4.2.4. Biomechanical advantage
A fourth new hypothesis for helical burrows relates to biomechanical
advantage. As a burrower excavates, it may be better able,
leverage-wise, to remove or rework sediment (soil) with a lateral (side)
stroke that results in the burrow bending left or right. This could
result in a savings in energy or better efficiency in excavation that
could offset the increased effort required, mathematically, to excavate
a helical burrow rather than a straight burrow (as calculated by Meyer,
1999 for Palaeocaster constructing Daimonelix ; but see
previous section on falling soil hypothesis). This pattern would also
result in easier removal of excavated material from the burrow for many
tetrapods and arthropods. Also, helical burrowing in stiffer, more
cohesive media (sediment, soil) appears to be a tendency observed by one
of us (STH) in during burrow construction by spiders and crayfish (also
see Hasiotis and Bourke, 2006). Such a result might also be expected for
tetrapods that construct helical burrows in stiff or firm, cohesive
soils (also see Hembree and Hasiotis, 2006; Riese et al., 2011).
There is currently no evidence for this hypothesis in any taxon.
However, Monod et al. (2013) hypothesized that different burrow
architecture between taxonomic groups of scorpions was due to behaviors
related to morphology: fossorial hormurids are pedipalp burrowers that
use the large, often rounded pedipalpal chelae to loosen the soil and
carry it out of the burrow, whereas the closely related scorpionoid
families are cheliceral burrowers that use their enlarged chelicerae to
loosen the soil and then scrape it out of the burrow using the legs
and/or metasoma (see references in Monod et al., 2013). Barrass (1963)
found the direction of the spiral was related to the asymmetry of the
crab’s claws such that the males with the major claw on the right exit
from the burrows spiralling counterclockwise, and vice-versa.
Testing the biomechanical hypothesis would minimally require observing
the digging strokes and understanding the biomechanics of burrow
excavation and preferably involve a comparison of energy required and
the efficiency of strokes that would create helical vs. straight
burrows. Toots (1963) provided an insightful treatment of the
fundamental biomechanical requirements of helical burrow construction by
considering the need for asymmetrical digging along the horizontal axis
and geotaxis and transverse gravity orientation (Toots, 1963). This
represents a good starting point for exploring the biomechanical
underpinnings of constructing a helical burrow, which may provide
insights into energetics and construction costs-benefits.