4.2.2. Anticrowding hypothesis
Many investigators have mentioned or addressed the possibility that helical burrowing could reduce crowding and subsequent interference that might otherwise occur if there were multiple burrows with straight runs or ramps within a discreet area (Martin and Bennett, 1977; Koch, 1978; Shorthouse and Marples, 1980; Myer, 1999; Gingras et al., 2008; Adams et al., 2016; Doody et al., 2015). For example, Martin and Bennett (1977) supposed that helical burrows in Paleocaster could save horizontal space and avoid neighboring burrows while maintaining a shallow incline, great depth and close packing of burrows. For scorpions, Koch (1978) mentions the avoidance of neighboring burrows under crowded conditions as a possible function, and Shorthouse and Marples (1980) hypothesized that helical burrows might decrease the risk of antagonism or cannibalism by reducing the encounters probability between scorpions from neighboring burrows. When discussing the function of Gyrolithes , Gingras et al. (2008) considered it likely that the similar helical burrows of thalassinid shrimps were a response to high population densities.
Myer (1999) used field data from Paleocaster -constructedDaimonelix to calculate that burrow interference with straight ramps or runs would lead to a low probability of a burrow interfering with another (5–8%). Adams et al. (2016) suggested that this hypothesis leads to the prediction that burrows in dense populations are more spiral than those in sparse populations; yet they noted that hormurid scorpions that construct simple, vertical burrows, occur in similar densities to those constructing burrows with spirals (Harington, 1978). Cambrian Gyrolithes examined by Laing et al. (2018) were relatively sparse, which may be due to them being part of larger burrow systems of Thalassinoides .
The anticrowing hypothesis is logical and supported by some evidence of high densities of burrows, but is extremely difficult, if not impossible, to directly test. An indirect test would be to characterize burrow types at different densities, but this would assume that the inclusion of spirals is a phenotypically plastic behavior, or that there has been behavioral evolution among populations leading to disparate burrow morphology.