Wall-following behavior: Its ultimate and proximate explanations, prevalence, and implications

Wall-following behavior is the movement along a wall exhibited by various animals. It is a conserved behavior among taxa, from unicellular organisms to humans, prone to selection pressure, and it is energetically cheap since it provides a homogenous trail and can help animals to follow the shortest distance between two points. There are three sets of explanations for this behavior. The first is that it is a defensive behavior meant to lead the animal to shelter or reduce predation risk, and which, at least in vertebrates, is a proxy of anxiety. The second explanation suggests it is an exploratory behavior, helping an animal either to exit an enclosed space or to orient in a novel (unfamiliar) environment, especially when vision is limited. As novel environments often induce stress, these two explanations are not mutually exclusive. Finally, the wall itself may offer some desired biotic or abiotic conditions, such as a higher prey availability moving along it or favorable microclimate conditions. Wall following is a variable behavior, easily affected by the test conditions, such as the test arena size (e.g., more in smaller arenas), shape, and illumination level. Standardization of its measurement is required to facilitate comparison among studies and species. The timing of examination plays a role too: Wall following often changes along development and with aging. Generally, females follow walls more often than do males. Furthermore, certain conditions experienced at a young age affect wall-following later, so the behavioral changes may be long-lasting. Wall following is correlated with a few other behaviors, such as a negative correlation with phototaxis and activity. We end our review by presenting some future research directions, such as examining wall-following behavior in predator-prey systems, examining whether and when wall-following is adaptive, and studying it in the context of urban ecology. Finally, there is a need to examine how common this behavior is in the wild as most studies have been conducted in the lab.