Human bipedalism and body-mass index

Area of Science:

• Comparative physiology
• Anthropometry
• Biomechanics

Background:

• Body-Mass Index (BMI), calculated as mass/height squared (M/H²), is a standard health indicator.
• Generalizing BMI to M/H^p allows for cross-species comparison of scaling principles.

Purpose of the Study:

• To determine the generalized BMI exponent (p) across diverse animal species, including humans.
• To investigate the unique scaling of human body mass relative to height.

Main Methods:

• Analysis of various human and animal (fish, whales, land mammals) population datasets.
• Calculation and comparison of the exponent 'p' in the generalized BMI formula (M/H^p).

Main Results:

• Non-human animals consistently exhibit a generalized BMI exponent (p) of approximately 3.
• Human infants show a p value greater than 2, while toddlers (1-5 years) display a p value around 2.
• This contrasts sharply with the p ≈ 3 observed in other animal species.

Conclusions:

• Human body mass scaling differs significantly from other animals, with p ≈ 2 in young children.
• The upright posture and bipedal locomotion of humans are proposed as key factors driving this distinction.
• A mechanical model supports the division between bipedal humans (p ≈ 2) and quadrupedal/other animals (p ≈ 3).