Hatchling turtles ingest natural and artificial incubation substrates at high frequency

Abstract

Geophagy is the consumption of hard objects with no caloric value (e.g. soil, sand, sediment) called gastroliths. This behaviour is widespread in animals, and among reptiles, geophagy has been reported in crocodilians and lizards and occasionally in chelonians. In this study, we calculated geophagy rates in snapping turtle (Chelydra serpentina) and painted turtle (Chrysemys picta) hatchlings under various incubation protocols, ranging from highly artificial to semi-natural conditions. Among multiple experiments where eggs were incubated atop the nesting substrate, 66% of painted turtle and 58–93% of snapping turtle hatchlings exhibited geophagy within 24 h of hatching. Snapping turtle hatchlings that experienced a simulated natural nest emergence had an 85–100% rate of gastrolith consumption. Hatchling snapping turtles from shallow simulated nests emerged earlier and had higher rates of geophagy compared with those from deeper nests. Given the high frequency, short time period (24~72 h) and variety of incubation protocols under which geophagy occurred, we suggest that this behaviour is intentional. We discuss multiple hypothesis for the adaptive and functional significance of geophagic behaviour with respect to chelonians, synthesize existing literature on chelonian geophagy and highlight the possible implications of hatchling turtle geophagy for ex situ captive breeding and head-starting programs. Given that hatchlings readily consume their incubation medium, caretakers should carefully consider the substrate their animals are exposed to. Future research should address how widespread geophagy is among hatchling turtles and the possible role of this behaviour for hatchling ecology and health, including effects on the gut microbiome. Animals regularly consume non-caloric foods, such as rocks and soils. This behaviour is exhibited by hatchling turtles, but why? Our literature review suggests that the ingestion of non-caloric foods by turtles is important for nutrition and, in particular, this behaviour may help establish gut health in hatchlings. Observational and experimental study demonstrated that nest characteristics in-part account for why hatchling ‘turtles eat dirt’. This has applications for hatchling ecology as well as captive rearing conservation programs, an increasingly common strategy for the conservation of these globally imperilled animals. This work complements several recent studies and review articles about geophagy in other major vertebrate groups (birds, mammals) and provides a comprehensive summary on the current state of knowledge of this behaviour for turtles.