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).