Primate Anatomy 2: Primate Locomotion and Sexual Dimorphism

Primate Anatomy 2: Primate Locomotion and Sexual Dimorphism

Types of Primate Locomotion

• Quadrupedalism

  • Digitigrade: Walking on toes, typical in terrestrial animals.

  • Palmigrade: Walking on the palms of the hands, common among some primates.

  • Knucklewalking: Walking on the knuckles of the hands, found in some primate species like gorillas.

Quadrupedalism Features

• Anatomical Characteristics:

  • Laterally located scapula: This anatomical adaptation allows for greater lateral movement of the arms.

  • Long, flexible lumbar spine: Facilitates movement and flexibility needed for locomotion.

  • Deep thorax: Provides necessary volume for respiratory efficiency.

  • Posterior foramen magnum: Positioned towards the back of the skull, indicating a more horizontal head position during locomotion.

  • Arms/legs same length: Physical proportion supporting quadrupedal movement.

Arboreal Quadruped Characteristics

• Body Features:

  • Laterally placed scapula: Enhanced lateral arm movement.

  • Forelimbs and hindlimbs: Short and of similar length, adapting for climbing.

  • Long tail: Serves as a counterbalance to maintain stability in trees.

  • Restricted shoulder joint: Limits range of motion to enhance climbing abilities.

  • Grasping feet: Adapted for holding branches and providing support.

Terrestrial Quadruped Characteristics

• Body Features:

  • Forelimbs and hindlimbs: Long and of similar length for efficient movement on the ground.

  • Reduced tail: Less necessary for balance on solid ground.

  • Short toes: Adapted to provide stability and support on varied terrain.

Vertical Clinging and Leaping (VCL)

• Characteristics:

  • Vertical posture: Adapted for leaping between trees.

  • Small to medium body size: Typically found in species like lemurs.

  • Long hands and flexible back: Essential for grasping branches and powerful leaping mechanics.

  • Long hindlimbs: Enhanced propulsion during jumps.

Suspension Locomotion

• Types of Suspension:

  • True brachiation: Swinging under branches using arms.

  • Semi-brachiation: Combination of climbing and brachiation, utilizing both arms and legs.

  • Slow climbing/quadrumanual: Climbing using all four limbs.

Features of Suspension Locomotion

• Anatomical Features:

  • Long upper limbs: Allows for efficient swinging and reaching.

  • Short, stable lumbar spine: Provides stability during climbing.

  • No tail: Adaptation for improved balance and movement.

  • Scapula on dorsum: Positioned on the back for increased arm mobility.

  • Good grip with long, curved fingers: Essential for holding onto branches.

  • Mobile wrist joint: Facilitates a greater range of motion.

  • Very long forelimbs: Adapted for brachiation.

  • Dorsally placed scapula: Improves leverage in swinging.

Bipedality

• Characteristics:

  • Longer legs: Adapted for upright walking at the expanse of the lower limbs.

  • Foramen magnum positioned inferiorly: Reflects vertical posture of the head.

  • Broad thorax and broad ilium: Supports weight distribution during walking.

  • Non-divergent hallux (big toe): Aligned with other toes for stronger forward propulsion.

  • Short, broad ilium: Enhances balance in an upright position.

  • Curved lower back: Provides shock absorption and balance.

  • Large head to femur: Proportionate to support bipedal movement.

  • Short fingers and long heel: Features facilitating efficient bipedal walking.

  • Short toes: Essential for balance and energy efficiency during locomotion.

  • Great toe aligned with other digits: Contributes to a steady foot strike during walking.

Intermembral (IM) Index

• Formula:

  • $IM = rac{(humerus + radius)}{(femur + tibia)} imes 100$

• Categories:

  • 50-80: Indicates bipedal or vertical clinging and leaping (VCL).

  • 80-100: Typical for various forms of locomotion.

  • 100-150: Suggests quadrupedal locomotion.

• Notes on Proportions:

  • Shorter limbs/longer trunk: Typical in arboreal quadrupeds for climbing.

  • Longer limbs/shorter trunk: Indicative of terrestrial quadrupedal species.

  • Trunk Defined: The trunk is considered from the sternum to the base of the tail or end of the sacrum.

Sexual Dimorphism and Intra-Sexual Selection

• Social Structures:

  • Monogamous: One male, one female pair bonding.

  • Polyandrous: One female mates with multiple males.

  • Polygynous: One male mates with multiple females.

  • Single-male, multi-female groups: Organized social structure with one dominant male.

  • Multi-male, multi-female groups: Allowed interactions among multiple individuals.

Canine and Body Size Dimorphism

• Male and Female Canine Length:

  • Male: 1.2, Female: 1.4 (relative canine size increases)

• Body Size Dimorphism:

  • Male body weight compared to female body weight marked at values like 1.0, 1.2, 1.4, which indicates how body sizes relate to gender within species.

Examples of Primate Species

• Orangutans (Pongo)

• Gorillas

• Bonobos and Common Chimpanzees (Pan)

  • Bonobo

  • Scientific Name: Pan paniscus

  • Average Body Weight: 74 lbs

  • Average Height: 4.2 feet tall

  • Chimpanzee

  • Scientific Name: Pan troglodytes

  • Average Body Weight: 100 lbs

  • Average Height: 4.3 feet tall

  • Male average weight: 132 lbs and height of 4.4 feet tall

  • Female average weight: 93 lbs and height of 4.1 feet tall

• Siamangs (Hylobates)

• Family Callithricidae

  • Example: Saguinus fuscicollis (Brown mantled tamarin)

  • Nature Library Reference: Picture credit to Thomas Marent, Minden / naturepl.com with identifier 00554621.