Jeffery et al. propose that a bicoded representation of three-dimensional space, in which horizontal and vertical dimensions are encoded in fundamentally different ways (i.e., metric and non-metric), is common to all vertebrate animals. Although bicoded spatial representation has implications for animals moving in all substrates, this commentary focuses on how the theoretical framework outlined by Jeffery et al. might be applied to nonhuman primates moving in natural multilayer environments and what techniques might be applied to this problem.

The neural evidence upon which Jeffery et al. base their conclusions comes largely from rats habitually moving within single layers of space or in multilayered, compartmentalized space (e.g., tunnels). The authors also describe animals that move in volumetric space (e.g., fish, birds) and the need for data regarding how these animals represent space in three dimensions. Applying Jeffery et al.'s framework to arboreal/terrestrial nonhuman primates, we find that the space in which these animals move presents significant challenges for generating spatial representation mosaics which Jeffery et al.'s examples do not habitually encounter. A nonhuman primate that moves both on the ground and in trees (e.g., chimpanzees, capuchin monkeys, macaques) is presented with the option of travel on substrates that may occur at a great variety of distances, angles, and heights from the horizontal plane of the animal's location at any given moment. Vertical, horizontal, and intermediately angled arboreal substrates comingle and contact substrates from neighboring trees. The animal must accurately estimate distance to cross open spaces by leaping or brachiation. These locomotor patterns are associated not only with horizontal, but also vertical displacements of the animal (e.g., Channon et al. Reference Channon, Crompton, Günther, D'Août and Vereecke2010). Considering Jeffery et al.'s Figure 12 in the context of a primate travelling in the trees, we might visualize similar arboreal bicoded map fragments that occur along numerous branches in many directions and extending into various distances from the central tree trunk. These bicoded map fragments meet and join fragments from neighboring trees, forming a web of metrically mapped spaces that, with increasing density of branches and variation in the branch angles, will approach metrically mapping both horizontal and vertical space (in the canonical orientation sense), assuming that the bicoded fragments are accurately joined in a mosaic.

Landscapes within which animals move consist of both vertical and horizontal components that animals act upon. Landscape components such as topography have an impact on the manner in which animals move through their environments (e.g., Mandel et al. Reference Mandel, Bildstein, Bohrer and Winkler2008). Animals moving primarily through a single layer of their environment (e.g., large grazing animals) are impacted by elevation, the one vertical component of the landscape upon which they move (Bennett & Tang Reference Bennett and Tang2006). Because moving uphill requires greater energy expenditure than moving on level ground (Taylor & Caldwell Reference Taylor and Caldwell1972), terrestrial animals may make changes in the horizontal nature of their movements in order to avoid a given area due to its vertical character (i.e., detouring around steep slopes). In contrast, animals that move in multilayer environments contend with elevation as well as other additional vertical components to their movement decisions (i.e., substrate height). For example, nonhuman primates and other vertebrates locomote in multilayer environments with a combination of arboreal and terrestrial substrates. The varying slopes of the multitude of potential substrates (i.e., branches, tree trunks, and terrestrial surfaces) and the locations to which these substrates convey present these animals with numerous options for movement. These options also allow the animal to exert greater control over the vertical component of its movement decisions. For example, continuous canopy forest would allow arboreal primates and other quadrupedal animals that move through arboreal habitats to travel at a constant height, minimizing vertical displacement, while the elevation of the forest floor rises and falls. We maintain that the use of non-compartmentalized multilayer environments requires a representation of space that is sufficiently accurate to allow for movement decisions in the vertical and horizontal dimensions as well as precise aerial swinging/leaping to distal substrates. Logistically, this spatial model may be hypothesized to include, at a minimum, precise heuristics regarding where, when, and how to swing or leap, or perhaps even a metric component of the vertical dimension.

In studies of the movement of nonhuman primates in multilayer environments, movement observations are frequently simplified for their analysis. Actual three-dimensional animal movements are reduced to two-dimensional displacements across a planar surface (Janson Reference Janson1998; Reference Janson2007; Normand & Boesch Reference Normand and Boesch2009; Noser & Byrne Reference Noser and Byrne2007b; Sueur Reference Sueur2011; Valero & Byrne Reference Valero and Byrne2007). Some prior studies have incorporated the vertical dimension of movement in discussions of nonhuman primate movement patterns by linking elevation to visibility of resource sites (e.g., Di Fiore & Suarez Reference Di Fiore and Suarez2007; Noser & Byrne Reference Noser and Byrne2007a). Draping the movements of nonhuman primates onto a digital elevation model of their habitat allows us to consider the energetic and viewpoint effects resulting from the vertical component of movement on a terrestrial substrate (e.g., Howard et al. Reference Howard, Bernardes, Nibbelink, Biondi, Presotto, Fragaszy and Madden2012). However, the greater vertical complexity of moving through multilayer environments (e.g., arboreal and terrestrial) and its effects on adaptive movement choice are not considered using this technique.

One technique that may accurately represent the arboreal substrates upon which nonhuman primates move is Light Detection and Ranging (LiDAR) technology. Terrestrial LiDAR is three-dimensional laser scanning that generates a hemispherical point cloud representing the returns of laser pulses from a ground-based vantage point (Beraldin et al. Reference Beraldin, Blais, Lohr, Vosselman and Maas2010). This high-density point cloud can be used in forest inventories and in measuring the detailed geometric characteristics of trees (Maas Reference Maas, Vosselman and Maas2010). Point clouds can be used to generate three-dimensional models of the canopy through which animals move. The use of this technique would allow researchers to catalog all possible substrates on a tree or group of trees, estimate the energetic costs of movement on those substrates, and even model animals' movement through a multilayer canopy based on heuristics or metric knowledge of vertical and horizontal spatial components of their environment. In this way, the framework proposed by Jeffery and colleagues may be tested against movements of animals in natural environments.