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First published online December 16, 2008
Journal of Experimental Biology 212, 153-154 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.025098
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Correspondence |
Response: Of ideas, dichotomies, methods, and data – how much do elephant kinematics differ from those of other large animals?
Structure and Motion Laboratory, The Royal Veterinary College, Hawkshead Lane, Hatfield, AL9 7TA, UK
jrhutch{at}rvc.ac.uk
Gregory Paul's reply demonstrates that our study's critique
(Ren et al., 2008
) of how
previous studies have dichotomized animal limb function into `flexed
vs columnar' categories was on the mark. He again divides limb
configurations into either `strongly flexed' or `highly extended' but where do
the boundaries between these categories lie? Do large animals supposedly
representing such categories, such as horses and elephants, differ appreciably
from each other?
A quantitative approach is needed to shed light on this question, as
postures are best described in terms of joint and segment angles. For example,
the angles of limb joints/segments at mid-stance (approximately corresponding
to maximal limb loading) in ambling elephants and trotting horses at
equivalent size-normalized speeds are shown in
Table 1. The data shown for
horses are firmly validated with radiographic studies, as we cited. Horse and
elephant thigh limb segment angles at mid-stance are quite similar whereas
humerus segments may be slightly more protracted in horses (we repeat that
elephant scapular motion remains to be measured). Furthermore, the wrist joint
angles of running horses and elephants are almost identical. Elephants have
30–60 deg. more flexed elbows, knees and ankles than horses do.
Similar results hold for other instances in the stride cycle or other speeds.
Where then do the supposed sharp divisions between `flexed' and
`columnar/extended' limb orientations lie?
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Quantitative data (Table 1) show that such dichotomies have no legs to stand on. Indeed, elephants' limbs could be said to be more flexed than horses' limbs during running – although we took a more cautious approach and described them as being roughly similar.
Paul repeats many of our points about the technical problems that face
interpretation of elephant (and other large animal) locomotion. His approach
(fitting a skeleton inside Muybridge's images of a single stride) is not
equivalent to ours (in which six digital cameras captured 240 images per
second at 1 mm accuracy, for 288 complete strides of 15 elephants, with joint
landmarks palpated and visualized, and thorough statistical analysis). Our 3-D
motion analysis approach is well accepted and validated as a reasonable
standard (e.g. Back et al.,
1995a; Back et al.,
1995b) despite the technical flaws we discussed; his is
unvalidated. Because Paul simply used Muybridge's data and did not provide a
reliable method or quantitative results (unlike the other studies we
discussed), we did not cite his small illustrations of an elephant in Paul and
Christiansen [fig. 5D in Paul and
Christiansen, 2000 (Paul and
Christiansen, 2000)]. Likewise, we did not cite countless images
of moving elephants in popular books. We felt all of these potential sources
of data were too unreliable.
Paul has provided no new information on the problem of skin motion for experimental studies whereas we treated the subject cautiously with a statistical analysis of marker placement and a restraint from quantifying proximal joint angles, which are expected to have the worst artifacts. In addition, we used defleshed cadavers, which lacked such artefacts, to show that horse and elephant limbs have broadly similar flexibility.
Paul tries to argue that the two methods are more or less equivalent and the problems so difficult that it's anyone's guess how elephant limbs move; the issue is `open to question' and `cannot be verified or refuted.' We find this attitude disturbingly anti-scientific and quite misleading as it implies comparably accurate methodology.
Our method of quantifying foot skeletal joint angles was not flawed for the
purposes of our analysis that was to estimate skeletal joint and segment
angles, not to estimate the centre of pressure, which essentially all
researchers have assumed would be more caudally positioned along the foot.
Paul appears to conflate where the centre of pressure is with where the axis
of foot motion is. The former is probably near mid-foot (our unpublished
data); the latter is along the third digit. Estimating skeletal motion from
the lateral side of the foot would underestimate ankle joint flexion and
confuse ankle abduction and flexion. Motion of the short digits of elephants
would not greatly change our results. Indeed our unpublished in vitro
and radiographic data support the assumption that the toes move very little
and that our markers were positioned on skeletal landmarks that enable
approximation of ankle joint motion. Paul repeats descriptions of the same
foot (and footpad) anatomy and function that we have described in three papers
on that subject (Weissengruber et al.,
2006; Hutchinson et al.,
2008; Miller et al.,
2008), so we do not see what he is critiquing. Again, he is
providing no new information.
