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.