4.1.6. Offspring escape hypothesis
The sole (new) hypothesis involving postnatal parental care proposes that the helix, by loosening the soil above the nest, facilitates hatchling escape of neonate monitor lizards (this hypothesis is new and may not apply to any other taxon). There is little doubt that the soil is less resistant in the excavated helix compared to the surrounding soil, which is often compact and firm. In fact, some helices were filled with very loose soils and in some the core of the helix even collapsed, leaving a cylinder-shaped section filled with loose soil (JSD, pers. obs.). Hatchling emergence or escape burrows were found for both deep-nesting species (Doody et al., 2018a, b). Hatchlings excavated escape burrows nearly straight upwards from the nest 2–4 m to the surface, rather than following their mother’s soil-filled burrow. Such deep nesting would challenge hatchlings to emerge considerable distances through resistant soils. Although emergence burrows were not carefully mapped, at least some of these burrows partly emanated through the helix (JSD, pers. comm.); burrowing through the helix would involve traversing ~1–3 m of softer soil than the surrounding firm soils (given that the helix begins at ~1.2–1.5 m below the surface). In short, the helix increases the probability of escaping by reducing the effort required by hatchlings.
Perhaps in support of this hypothesis, the low clutch size (e.g., 3–8 eggs; Doody et al., 2020) of an animal nesting very deep might indeed select for parental effort for careful nest excavation to facilitate hatchling escape. While most ground-nesting reptiles deposit eggs <30 cm below the surface, these lizards nest 2–4 m deep in firm soils, seemingly requiring considerable energy for a small group of hatchlings to excavate one emergence burrow. In only one of many nests did we observe multiple emergence burrows emanating from the same nest—-there were two. The position of the helix directly above the nest supports a useful function, but one could ask why the entire burrow is not a helix (although perhaps loosening the soil for half the emergence distance is enough to facilitate successful excavation and escape). No other species can shed light on this hypothesis because none are known to lay eggs or possess emergence burrows excavated by hatchlings.
The recent finding of fossilized neonate Diictodon skeletons with adults in burrows assigned to Daimonelix suggest that they could have served as brood chambers; whether Diictodon bore live young or laid eggs is still under debate (Smith et al., 2021). Pocket Gopher (Geomys ) nests have also been found associated with helical burrows (Brown and Hickman, 1973; Wilkins and Roberts, 2007). Although the helix could be associated with brooding or eggs in some species other than monitor lizards, the open Daimonelix burrows ofPaleocaster and Diictodon do not support the idea of the helix loosening the soil for neonates as posited by the hatchling escape hypothesis.
Otherwise, there is little support for hatchling escape as a general explanation for helical burrows. This hypothesis could be directly tested by carefully excavating hatchling escape burrows to determine if they typically emanate through the helical portion of the mother’s burrow. If they do, measuring the energetic costs of the hatchling escape through resistant (no helix) soil vs. less-resistant soil (helix) in the laboratory would be ideal. Measuring the energetic cost of the mother’s excavation of a helical nesting burrow would also provide context for understanding any energetic benefit to hatchlings (see Rusli et al., 2016 for a relevant example with sea turtle hatchlings).