Singing snappers - Hatchling vocalizations are associated with beneficial social interactions in subterranean nests of a freshwater turtle

Claudia N. Lacroix, Ecology & Evolutionary Biology, University of Toronto, Toronto, ON, Canada, Christina M. Davy, Wildlife Research and Monitoring Section, Ontario Ministry of Natural Resources and Forestry, Peterborough, ON, Canada and Njal Rollinson, Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, ON, Canada

Background/Question/Methods
Social behaviour in non-avian reptiles is largely underestimated, and until recently, turtles were thought to be voiceless. Despite this belief, recent work suggests that freshwater turtles communicate using vocalizations, and theory suggests that this behaviour may underlie complex social behaviours. For instance, hatchling vocalizations among turtles may be important for hatchling communication in early life, perhaps to encourage hatching synchrony and coordinate group emergence from subterranean nests (the ‘Social Facilitation Hypothesis’). Using the broadly distributed Snapping Turtle (Chelydra serpentina) as a model organism, we examined whether vocalizations function in hatchling communication. First, we characterized the vocal repertoire of hatchling turtles by recording the audio of a turtle nest and examined vocalizations produced within three distinct hatching periods (pre-pipping, pipping, and emergence). Second, we examined whether the presence of siblings during hatching and nest emergence increases fitness (a basic pre-requisite of the Social Facilitation Hypothesis) by manipulating egg burial depth (shallow or deep) and sociality (presence or absence of siblings) in a 2 x 2 factorial design. Finally, we experimentally tested a component of the Social Facilitation Hypothesis by examining whether vocalizations facilitate hatching synchrony, playing vocalizations to late-stage embryos, and evaluating whether vocalizations modify hatch timing.
Results/Conclusions
First, we demonstrate that C. serpentina hatchlings have a vocal repertoire: we detected 261 vocalizations of 5 different types in a simulated nest environment. Most vocalizations occurred in the 24 hrs following egg pipping, where more complex vocalization types were observed after hatching. Next, we demonstrate that hatchlings benefit energetically from emerging with siblings, where eggs in the presence of siblings hatched 1.6 days earlier and lost 0.5g less weight. This study is the first to demonstrate that snapping turtle hatchlings can modify development timing within the egg and confirms that siblings reduce energy expenditure during nest emergence. However, contrary to a leading hypothesis that vocalizations cue hatching synchrony, our results show that hatchlings did not modify hatch timing with respect to vocalization playback. The present study raises the possibility that vocalizations do not synchronize hatching, but that hatchling social behaviour still plays a role in nest emergence. We provide incremental support to the Social Facilitation Hypothesis while contributing to a growing literature on the adaptive significance of reptile sociality. By studying a lineage that greatly differs from well-studied vertebrates, this work generates fresh insight into factors that drive the origin and persistence of social behaviour in vertebrates.