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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?

Bikke the Chipmunk Yawning and Stretching

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    $\begingroup$ Not just mammals. Insects and reptiles do this too. $\endgroup$ – Brock Adams May 29 '15 at 23:21
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    $\begingroup$ Human soldiers do it, too. And many animals live in what amounts to a war zone. $\endgroup$ – Jonas May 30 '15 at 10:54
  • $\begingroup$ Are they actually pausing more than larger animals? I mean, if I speed up videos of elephants they look snappy with intermittent movements (or e.g. some of the people in this time lapse), and if I slow down videos of chipmunks they look smooth and continuous like larger animals (sort of like this). I think it's just a relative time thing? $\endgroup$ – Jason C May 31 '15 at 5:02
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    $\begingroup$ Just to add an interesting (but exceptional) example, tiger beetles move so fast that they are essentially blind during movement; they dash in short straight lines, then must stop to locate their prey and make another dash. Not the case for most animals, but a fun one. $\endgroup$ – Steve Heim Dec 25 '16 at 8:36
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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

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  • $\begingroup$ I have no evidence for this, but I would also wager that stopping will improve your ability to scan your environment for predators. $\endgroup$ – agweber May 29 '15 at 19:01
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    $\begingroup$ Motion not only greatly increases your chances of being seen by something that wants to eat you (or something you want to eat), but it also increases your chances of being heard (or vibrationally detected), and/or smelled. $\endgroup$ – Brock Adams May 29 '15 at 23:27
  • $\begingroup$ Well, this was pretty obvious answer. But why does this only apply to tiny creatures like mice and insects? $\endgroup$ – Tomáš Zato Nov 3 '15 at 15:49
  • $\begingroup$ @TomášZato - I've never said it did. However, I agree a PS might be in place. What about the postscriptum add-on? Does that satisfy your critiques? (Why do I hate downvotes so much :-) $\endgroup$ – AliceD Nov 4 '15 at 0:10
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    $\begingroup$ @TomášZato it doesn't, it is just that for larger animals the time between pauses is long enough people tend not to notice, many prey animals pause and scan for predators. also in larger animals movement between food sources is less common since they are both more energy efficient and tend to have different feeding mechanisms, so pauses are harder to notice. royalsocietypublishing.org/doi/pdf/10.1098/rspb.2000.1388 $\endgroup$ – John Mar 28 at 13:27
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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:

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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.

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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.

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  • $\begingroup$ I think the question is not on why they are fast, or accelerate quickly, but why they move intermittently. $\endgroup$ – AliceD May 30 '15 at 10:15
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    $\begingroup$ I think this rather explains why larger animals do not make snappy movements: because all that accellerating and decellerating consumes too much power. $\endgroup$ – Hagen von Eitzen May 30 '15 at 12:24
  • $\begingroup$ Difference in snappiness may result from difference in the ability to accelerate your movements. Larger animals do not make snappy movement because they can't while smaller animals can. It is true it doesn't explain why they would not slow down their movement. I would tend to think that it is simply generally beneficial to move fast. $\endgroup$ – Remi.b May 30 '15 at 14:48
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    $\begingroup$ @AliceD Maybe it's related? I mean, watching the OP's linked video, at least, it seems like being able to do things more quickly (interpreted as "snappy") leaves more time to do nothing in between. I know it's different, but if I could move 10x faster than I could now, I'd probably spend a bit more time not moving at all, since I already accomplished the intended movement and wouldn't fill time with unnecessary movements for no reason. I totally am pulling this comment out of my butt, though, so I dunno. $\endgroup$ – Jason C May 31 '15 at 3:56
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    $\begingroup$ (Do bigger animals pause a lot too but on a slower scale? Like, if something makes a noise, I turn my head to look. Then I pause. Then I resume what I was doing. Does that count as intermittent but slower, same thing but not "snappy" ["snappy" just meaning relatively fast compared to the observer, really]?) $\endgroup$ – Jason C May 31 '15 at 3:58
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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.

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From the systems theory point of view it is because they have a lot less inertia, so their feedback systems operate much more rapidly, with a 'snap' action.

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  • $\begingroup$ The question body elaborates on the question: "...pausing between each movement". The question is not on why it is fast per se, or why they accelerate quickly, it is about the pauses in-between movements that is asked for. $\endgroup$ – AliceD May 30 '15 at 10:08
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    $\begingroup$ Could you expand further on this answer, as at the moment it's more of a comment. $\endgroup$ – Rory M May 31 '15 at 11:13
  • $\begingroup$ @AliceD They get to the next state and attain equilibrium more quickly so there's less to do in between. It's not inconsistent. $\endgroup$ – user207421 Jun 3 '15 at 12:12
  • $\begingroup$ @RoryM I don't see that it's 'more of a comment' at all. $\endgroup$ – user207421 Jun 3 '15 at 12:13
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    $\begingroup$ @EJP - RoryM is right - it's really more of a comment - a one-liner. E.g., who are they, and "Have less inertia than who? Without inferring a lot of things your answer doesn't make much sense and leaves room for interpretation. $\endgroup$ – AliceD Jun 3 '15 at 12:17

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