Study examines link between giraffe limb length and walking efficiency
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ublished in the journal 'Proceedings of the National Academy of Sciences' (PNAS) on July 7, a study examines trade-offs between limb length and walking efficiency in giraffes. In many mammals, long, straight limbs confer a mechanical advantage that enhances speed and muscle efficiency.
However, the extremely long limbs of giraffes, which help in eating, may not promote athleticism. Christopher Basu and John Hutchinson produced musculoskeletal models of giraffe forelimbs and two related species Okapia johnstoni, a living species, and Sivatherium giganteum, an extinct species.
The authors measured muscle efficiency in walking giraffes using motion capture and force data, and estimated efficiency in the other two species. The muscle efficiency of giraffes was four times lower than expected based on data extrapolated from a variety of small animals, indicating that giraffe muscles must generate relatively large forces. The muscle efficiency varied across the three species tested; the giraffe and Sivatherium have similar body mass, but the giraffe has lower muscle efficiency.
The giraffe and okapi have similar muscle efficiency, even though the giraffe has 4-6 times the mass of the okapi. The results suggest that, at a certain size, the increase in muscle efficiency associated with increased body mass reaches a plateau. According to the authors, tall animals are likely to have relatively lower muscle efficiency, unless a parameter known as the muscle moment arm, which is the distance between the muscle and the pivot point of the joint, is proportional to their limb length.
Giraffes are the tallest living animals, using their height to access food unavailable to their competitors. It is not clear how their specialized anatomy impacts their athletic ability. We made musculoskeletal models of the forelimbs from a giraffe and two close relatives and used motion-capture and force data to measure how efficient they are when walking in a straight line. A horse, for example, uses just 1 unit of muscle force to oppose 1 unit of force on the ground. Giraffe limbs are comparatively disadvantaged--their muscles must develop 3 units of force to oppose 1 unit of force on the ground. This explains why giraffes walk and run at modest speeds.
Giraffes (Giraffa camelopardalis) possess specialized locomotor morphology, namely elongate and gracile distal limbs. While this contributes to their overall height and enhances feeding behavior, we propose that the combination of long limb segments and modest muscle lever arms results in low effective mechanical advantage (EMA, the ratio of in-lever to out-lever moment arms), when compared with other cursorial mammals. To test this, we used a combination of experimentally measured kinematics and ground reaction forces (GRFs), musculoskeletal modeling, and inverse dynamics to calculate giraffe forelimb EMA during walking. Giraffes walk with an EMA of 0.34 (+-0.05 SD), with no evident association with speed within their walking gait. Giraffe EMA was about four times lower than expectations extrapolated from other mammals, ranging from 0.03 to 297 kg, and this provides further evidence that EMA plateaus or even diminishes in mammals exceeding horse size.
We further tested the idea that limb segment length is a factor which determines EMA, by modeling the GRF and muscle moment arms in the extinct giraffid Sivatherium giganteum and the other extant giraffid, Okapia johnstoni. Giraffa and Okapia shared similar EMA, despite a four to sixfold difference in body mass (Okapia EMA = 0.38). In contrast, Sivatherium, sharing a similar body mass with Giraffa, had greater EMA (0.59), which we propose reflects behavioral differences, such as a somewhat increased capability for athletic performance. Our modeling approach suggests that limb length is a determinant of GRF moment arm magnitude and that unless muscle moment arms scale isometrically with limb length, tall mammals are prone to low EMA.
We have highlighted that giraffes use lower-than-expected effective mechanical advantage, as their musculoskeletal morphology (such as the ulna's olecranon process) is insufficient to maintain the observed trend in EMA in animals up to 300 kg. Our results from an analysis of modeled GRF moment arms and muscle moment arms suggested that giraffes' EMA is similar to okapis, a giraffid with lower body mass and more plesiomorphic locomotor traits.
Low EMA was not ubiquitous among the giraffids, as S. giganteum was predicted to have greater EMA, but still low compared to smaller mammals, even horses. The differential EMA between Sivatherium and Giraffa may reflect behavioral or athletic differences between these two similarly sized giraffids, which more elaborate methods, such as simulations, could test. While giraffes' feeding ability is driven by extreme height, it appears that extreme cursoriality has come with a functional trade-off with locomotor performance.
✔️ Study examines link between giraffe limb length and walking efficiency
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