Evidence for atypical nest overwintering by hatchling lizards, Heloderma suspectum

Abstract

The timing of reproductive events (e.g. oviposition and hatching) to coincide with favourable seasonal conditions is critical for successful reproduction. However, developmental time may not match the duration between the optimal time for oviposition and the optimal time for hatchling survival. Thus, strategies that alter the time between oviposition and hatchling emergence can be highly advantageous. Arrested development and the resulting extension of the duration between oviposition and hatching has been widely documented across oviparous amniotes, but nest overwintering by hatchlings has only been documented in aquatic chelonians that live where winters are quite cold. Herein, we present a compilation of evidence regarding reproductive phenology by hatchlings of the Gila monster (Heloderma suspectum), a lizard inhabiting the Sonoran Desert of North America. Our data demonstrate that (i) Gila monster hatchlings from eggs oviposited in July do not emerge from their nests until late spring or summer of the following year, yet (ii) Gila monster eggs artificially incubated at field-relevant temperatures hatch in 4-5 months. Furthermore, we describe a fortuitous excavation of a hatching Gila monster nest in late October, which coincides with the artificial incubation results. Together, these results provide strong support for the existence of overwintering in the nest by a lizard, and suggest that this reproductive strategy should be explored in a broader array of taxa.

Keywords: development; diapause; hatching; overwintering; reproduction; reptile.

Figure 1.

Thermal profiles used to incubate Gila monster eggs in captivity. The solid line represents the norm for optimal artificial incubation (control), while the dashed line is the temperature profile recorded from a nest in the wild.

Figure 2.

Emergence mass relative to date of emergence for nest 1 (open circles, dashed line) and nest 2 (open squares, solid line). When data for the two clutches were pooled, Pearson's correlation coefficient showed emergence mass decreased as hatchlings emerged later in the year (t₁₁ = −3.1146; p < 0.01; correlation coefficient = −0.6846). See text for additional statistics.

Figure 3.

Incubation duration was similar for eggs incubated at a constant 29.3°C (black diamonds and solid line) and eggs incubated under a natural regimen (open squares and dashed line) (p = 0.234). However, Welch's t-tests showed hatchlings incubated under the natural thermal cycle took longer to exit the egg after pipping (control: 1.3 ± 0.2 days; experimental: 10.2 ± 0.9 days; t₆ = −8.2346; p < 0.001), and incubation duration did not affect the time to exit the egg after pipping (p = 0.077).