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I was watching a nice little video on youtube but couldn't help but notice how snappy smaller animals such as rats and chipmunks move. By snappy I mean how the animal moves in almost discrete states pausing between each movement.
Is this a trivial observation or something inherent in the neuro-synapse or muscular make-up of these animals?
Short answer
Intermittent locomotion can increase the detection of prey by predators (e.g. rats), while it may lead to reduced attack rates in prey animals (e.g., rats and chipmunks). It may also increase physical endurance.
Background
Rather than moving continuously through the environment, many animals interrupt their locomotion with frequent brief pauses. Pauses increase the time required to travel a given distance and add costs of acceleration and deceleration to the
energetic cost of locomotion. From an adaptation perspective, pausing should provide benefits that outweigh these costs (Adam & kramer, 1998).
One potential benefit of pausing is increased detection of prey by predators. Slower movement speeds likely improve prey detection by providing more time to scan a given visual field.
A second plausible benefit is reduced attack rate by predators. Many predators are more likely to attack moving prey, perhaps because such prey is more easily detected or recognized. Indeed, motionlessness (‘freezing’) is a widespread response by prey that detect a predator.
A third benefit may be increased endurance. For animals moving faster than their aerobically sustainable speeds, the maximum distance run can be increased by taking pauses. These pauses allow the clearance of lactate from the muscles through aerobic mechanisms.
PS: If you mean with 'snappy' not only that small animals move intermittently, but also 'fast', then Remi.b's answers nicely covers the story why small critters are quick. Basically, it comes down to Newton's second law, namely acceleration is inversely proportional to mass (a = F/m), but the size of muscle power is not. Hence, bigger animals have more mass and need a lot more force to build up to accelerate at the same speed. That build up of force needs time (ever witnessed the vertical lift-off of a space shuttle?) Hence, small critters accelerate quicker and allow them to move 'snappy'.
Reference
- Adam & kramer, Anim Behav (1998); 55: 109–117
In addition to @AliceD's excellent answer, I would like to add that a simple mechanistic relationship between body size and "snappiness" may explain the observed pattern.
Basics of biophysics
Difference in snappiness may result from a difference in the ability to accelerate your movement.
An increase in body size over one dimension (=body length) $x$ scales with an increase in volume and mass of the order of $x^3$. Muscle strength is dependent on the cross-sectional area of the muscle and therefore by a linear increase of $x$, strength increases by $x^2$.
Digging back to high school physics lectures, $F=Ma \left(\text{or } a=\frac{F}{M}\right)$ , where $F$ is a force, $M$ a mass and $a$ an acceleration. With an increase $x$ of the individual size (over one dimension, the length of the individual if you prefer), the force increases by $x^2$ and the mass increases by $x^3$, therefore the acceleration gets multiplied by $\frac{x^2}{x^3} = \frac{1}{x}$. Of course, nature is not all that easy, but if we assume it is, then the relation between the acceleration of movements and body size (=body length) should look like this:
An animal that is 4 times as big should make accelerate its arms and legs 4 times less.
Contrasting to @AliceD's answer
@AliceD listed plausible hypotheses for why there might have a selection for snappy movement in small animals but simpler explanation based on the physical properties of bodies might be enough to explain the observed pattern. It would be interesting to know exactly what the relationship between body size (=body length) and "snappiness" look like in nature to further this discussion.
Fun Facts
The mantis shrimp (see below picture) is able to accelerate its claws up to 10,400g (reference). More information about g (unit of acceleration) here. In short, a typical human can support up to 5g. 10,400g is absolutely insane! To offer some comparison: a baseball gets an acceleration that is only about a third of that of the mantis shrimp's claws. Mantis shrimps use their claws to break open clams. Here is a video. According to wikipedia, due to the friction of the claws against the water, small bubbles are formed and the temperature at the edge of those bubbles can go up to several thousands of °C.
Apparently, a jellyfish stinger can go up to 5,400,000g (reference) which appears to be about 30 times more than a bullet. You will find a list of examples of things that undergo various levels of acceleration on this wikipedia list.
Layman's hypothesis, but... It seems an animal using intermittent movement alternates between a hard-to-see state and a hard-to-catch state, with some degree of safety in both. Operating at a slower, sustained pace offers neither of these benefits and significantly less safety.
Wondering if "small mammals" is the right descriptor. The obvious examples seem to be rodents. Also vague: cats are small relative to horses, but they're not intermittent.
Maybe another way to look at this is "When/why does an animal uses their maximum speed?" Rodents seem to do it as a matter of routine locomotion, most mammals do it only when actively fleeing or pursuing.
Small lizards seem more akin to rodents in this regard than not-quite-as-small mammals.
