Manatee calf call contour and acoustic structure varies by species and body size

Abstract

Vocal activity and signal characteristics of mammals are driven by several factors that result in both stability and plasticity over multiple time scales. All three extant species of manatee communicate with several calls that are especially important for maintaining contact between cows and calves. Determining if calf calls differ across manatee species will provide insights into the evolution of species-specific acoustic communication traits. We investigated the interspecific differences in the vocalizations of calves of Amazonian manatees (Trichechus inunguis) and the two subspecies of the West Indian manatee (T. manatus). Vocalizations of individual calves were recorded in rehabilitation centers in Brazil, Puerto Rico, the United States, and Mexico. The acoustic structure of calls produced by manatee calves varied between species and with body size. Amazonian manatee calves produced shorter calls with multiple notes at higher frequency while West Indian calves produced modulated calls that were lower in frequency and longer in duration. Smaller West Indian calves produced frequency modulated, hill-shaped calls that flattened with an increase in body length. Our results provide evidence for divergence in the ontogeny of vocalizations across T. manatus and T. inunguis and suggest variation in body size contributed to the evolution of differences in the characteristics of their calls.

Figure 1

Schematics showing how the acoustic parameters of the vocalizations of manatee calves were extracted. The spectrogram on the left shows a single manatee call with the fundamental frequency of the call boxed in. The same box is expanded in the top right to show how duration (ms), frequency modulation (kHz) (peak frequency maximum minus peak frequency minimum), start frequency, and end frequency (kHz) were measured. The shaded area overlapping the call illustrates the power spectrum slice expanded in the bottom right showing the measurement of the power at the minimum and maximum frequencies (kHz) of the selected call. All measurements were conducted in Raven 1.6. Spectrogram parameters: DFT: 512 point; Hamming window; 50% overlap.

Figure 2

Spectrograms of vocalizations recorded from individual calves in this study from the Amazonian manatee (Trichechus inunguis) and the two subspecies of the West Indian manatee (T. manatus)—the Antillean manatee (T. m. manatus) and the Florida manatee (T. m. longirostris).

Figure 3

Manatees call frequency and temporal variation by populations. Box plots display medians, first and third quartiles, as well as standard error.

Figure 4

Ontogenetic changes from a hill-shape to a linear call contour of the vocalizations of the Florida manatee (T. m. latirostris) and the Antillean manatee (T. m. manatus). The Florida and Antillean manatee are subspecies of the West Indian manatee (Trichechus manatus),