How does the trunk differ between humans and other mammals

Humans are built for bipedalism (upright walking). Most mammals are built for quadrupedalism (moving on four legs). The key differences show up in the spine, pelvis, and rib cage.

Spine

Humans

• S-shaped spine: thoracic kyphosis (outward) + lumbar lordosis (inward).
• Acts like a shock absorber and keeps the head balanced over the trunk.
• Transfers weight efficiently to the legs.
• Lumbar vertebrae are larger to carry upper-body load.

Other mammals

• Spine forms a single gentle arc.
• Works like a spring for four-legged propulsion and absorbs forces horizontally.
• Lower center of gravity with weight spread across four limbs.

Pelvis

Humans

• Short, broad, bowl-shaped pelvis.
• Supports internal organs in an upright body.
• Iliac blades rotated backward; large sacroiliac joint stabilizes the upright trunk.

Other mammals

• Longer, flatter, narrower pelvis.
• More gracile with a narrow sacrum and smaller sacroiliac surface.

Rib cage

Humans

• Broad and shallow; reduced front-to-back depth.
• Brings center of gravity closer to the spine for better balance when walking.
• Creates a narrow waist.
• During gait, thorax rotates opposite to the pelvis, cutting wasted motion.

Other mammals

• Deeper, funnel-shaped rib cage that flares at the bottom.
• Extra space for a larger digestive tract.
• In bipedal bouts (e.g., chimpanzee), thorax and pelvis rotate together, unlike humans.

Functional consequences

• Bipedal efficiency (humans): Excellent for energy-saving walking and endurance running, with trade-offs.
• Spinal problems (humans): Curved, load-bearing spine is prone to lower back pain—rare in quadrupeds.
• Childbirth (humans): Narrower pelvis aids walking but makes delivery more difficult.
• Mammalian diversity: Other species tune their trunks for different tasks. Example: the elephant’s trunk is a muscular, boneless fusion of nose and upper lip—built for grasping, not support.