4.2.3. Vertical patch hypothesis
A third new hypothesis reflects potential heterogeneity in media stiffness in the vertical vs. horizontal planes. Each soil horizon tends to be more homogeneous in composition compared to overlying and underlying in a particular area due to the abiotic and biotic processes that form them (Kraus, 1999; Brady and Weil, 2002; Hasiotis, 2007). Overall, there is greater heterogeneity vertically in a soil profile than laterally, where the tendency is for the surface horizon (i.e., A horizon) is looser with a greater amount of organic matter compared to the subsurface horizon (e.g., B horizon), which is the zone of accumulation of clays and other minerals that make it firmer and more compact. Thus, deep burrowing in the form of a vertical burrow or tightly helical burrow will incur a higher energetic cost to construct the burrow. However, this cost is offset with the benefit of (1) greater relative humidity and soil moisture (microclimate amelioration) combined with (2) greater soil density (firmness and consistency) to ensure a lower chance of soil collapse (i.e., escape of hatchlings) that also doubles as better burrow construction in that sediment-filled burrows result in predator avoidance.
In the case that shallower optimal or suitable sediment layers predict similar layers directly below, it could benefit creators to construct helical burrows once those shallower sediment conditions have been discovered, rather than chance burrowing into suboptimal or unsuitable conditions by excavating angular burrows. Relevant conditions could be sediment or soil friability, hardness, or some biotic factor such related to food or farming. In the case of deposit feeding or microbial farming here could be post-construction benefits (see details for those hypotheses). With the monitor lizards, most nesting areas are communal and traditional, possibly due to the reduction in excavating costs associated with constructing burrows in soil already loosed by conspecifics (Doody et al., 2015; 2018).