Buried Away: Vocalising Turtles and Cooperative Hatching Behaviour

Reptiles are not known for their behavioural complexity, and until recently I bought into that belief. In my lower year undergraduate classes, behavioural ecology and herpetology were taught separately. So, when I asked my current supervisor, Dr. Njal Rollinson, if I could do a third-year research thesis, I was shocked when he proposed a project on turtle vocalisations. First of all, turtles vocalise? And second of all, I can study behaviour AND turtles? This idea largely came from a video of sea turtle vocalisations that was featured on an episode of David Suzuki’s The Nature of Things. I was more than happy to run this project, and so my journey with turtle vocalisations began.

Turtle vocalisations are a phenomenon that has only been studied in depth in the past 10 years or so. Although we do not know why turtles vocalise, research with crocodilians indicates that crocodilian hatchlings vocalise to cue parental care, synchronize hatching, and/or coordinate nest emergence. It’s possible that turtles vocalise to communicate amongst each other for the same reasons as crocodilians. We were intrigued by this idea and wanted to see if Snapping Turtles (Chelydra serpentina) were also vocalising within the nest. So, in the summer of 2019, we brought Snapping Turtle eggs from the Algonquin Wildlife Research Station (AWRS) to the University of Toronto. We buried a clutch of eggs in an aquarium filled with sand, added a microphone, began recording audio within the nest, and waited.

We also decided to start a second project to compliment this research and investigate whether Snapping Turtles hatch earlier in a group and whether group emergence provided an energetic benefit. The idea behind this is that, if turtles do not vocalise, maybe there is another mechanism that allows them to hatch at the same time. I hauled what felt like tonnes of sand from the shore of Lake Sasajewun at the AWRS, brought it back to Toronto, spent hours sifting out giant rocks and then filled dozens of jars with sand. I then buried Snapping Turtle eggs in the jars to conduct a hatching experiment in which I manipulated two factors: egg depth (i.e., shallow or deep) and the presence of siblings (i.e., hatching alone or hatching in a group of 10 eggs).

September and October were hectic. Turtles were hatching left, right, and centre, with several hatchlings emerging from their nest each day (an almost 90% hatching success rate!). In our emergence experiment, we found that turtles hatched earlier and reduced the amount of energy used to escape the nest in the presence of siblings. By emerging as a group, hatchlings emerge earlier in the season, emerge with more energy reserves, and consequently may experience a higher chance of survival. Based on our knowledge of turtle ecology, incubation timing is largely dependent on temperature, with turtle eggs at the top of the nest developing faster than eggs at the bottom. This led us to question: if hatchling development rates differ between the top and bottom of the nest (because of a temperature gradient), yet hatching and nest emergence are synchronous, how do hatchlings hatch at the same time?

It turns out that Snapping Turtles vocalise within their nest! We found that turtles produced 6 distinct vocalisation types as they hatch and climb towards the surface of the nest. This suggests that important information may be being communicated after they hatch. Are vocalisations a cue for synchronous hatching and/or emergence behaviour? Being curious herpetologists, we wanted to further explore this question. We took our research one step further and experimentally tested whether vocalisations cue hatching. To do so, in Fall 2020, we played pre-recorded vocalisations to eggs near the end of the incubation period and recorded hatch timing. We then compared hatch timing between eggs that did and did not receive vocalisation playback. Contrary to a leading hypothesis that vocalisations cue hatching synchrony, our results show that hatchlings did not modify hatch timing with respect to vocalisation playback. If vocalizations do not synchronize hatching, then why are hatchlings vocalising?

Perhaps vocalisations play a role in another aspect of hatchling social behaviour, such as nest emergence or post-nest emergence activities. There is still much work to be done and much that we don’t know about turtle social behaviour and vocalisations. Luckily, I will be pursuing a master’s thesis this fall, during which I plan to investigate several aspects of turtle behaviour. With hope, I will have more turtle vocalisation and behaviour stories to report on in the future, perhaps with a larger sample of turtle species.

For now, I would like to thank my supervisor Dr. Njal Rollinson for the ongoing support, mentorship and for giving me the opportunity and creative freedom to work on this project (especially as an undergrad with little to no experience!). I would also like to thank Dr. Christina Davy for the ongoing help, mentorship and always being available to troubleshoot the daunting task of quantifying this obscure thing called ‘turtle vocalisations’.

References

Lacroix C., C.M. Davy and N. Rollinson. 2020. How Snapping Turtle vocalizations are associated with energetic benefits during nest emergence [Conference presentation]. British Ecological Society Festival of Ecology. https://youtu.be/pRk67vmkmoI