- 14 Aug 2003
- This page is dedicated to discussion on leg number. What are the advantages/disadvantages of having 4 legs or 6? What are the main trade-offs? Perhaps we should take into account the goals of the robot and what we plan to do with it later (when changing the number of legs will be much more difficult).
4 Legged Design:
- Lighter, if legs are the same as with 6 legged robot
- Every creature bigger than an insect uses only 4 legs (+dextrous tail)-> Must be some good reason for this
Every creature with a backbone has 4 legs. This is an evolutionary constraint of the way vertebrates develop from embryos. An internal skeleton has allowed vertebrates to be terrestrial (support against gravity), as the exoskeleton has allowed arthropods (including spiders, crabs, and insects) to be terrestrial.
In comparison to vertebrates, the developmental genetic control (HOX genes) in arthropods includes much more flexible serial homology (repetition of segments), and much more flexibility in leg number. However, an exoskeleton scales poorly in comparison to an endoskeleton with respect to mass and gas transport at the skin.
Seeing that many animals have chosen to adapt their spinal column into a useful tail (which is similar to an additional leg), and that there is probably no other feasible way for a vertebrate to sprout another limb, perhaps vertebrates could have been better off with more limbs. In other words, some vertebrates went from 4 -> 5 limbs, and had to stop because of the limitations of genetic mutations. Without such limitations, who knows how many limbs they would choose as optimal?
Q: Why do large animals have 4 legs?
A: Because large animals are vertebrates and vertebrates are constrained to have 4 limbs.
Q: Why are insects small?
A: Because insects have exoskeletons that become heavy at large body sizes, and because the tracheal ventilation system in insects does not scale well at large body size because of diffusion limitation.
THIS IS THE REAL QUESTION:
Q: Why don't arthropods have 4 legs?
A: Developmentally, they could. Losing limbs is not difficult for arthropods. Indeed some insects and spiders have fewer functional legs. However, 6 legs almost certainly has adaptive advantages relative to fewer. Bob's group has shown many of these advantages.
Arthropods can lose legs in everyday interactions with the environment, and have developed ways to regrow the legs so that they aren't handicapped for long. A 3-legged crab would not stand a chance, while a 7 or even 6-legged crab can still operate well. Is this a primary advantages to having more legs, or are there other good reasons?
- May require flexible body (look at flexure of Gecko spine) -> This may make carrying a payload more difficult
- Location of center of gravity is more critical -> Moving equipment such as a camera may offset this balance
6 Legged Design:
- Greater security (if one leg slips, no catastrophe)
- Fewer DOF for legs may be possible -> Lighter legs
- Front legs could be used for other things , in addition to climbing (manipulating objects, transitioning from horizontal walking to climbing a steep surface)
- Heavier, if same leg design
Testimonials/ Gut Feelings: