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.
Using the tip of the toe also exaggerates apparent rotation of the ankle because the rotation of the toe segments is being added to the total. Hildebrand and Hurley [see their figure 1 (Hildebrand and Hurley,1985)], 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.