## 2 Related Work
### Spatio-Temporal Studies  
Events in physically dynamic environments, specifically with spatial constraints, are related to each other according to the first law of geography by [PERSON] (2016):  
...'everything is related to everything else, but near things are more related than distant things'...  
Note that the same law is also applicable to more recent events in time. Thus, vehicular networks have an inherent interaction with their physical environment in which entities move; consequently, in addition to recent events that happen in time, the near spatial closeness of event also impacts the network behavior and current topology. Furthermore, the exchanged data describe these events. Therefore, the term spatio-temporal constraints in communication refers to the relationship between the space and time in which the events happen, and this needs to be communicated for the system to be useful.  
In the literature, three basic behaviors and the combination of spatio-temporal phenomena in nature are presented: affectation, degradation, and propagation (2016; 2017). These behaviors are detailed in Table 2, as well as analogue or equivalent bio-inspired behaviors examples:  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Behavior** & **Characteristics** & **Bio-Inspired Examples** \\ \hline \multirow{3}{*}{Affectation} & Maintains a sustained intensity, action or influence into a & \multirow{3}{*}{Mammals’ immune system} \\  & focalized location during a period of time & \\  & Similar to affectation, however, the intensity, action or & \\ \multirow{2}{*}{Degradation} & influence varies in space and/or time, or else, it depends on & \\  & the beholder’s perspective & \\ \multirow{2}{*}{Propagation} & Dissemination of the intensity, action, or influence through & \\  & a physical environment and with continuous change and & \\  & during the time the effect lasts & \\ \multirow{2}{*}{Combined behaviors} & Spatio-temporal combinations of the behaviors or the & \\  & observance of more than one at the same space and/or time & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison and description of natural spatio-temporal behaviors.  
Additionally to the spatio-temporal constraints, we state that spatio-temporal coupled entities are those that enable these constraints, i.e., they exist as message transmitters and receivers and share a region in space during a period of time, while feasible communication among them is achieved. If one condition, either space or time, is not satisfied, even if the other is satisfied, then the coupling is not achieved.  
Finally, unlike routing and cluster approaches, previous knowledge among entities or discovery or location services is not mandatory, and the message receivers are determined by the space-time constraints, as in stigmergic communication, which is a type of indirect communication in which individuals take advantage of the environment by changing it [38]. Social insects such as ants use stigmergic communication through pheromones by means of traces to food sources in a spatial way, and the pheromone will lasts as long as it is reaffirmed depending directly on the time validity of the food source [9; 10]. Therefore, stigmergic communication through pheromones is considered a spatio-temporal behavior (degradation), as noted in Table 2. Furthermore, communicating entities are defined through these spatio-temporal constraints; however, these entities become secondary as the due communication is established in order to achieve the common task of food retrieval or dissemination of emergency messages, in the cases of described social insects and vehicular networks, respectively [11; 12].