4.1.1. Antipredator hypothesis
Perhaps the most obvious reason to construct a more complex, helical burrow is to reduce the threat of predation of the inhabitant(s) (Martin, 1994; Doody et al., 2015). Accordingly, we could not rule out this explanation for helical burrows of any species (Table 1). Any burrow refuge could decrease predation, but a spiral could slow or confuse a predator in pursuit of the inhabitant as it attempts to escape down the burrow. The helical burrow could also, depending on size and mobility of the predator, prohibit the predator from reaching the inhabitant, its eggs, or offspring. For example, the helical burrows of the pocket gopher Geomys pinetis may slow down or confuse predators such as weasels or snakes (Brown and Hickman, 1973). Myer (1999), however, noted that while predators that could fit intoPalaeocastor burrows may have been too long and/or not flexible enough to follow the beavers down into the tight helix, snakes and weasels could easily access helical burrows, and such predator fossils as Zodiolestes daimonelixensis (a prehistoric weasel) have been found in Palaeocaster burrows (Martin, 1989; 1994). Elsewhere, helical burrows have been speculated to help thwart monitor lizard and fish predators of scorpions (Urodacus ) and shrimp (Axianassa ), repsectively (Koch, 1978; Felder, 2001). According to Adams et al. (2016), the cost of predator excavation may increase disproportionately the deeper and more tortuous the burrow, as sand caves in and the tunnel becomes increasingly harder to follow.
In the case of monitor lizards (Varanus panoptes and V. gouldii ) that construct helical burrows solely to lay eggs, the helix could thwart egg predators. In support of predator exclusion geometry, monitor helical burrows are very tight and regular. No egg predators have been identified for these lizards, but the most likely predator would be conspecific males (Doody et al., 2015). The monitor eggs at the burrow terminus would be difficult to detect and reach by any predator because the burrows are 2–4 m deep and soil-filled (Doody et al., 2015; 2018a, b; 2020), but perhaps the addition of a helix would further frustrate a predatory monitor lizard. Alternatively, Doody et al. (2015) speculated that perhaps the helix in monitor lizards evolved as a deterrent to a now-extinct predator, such as Thylacinus orMegalania (Clode, 2009).
There is no real evidence for an antipredator function of helical burrows, but testing this hypothesis would be difficult for most species, especially in situ , and not possible for ichnotaxa with no modern homologs. In the monitor lizards, field experiments comparing nest predation rates between natural nests and artificial nests without a helix could be a useful indirect test, as could creating baited, artificial helical burrows to see if (marauding) male monitor lizards could navigate the burrows.