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.