Three‐Legged Locomotion and the Constraints on Limb Number: Why Tripeds Don’t Have a Leg to Stand On

Abstract

Three-legged animals do not exist today and such an animal is not found in the fossil record. Which constraints operate to result in the lack of a triped phenotype? Consideration of animal locomotion and robotic studies suggests that physical constraints would not prevent a triped from being functional or advantageous. As is reviewed here, the strongest constraint on the evolution of a triped is phylogenetic: namely, the early genetic adoption of a bilaterally symmetrical body plan occurring before the advent of limbs. Presumably, this would greatly constrain any three-legged animal from ever evolving. Tripedalism is employed only by a few animals, but many use a tripod stance while engaged in a variety of activities. Because terms are often used interchangeably in the literature, a standardization of locomotion terminology is proposed. Understanding the constraints behind "forbidden" phenotypes forces us to confront gaps in our evolutionary understanding of which we may be unaware.

Overcoming a ‘forbidden phenotype’: the parrot's head supports, propels and powers tripedal locomotion

“…One well-known, but anecdotal [6,11,12], example of tripedal locomotion is the climbing behaviours of parrots (Order: Psittaciformes) [1,6]. Unable to use their wings as grasping forelimbs, parrots have evolved to be resourceful climbers by co-opting their feeding apparatus as an additional 'limb' [4].…”

Overcoming a “forbidden phenotype”: The parrot’s head supports, propels, and powers tripedal locomotion

“…The silhouettes placed on the right correspond with distinct events during a stride. (a) Average (top) and peak (bottom; box and whisker plots) propulsive/braking substrate reaction force (SRF) demonstrate that the beak (orange) provides equal propulsive force to the hindlimb (blue) and is comparable to human (1) and primate (2) forelimbs during climbing. (b) Tangential SRF illustrates that the beak provides an equal but opposite pulling force (top) to the pushing force of the hindlimb to balance on the vertical substrate.…”

“…There is no odd-legged vertebrate alive today, nor is there any evidence of such a creature ever existing in the fossil record (1). This “forbidden phenotype” is inherently reliant on the presence of a midline limb, but phylogenetic constraints of bilateral limb development circumvent such a possibility (1). Despite the lack of odd-limbed tetrapods, tripedal and pentapedal gaits have emerged in multiple lineages, including mammals and birds (1–3).…”

“…This “forbidden phenotype” is inherently reliant on the presence of a midline limb, but phylogenetic constraints of bilateral limb development circumvent such a possibility (1). Despite the lack of odd-limbed tetrapods, tripedal and pentapedal gaits have emerged in multiple lineages, including mammals and birds (1–3).…”

“…One well-known, but anecdotal (but see 2), example of tripedal locomotion is the climbing behaviors of parrots (Order: Psittaciformes) (1, 4). Unable to use their wings as grasping forelimbs, parrots have evolved to be resourceful climbers by co-opting their feeding apparatus as an additional “limb” (2).…”