We showed how elephant ankles dorsiflex, then plantarflex during the stance phase (as in most other mammals with spring-like ankles), exhibiting classic spring-like kinematics. Paul provides no evidence or coherent argument to falsify our measurements; the statement that elephant ankles are `too short and inflexible' is a non-sequitur. Precisely how long would they need to be and what kind of quantitative motion would they need to exhibit to be spring-like? Specifically, how much more `functionally columnar' is an elephant's foot than a horse's foot, if the skeletal kinematics are less columnar in elephant feet? Certainly more data on foot mechanics for elephants and other large animals would be useful and our ongoing studies are fulfilling this need.
Whereas few studies have provided reliable new data, we cited numerous
studies (e.g. Paul and Christiansen,
2000) that have talked about elephant locomotion without measuring
or carefully considering empirical data. Some studies hung elaborate ideas
about broader patterns in animal locomotor evolution on tidy categories like
`flexed' and `columnar.' In our introduction, we first cited some antiquated
notions about elephant limbs as being inflexible or simply column-like and
then noted `ridiculous as those fallacies may seem to contemporary scientists,
elephant posture and gait remain misunderstood, partly because of their
strange anatomy and partly because of little rigorous measurement of elephant
locomotion' [p. 2735 in Ren et al. (Ren et
al., 2008)]. That sentence summarizes the current dialogue quite
well.
Morphology can matter; indeed we agree with Paul that the locomotor differences between elephants and other animals may hinge upon anatomy and other factors, so we suspect that there is much more to the mystery than simple dichotomies and qualitative anecdotes. For example, Fig. 1 supports the inference that previously perceived differences in posture between `cursorial' horses and `graviportal' elephants are largely due to different limb proportions, i.e. relative lengths of the proximal and distal limbs, not more flexed limbs in horses.
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Gait analysis of elephants and other large animals needs more data but Paul offers none. Ideas about animal locomotion are not enough, especially when weighed down with ponderous conceptual baggage and antiquated methodology. To contribute to animal locomotor science, researchers must generate novel data with reliable methods. We stand by our original methods and data.
References
Back, W., Schamhardt, H. C., Savelberg, H. H. C. M., van den Bogert, A. J., Bruin, G., Hartman, W. and Barneveld, A. (1995a). How the horse moves: 1. Significance of graphical representations of equine forelimb kinematics. Equine Vet. J. 27,31 -38.[Medline]
Back, W., Schamhardt, H. C., Savelberg, H. H. C. M., van den Bogert, A. J., Bruin, G., Hartman, W. and Barneveld, A. (1995b). How the horse moves: 2. Significance of graphical representations of equine hind limb kinematics. Equine Vet. J. 27,39 -45.[Medline]
Hutchinson, J. R., Miller, C. E., Fritsch, G. and Hildebrandt, T. (2008). The anatomical foundation for multidisciplinary studies of animal limb function: examples from dinosaur and elephant limb imaging studies. In Anatomical Imaging: Towards a New Morphology (ed. H. Endo and R. Frey), pp.23 -38. Tokyo: Springer-Verlag.
Miller, C. M., Basu, C., Fritsch, G., Hildebrandt, T. and
Hutchinson, J. R. (2008). Ontogenetic scaling of foot
musculoskeletal anatomy in elephants. J. R. Soc.
Interface 5,465
-476.
Paul, G. S. and Christiansen, P. (2000).
Forelimb posture in neoceratopsian dinosaurs: implications for gait and
locomotion. Paleobiology
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.
Weissengruber, G. E., Egger, G. F., Hutchinson, J. R., Groenewald, H. B., Elsässer, L., Famini, D. and Forstenpointner, G. (2006). The structure of the cushions in the feet of African Elephants (Loxodonta africana). J. Anat. 209,781 -792.[CrossRef][Medline]
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