4.3. Potential evolutionary sequence of deep, helical burrowing behavior in some monitor lizards
We can reconstruct the putative evolution of nesting behavior in these species using the discussions in previous sections. Large monitor lizards that lay large eggs that require long incubation periods (6–9 months; Horn and Visser 1989; 1997) that must stretch over dry seasons in species inhabiting arid areas, at least in Australia (Doody et al., 2015; 2018). These species have evolved the behavior of nesting much deeper than any other reptile (Doody et al., 2014; 2015; 2018). The cheapest way, energetically, with regards to distance only , for a monitor lizard to nest 2, 3, or 4 m deep is to construct a vertical tunnel straight below the site they have chosen (White, 2001, referring to scorpions). However, a lizard cannot excavate a burrow straight down because the soil continues to fall in on itself. For the lizard to remove the soil from the burrow once they are deeper than 1 m is effectively impossible because the soil would need to be thrown upwards out of the burrow a considerable distance with efficiency. To our knowledge, monitor lizards cannot carry or transport soil other than kicking or dragging it on the surface with their limbs, head, and neck. So, a deep, straight vertical burrow is physically impossible because the creator could not get the loose soil out of the way to allow continued burrow construction.
A physically manageable but more energetically expensive (distance-wise) approach would be to excavate an inclined (straight) burrow run at an angle that would prevent soil from falling back down once loosened. The mean incline for V. panoptes entrance burrows was 8° (Doody et al., 2015). If the burrow is to be 3 m deep, with an angle of 8°, solving for the opposite side of a right triangle yields a horizontal distance of the nest from the burrow entrance of 19 m (13 m if 2 m deep, 26 m if 4 m deep). This is a considerable distance from where the mother selected a suitable patch of soil, creating risk that she might encounter more resistant soils that would be more costly to burrow through. In support, both V. panoptes and V. gouldii nest communally and traditionally, apparently taking advantage of soil loosened by conspecifics by nesting in a discreet area of soil that is softer than the surrounding area (Doody et al., 2015). Increasing the angle of incline (steeper) would decrease the horizontal distance of the nest from the burrow entrance, but at some point, the incline allows soil to fall back into the burrow. As noted earlier, continually removing soil that is falling back into the burrow is energetically expensive and probably impossible at depths greater than one meter. This cost could be large enough to offset or even outweigh the cost of constructing a helix (calculated by Myer, 1999). Steeper inclines would at some point be prohibitive (as with the vertical burrow above).
The possible solution was the construction of a helix, which is physically manageable, and possibly energetically equivalent or superior to a straight incline and would bring the creator straight down into the intended nesting area with loosened soil. Perhaps there were intermediates that resembled a zigzag or switchback pattern; these could eventually ‘tighten’ into a helix. Stopping the falling soil might be especially needed useful for monitor lizards because they do not remove the soil from the burrow, except for the first 0.5 m of the entrance run.
Conclusions
Our near-exhaustive review was the first to consider all taxa when addressing the evolution and function of, and costs and benefits to, helical burrowing in animals. Our examination of the fit of 10 hypotheses to numerous living and extinct taxa failed to find compelling evidence for one general hypothesis for why animals construct helical burrows. Only two hypotheses—-antipredator and biomechanical advantage—-could not be rejected for any species, although six of the hypotheses could not be rejected for most species (possible in 86–100 % of spp. ). Thus, one or more of these could explain the behavior of helical burrowing in most species. Four of these six were construction hypotheses, raising the possibility that helical burrowing might have evolved without providing post-construction benefits. Our analysis did eliminate four hypotheses - increased drainage, deposit feeding, microbial farming, and offspring escape – as explanations for helical burrowing behavior in the majority of taxa (possible in 5–48% of spp.). The extended phenotype of helical burrowing may have evolved for a diversity of reasons. Further observations of helical burrowing in different biotic and abiotic contexts, and in particular, experiments, could in some cases eliminate or provide support for some of the hypotheses, while other hypotheses are difficult to test, or not directly testable.
Tables
See attached file.