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).