Anatomical Adaptation for Cursorial Locomotion
(Designed for Running)
Running is mainly about speed -- obtained by combining stride rate and stride length. Stride rate is favored by reducing limb mass (especially distally). Stride length is gained by increasing limb length (anatomically or physiologically). Rate and length are antagonistic, i.e., enhancing one compromises the other.
Domestic mammals exhibit cursorial (running) adaptations to varying degrees:
Carnivores require multipurpose limbs (both running & manupilating). Carnivores are fast because they have flexible trunks, which is possible because they have a meat diet (which is why they need multipurpose limbs in the first place).
Herbivores, with roughage diets and bulky abdominal viscera, have relatively limited trunk flexion, but their limbs are freed to be devoted entirely to locomotion. Equine limbs have become so specialized, they resemble "machines" (reliance more on bone & ligament and less nerve and muscle).
Thus locomotion and digestion are linked in evolution. This document considers:
Anatomical adaptations for running (forelimb)
Forelimb vs. hindlimb differences
Carnivore vs. herbivore digestive tracts
Muscle/Skeletal Specialization of Forelimbs
Anatomical limb design involves compromises between flexibility (rotation & dexterity) and stability (joint fixation & secure contact with a surface). Anatomical specialization has evolved to facilitate different functional roles, including the different roles of thoracic and pelvic limbs (compared below).
Generalized limb anatomy (evident in many mammals, reptiles, and birds)
- scapula positioned dorsally on a flat back
- shoulder joint fixed & capable of circumduction (ball and socket joint)
- two bones in forearm, enabling rotation while providing stability
- ulna firmly attached to the elbow (loosely attached to the wrist)
- radius firmly attached to the wrist (loosely attached to the elbow)
- plantigrade contact with the ground
- multiple digits (typically five)
- unguis (nail, claw, hoof)
Limb adaptations running (to enhance stride length/rate)
- scapula positioned laterally on a mediolaterally flattened thorax (which lengthens stride)
- shoulder joint free to move (becomes effectively a hinge joint stabilized by muscles)
- brachium remains highly muscular & relatively short (keeps limb mass proximal)
- radius becomes the dominant bone in the antebrachium (stability favored over rotation)
- reduction of digits in the manus & pes (stability favored over manipulative dexterity)
- conversion of musculature to ligament (reduced mass & reliance on passive mechanics)
- elongation of the manus/pes (particularly the metacarpus/metatarsus and digits)
- ground contact made with digits (digitigrade) or hoof (unguligrade) (vs plantigrade)
Forelimb vs. Hindlimb Differences
Forelimbs and hindlimbs have different roles in cursorial quadrupeds. Consequently they are anatomically different:
Forelimbs (thoracic limbs) carry 60% of static body weight. They are designed to catch body weight as it is thrown forward by pelvic limbs. Forelimbs improve gait efficiency by minimizing wasteful up/down energy expenditure (they absorb kinetic energy of downward movement , store it as potential energy in stretched ligaments, and in turn convert that to upward kinetic energy). Anatomical features of the forelimb include:
- shorter and straighter than the pelvic limb
- connected to the trunk only by muscle and ligament (not bone to bone articulation)
- broader, more rounded hoof (horse)
Hindlimbs (pelvic limbs) are the "motors" of locomotion. They drive the trunk forward and propel the body up/over obstacles during jumps. Anatomical features of the hindlimb include:
- longer and more angular than the thoracic limb
- osseous articulation to the trunk (axial skeleton) through a sacroiliac joint
- musculature capable of simultaneously extending hip, stifle, and hock
- narrower, more pointed hoof (horse)
Carnivore vs. Herbivore Digestive Tracts
The different diets of domestic mammals have associated anatomical, physiological, and behavioral implications. Locomotor anatomy and behavior are linked to diet, the nature of the digestive tract in the abdomen has implications for locomotion. Features of three types of digestive tracts are:
Canine digestive tract
- simple glandular stomach
- small cecum and simple colon
- small abdominal volume especially when fasted
- high protein diet
Ruminant digestive tract (cattle, sheep, goat, deer, etc.)
- huge stomach with four compartments (fermentation chambers and a glandular compartment)
- large cecum and coiled ascending colon
- large abdominal volume even if fasted
- rumen flora and fermentation products supplement a roughage diet
Equine digestive tract
- simple stomach (half glandular)
- huge cecum and ascending colon
- large abdominal volume even if fasted
- roughage diet requires a protein source (obtained via grain or coprophagy)
The carnivore lifestyle requires limb flexibility and control for grasping and manipulating prey and defending self, offspring, and territory. Thus limb elongation, bone reduction, digit elimination, and conversion of muscle to ligament is moderated. The unguis (nail) in not used for locomotion and remains available as a "tool". To catch long-leg prey, the carnivore must obtain long stride length physiologically, by trunk flexion and extension (which requires much energy expenditure).
The equine exhibits extreme anatomical specialization for running. It walks on the hoof of a single digit of an elongate manus/pes in which muscle is replaced by ligament. The digestive apparatus of the horse predisposes it to continual grazing (in the wild). Accordingly, the horse has evolved ligament structures that facilitate such a lifestyle, including prolonged standing with minimal expenditure of muscular energy (stay apparatus), mechanical joint linkage (reciprocal apparatus), and mechanical energy conservation through potential/kinetic energy exchange (e.g., fetlock translation).