studies by Paul and Christiansen [see their figure 5B–D
(Paul and Christiansen, 2000)]
and Hildebrand and Hurley [see their figure
1 (Hildebrand and Hurley,
1985)] cannot be verified or refuted.
Ren and colleagues have not shown that elephant limbs are not markedly more columnar and otherwise distinctive from those of running mammals and birds. At most they have provided additional but not definitive evidence that elephant knees are significantly flexed especially when fast ambling, and that minor foot rotation occurs during the propulsive stroke. This does not alter the fact that elephant limbs are radically divergent from those of other extant large land animals, being overall less flexed and having short, massive distal segments, with the hindfoot especially short and limited in flexibility. As a result of this uncommon limb form elephants are restricted to an exceptionally slow ambling gait that does not include an entirely suspended phase. Conversely, elephant joint flexion during the propulsive stroke is limited because their limb excursion arcs are modest due to their combination of slow speed and large size. All ungulates and large birds use their more flexible limbs to achieve a full-suspended phase run, which in turn requires more extensive joint flexion and rotation during the propulsive stroke because limb excursion arcs are higher. Although ancient authors exaggerated the columnar rigidity of elephant legs, they correctly recognized that their limbs are dramatically different from those of faster animals. Conversely Ren and colleagues exaggerate the commonality of large animal limb form and function based on data that – although useful – is less reliable than they present because unavoidable data-gathering limitations prevent a truly detailed examination of large animal locomotion; only improved technologies for imaging large animal interior anatomy can solve the problem. When it comes to restoring peak locomotary performance, morphology continues to matter.
References
Alexander, R. M. and Pond, C. M. (1992). Locomotion and bone strength of the white rhinoceros. J. Zool. 227,63 -69.[CrossRef]
Alexander, R. M., Maloiy, G. M., Hunter, B., Jayes, A. S. and Nturibi, J. (1979). Mechanical stresses in fast locomotion of buffalo and elephant. J. Zool. 189,135 -144.[CrossRef]
Bakker, R. T. (1986). Dinosaur Heresies. New York: William Morrow.
Christiansen, P. and Paul, G. S. (2001). Limb bone scaling, limb proportions, and bone strength in neoceratopsian dinosaurs. Gaia 16,13 -29.
Gambaryan, P. P. (1974). How Mammals Run. New York: Wiley and Sons.
Granger, W. and Gregory, W. K. (1936). Further notes on the gigantic extinct rhinoceros, Baluchitherium, from the Oligocene of Mongolia. Bull. Amer. Mus. Nat. Hist. 72, 1-73.
Hildebrand, M. and Hurley, J. P. (1985). Energy of the oscillating legs of a fast-moving cheetah, jackrabbit, and elephant. J. Morphol. 184,23 -31.[CrossRef][Medline]
Kingdon, J. (1979). East African Mammals: vol. III part B (Large Mammals). Chicago: The University of Chicago Press.
Muybridge, E. (1957). Animals in Motion. New York: Dover Publications.
Osborn, H. F. (1942). The Proboscidea V I. New York: The American Museum of Natural History.
Paul, G. S. (1998). Limb design, function and running performance in ostrich-mimics and tyrannosaurs. Gaia 15,257 -270.
Paul, G. S. and Christiansen, P. (2000).
Forelimb posture in neoceratopsian dinosaurs: implications for gait and
locomotion. Paleobiol.
26,450
-465.
Ren, L., Butler, M., Miller, C., Paxton, H., Schwerda, D.,
Fischer, M. S. and Hutchinson, J. R. (2008). The movement of
limb segments and joints during locomotion in African and Asian elephants.
J. Exp. Biol. 211,2735
-2751.
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati 
Twitter What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
