Manatees, often referred to as sea cows, are large, gentle aquatic mammals found in coastal waters and rivers. Despite their imposing size, these herbivores navigate their underwater world using a surprisingly intricate system of communication. Their acoustic environment plays a significant role in their daily lives, allowing them to interact and convey information.
Discovering Manatee Vocalizations
Manatees produce a diverse range of sounds, primarily through their vocal folds in the larynx. Researchers have categorized these vocalizations into five distinct types: squeaks, squeals, high squeaks, chirps, and squeak-squeals. Squeaks, high squeaks, and squeals are the most frequently observed, with squeaks dominating adult communication.
These vocalizations are high-pitched, resembling chirps, whistles, or squeaks. Spectrogram analyses reveal manatee sounds are frequency-modulated, meaning their pitch changes over time, and last for a short duration, ranging from 200 to 800 milliseconds. They also contain multiple harmonics, which are additional frequencies. The fundamental frequencies of these calls can vary, with Florida manatees producing sounds between 1.17 and 2.83 kilohertz, while Antillean manatees can range from 0.64 to 5.90 kilohertz.
Calves produce high-pitched, “hill-shaped” squeaks, which appear as characteristic contours on a spectrogram. As a calf matures, these “hill-shaped” calls gradually flatten, transitioning into the more linear adult squeak. Manatees also have an impressive hearing range, perceiving sounds from 400 to 46,000 hertz, with their most sensitive hearing between 16,000 and 18,000 hertz. Some vocalizations extend into the ultrasonic range, above 20 kilohertz, with some Antillean manatee calls reaching up to 88.3 kilohertz.
The Meaning Behind Manatee Sounds
Manatee vocalizations serve various communicative purposes, facilitating essential behaviors within their loosely structured social groups. A primary function is maintaining contact, especially between a mother and her calf, which forms the strongest and most prolonged social bond among manatees, lasting up to two years. Mothers and calves rely on vocalizations to locate each other, with cows capable of responding to their calf’s squeals from over 61 meters (200 feet) away. Calves are more vocal than their mothers, particularly when separated, using specific contact calls to prompt reunion.
Beyond mother-calf interactions, manatee sounds are used in broader social contexts and to convey emotional states. Squeals, for instance, are associated with playful interactions and may indicate excitement or aggression. Manatees may also alter the length of their calls, extending them when they experience fear, or modify the pitch of their vocalizations during periods of play. These subtle variations in call structure are believed to convey different meanings, allowing for nuanced communication within their temporary aggregations.
Vocalizations also play a role in coordinating movements and recognizing individuals. Manatees increase their calling rates when traveling or rejoining other individuals, suggesting a function in group cohesion. While their social structure is fluid, with groups forming temporarily for a few hours to days, these sounds help facilitate interactions within these gatherings. Despite their extensive use of sound for communication, manatee vocalizations are not thought to be used for navigational purposes.