First investigation on the behavioural contexts of underwater sound production in juvenile green turtles, Chelonia mydas

Although sea turtles have long been considered deaf and silent, recent studies have confirmed their ability to hear and produce sounds. A study revealed that juvenile green sea turtles Chelonia mydas, can produce ten distinct types of sounds, with some of them potentially involved in social communication. To investigate the potential biological function of these sounds, we used video recordings associated with acoustic recordings of 23 free-ranging juvenile green turtles and we examined the co-occurrences of sounds with behaviours or external events. Our study revealed that most of the sounds were produced when the sea turtles were resting or swimming. However, four sound types were produced in more specific contexts. Long sequences of rumbles were recorded after sunset and mainly during resting. All these rumbles appear to have been produced by several individuals recorded simultaneously, suggesting that rumbles may be used for social interactions. The frequency modulated sound was highly associated with scratching behaviour and thus may express a positive emotion. Finally, the long squeak was produced by individuals in the presence of human beings, and the grunt was occasionally heard when green turtles were vigilant or approaching a conspecific. These two sounds may be the first evidence of an alarm or warning signal for intra-specific communication in green turtles. Our results mark a significant milestone in advancing the understanding of sound production in the behavioural ecology of sea turtles. Further investigations are now required to test the hypotheses suggested by our findings, which could significantly contribute to their conservation.