Roars, Rumbles, and Resonance: A Systematic Review and Meta-Analysis of Crocodylian Acoustic Signals

Abstract

Crocodylians are highly vocal reptiles, possessing a complex acoustic signalling system including vocal and non-vocal signals used for courtship, mating, mediating conflict, and providing maternal care. Despite this, research on crocodylian acoustic signalling remains infrequent, with methodologies and terminology varying widely across studies. Here, we systematically review the literature and standardise crocodylian acoustic signal descriptions, measurements, and terminology to establish a consistent foundation for future research. The compiled dataset included 22 publications, with 623 acoustic signalling observations and 53 distinct parameters measured across various species, life stages, and contexts. The American alligator (Alligator mississippiensis) was the most frequently studied species and distress calls the most commonly recorded signal type. Significant variation existed in parameters measured across studies, with dominant frequency, call duration, and fundamental frequency the most common. We synthesised acoustic parameters from available publications into comparable values and units, and provide these as a centralised database along with a standardised ethogram including representative spectrograms, and a glossary of commonly used terms facilitating clearer cross-species comparisons. Significant foundational level data gaps were identified with many species lacking defined repertoires, and notably, underwater acoustic signalling was rarely incorporated. We recommend shifting future research focus from distress calls to include a wider range of spontaneously produced acoustic signals, by individuals of known sex and life stage. The inclusion of a broader set of underrepresented acoustic parameters will also enable better cross-species comparisons, and identification of encoding in crocodylian acoustic signals. We further promote the reanalysis of existing data incorporating these key parameters, along with increased collaborative efforts, to add valuable insights informing conservation without requiring additional fieldwork. Such strategies will support future research into crocodylian acoustic communication and guide the development of more effective monitoring techniques such as passive acoustic monitoring and machine learning as tools for conservation and management of crocodylians.