Interspecific Scaling of the Morphology and Posture of the Limbs during the Locomotion of Cats (Felidae)

1. What they did

A long-standing idea in biology is that bigger mammals walk on straighter, more upright legs than small ones. Most of the evidence comes from comparing very different animals, such as mice and elephants, where differences in body plan could hide the effect of size alone. The authors asked whether the idea holds within one closely related group: the cats.

They videotaped 25 captive cats of nine species walking past a side-on camera: domestic cat, serval, ocelot, Canadian lynx, leopard, cheetah, cougar, lion and tiger. The cats ranged from under 4 kg to nearly 200 kg. Paper stickers on the fur marked the shoulder, elbow, wrist, hip, knee, ankle and toe joints. Frame by frame, the authors measured the length of each leg bone, the angle of each joint, the angle of each bone from vertical, and the height of the shoulder and hip, at two moments: when the foot first touched down (footfall) and halfway through its time on the ground (midstance). They chose only strides with a similar walking gait and then tested whether any of these measures changed with body mass.

2. What they found

Body shape

• Lions and tigers had limbs and bodies about three times as long as domestic cats. The heaviest species were about 46 times the mass of the lightest.
• In every species the hind legs were longer than the front legs.
• Apart from overall size, most species had very similar limb proportions.
• Cheetahs and lynx had long legs for their mass. Cheetahs did not have long legs for their body length.
• Servals, often described as long-legged, did not have unusually long legs for their mass or body length, though their foot bones (metacarpals and metatarsals) were relatively long.

Walking

• All cats used a walk in which each foot was on the ground for about 64% of the stride on average. Larger species took fewer strides per second, but speed and relative stride length were not closely linked to size.
• The pattern of leg movement was the same in all species. For much of the time a foot was on the ground, the elbow, knee and ankle stayed bent at about 135 degrees. These joints were never fully straightened at any point in the stride.
• Legs were most upright at midstance, with the foot nearly under the shoulder or hip. Even then, shoulder and hip height were usually under 80% of total leg length, so the legs were never fully straight.
• Of 24 angles measured at footfall and midstance, only three changed significantly with body mass (elbow angle, humerus angle and metatarsal angle, all at midstance). The authors note that even a modest correction for multiple tests would make these non-significant.
• Larger cats did not walk with more upright legs than smaller cats.
• The leopard had the most crouched front and hind legs of any species, and the ocelot had among the lowest shoulder and hip heights.

3. What they concluded

• Over a nearly 50-fold range in body mass, cats walk with almost the same moderately upright leg posture. Unlike in broad comparisons across mammals, bigger cats do not have straighter legs.
• Because leg posture hardly varies, neither the cats’ family tree nor their ecology predicts much about it. The authors suggest this posture was inherited from a fairly large ancestral cat and has stayed the same as different cats evolved to be bigger or smaller.
• The slightly more crouched posture of leopards and ocelots may be linked to their climbing habits, as many animals crouch on branches and slopes.
• Cheetahs (built for very fast sprints), lynx (with snowshoe-like feet) and servals (tall hunters of long grass) did not have especially distinctive walking postures.
• Whether this sameness is unusual or typical for closely related groups of mammals will only become clear once similar data are collected for other groups.

6. Other technologies, skills or innovations

• Side-on digital video (JVC GR-DVL 9800, 30 frames per second) with a reference grid in view for scale.
• Temporary paper stickers placed on the fur over each joint, after feeling and moving the joint to find it.
• Frame-by-frame motion analysis software (DgeeMe) to measure joint and bone angles.