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.