Frequently asked questions about Mobility

What is the difference between Mobility and Movement?

Mobility can be defined as the degree of movement of an animal’s body independent of spatial displacement of the center or any other body point, which is measured by Movement. ‘Independent’ does not mean that mobility is corrected for the center-point position, it means that the calculation does not use the x,y coordinates of the animal (they are not in the equation used to calculate it). Mobility is calculated 100% independent of movement of the coordinates identified as the center-point (or the nose/tail point). That means that the center-point can have zero movement but high mobility.

For example, imagine that you are tracking a rat in an open field. When the rat stands still and grooms, the center of gravity does not move, therefore there is no spatial displacement of the subject (the current state for the Movement variable is Not moving), however the rat’s head and forelimbs move, resulting in changes in the surface area. Although there is no spatial displacement of the body, the current state of the Mobility variable is Mobile (depending on the threshold value).

Does Mobility depend on the size of the subject vs. arena?

No. It is dependent on the size of the subject only. Keeping resolution constant, the smaller the subject, the smaller the number of pixels that form its image, the more likely that any small movement results in a change in area that is detected as Mobility.

Does Mobility depend on the video resolution?

Yes. The higher the video resolution, the greater the number of pixels that form the image of the subject. Therefore, the less likely that a small movement of the subject results in an abrupt change in area.
However, this effect is present at very low resolution (for example, when the subject is less than 100 pixels large). At resolutions provided by video files, this effect is negligible.

Does Mobility depend on the sample rate?

Yes. Since Mobility is detected from the change in area of the subject between two samples, and the change in area depends on how frequently the subject area is acquired (that is, the sample rate), Mobility depends on the sample rate. All being equal, the higher the sample rate, the smaller the change in subject area.

In the example below, the Immobile threshold was set to 2%, and the Highly mobile threshold was set to 10%. A certain movement pattern detected with a sample rate x determines a change in area of around 15%, which causes EthoVision consider the subject Highly mobile.

If the sample rate doubles, more samples are captured in the same time interval, therefore the same movement results to a smaller change in area between samples. For the intermediate samples, EthoVision XT considers the subject Mobile since the change in area is smaller than 10%. The proportion of samples where the subject is considered Mobile increases relative to the proportion of samples where the subject is considered Highly mobile.

As a general rule, the higher the sample rate, the lower the Immobile and Highly mobile thresholds must be.

Does Mobility depend on track smoothing?

The results of Mobility may differ depending on whether you set a Minimum Distance Moved filter (see The Minimal Distance Moved smoothing method). When you apply this filter, a sample in the track that is very close to that at the previous time stamp is given the same coordinates as the previous one. When that occurs, the Area change is also set to zero for that sample. This results in a lower value of the average Mobility.

In contrast, other track smoothing methods do not influence Mobility. See Smooth the Tracks

If you want to preserve the values of Mobility when the animal is almost completely still, instead of using the Minimal Distance Moved filter select the data based on Movement. In the Data profile, choose Nesting > Movement. Under Calculate nesting for, select Not moving and set the threshold velocities in such a way the animal is considered Not moving when its velocity is lower than the Stop velocity value. See Nesting over Movement. Next, calculate Mobility.