Dolphins routinely use sound for social purposes, foraging and navigating. These sounds are most commonly classified as whistles (tonal, frequency modulated, typical frequencies 5-10 kHz) or clicks (impulsed and mostly ultrasonic). However, some low frequency sounds have been documented in several species of dolphins. Low frequency sounds produced by bottlenose dolphins (Tursiops truncatus) were recorded in three locations along the Gulf of Mexico. Sounds were characterized as being tonal with low peak frequencies (mean = 990 Hz), short duration (mean = 0.069 s), highly harmonic, and being produced in trains. Sound duration, peak frequency and number of sounds in trains were not significantly different between Mississippi and the two West Florida sites, however, the time interval between sounds within trains in West Florida was significantly shorter than in Mississippi (t = -3.001, p = 0.011). The sounds were significantly correlated with groups engaging in social activity (F=8.323, p=0.005). The peak frequencies of these sounds were below what is normally thought of as the range of good hearing in bottlenose dolphins, and are likely subject to masking by boat noise.

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.

Bottlenose dolphins’ phonations are commonly divided into three major categories: whistles, echolocation clicks, and burst pulses. Other categories are often mentioned and described as yelps, squawks, barks, and low-frequency narrowband sounds. Here, we report the occurrence of low-frequency tonal phonations with fundamental frequency within the 500–2000 Hz range, accompanied by numerous harmonics. Recordings were made using 48 and 192 kHz sampling rates and a 100 Hz high-pass filter in the presence of free-ranging bottlenose dolphins in Mississippi Sound, MS. Auditory sensitivity of the bottlenose dolphins has been studied extensively, and the results indicate that dolphins have their best hearing sensitivity in the higher-frequency range (15–110 kHz). Low-frequency sounds (above 75 Hz) can be detected as well, and it has been suggested that this detection mechanism may be entirely different from that used for higher frequencies and may even include mechanoreception. The occurrence of low-frequency phonations in wild dolphins indicates that low-frequency detection may play an important role in the animals’ everyday activities.

Investigations of the bottlenose dolphin's acoustic repertoire have mainly focused on whistles and echolocation clicks. However, despite their widespread occurrence, burst pulses have not received much attention. The primary function of burst pulses remains unknown and a comparative analysis can be used to advance our knowledge of the function of burst pulses. We recorded the acoustic repertoires of free-ranging bottlenose dolphins in the Tampa Bay and Mississippi Sound areas using broadband recording equipment. The two habitats differ in their environmental features, including water temperature, ambient noise levels and water turbidity. Our preliminary data show that several acoustic parameters such as peak frequency and center frequency of burst pulses differ among habitats. We also report other acoustic parameters, such as the number of clicks, inter-click interval, 3-dB and rms bandwidth. In our ongoing study, we will examine relationships between environmental features of the habitats and the acoustic characteristics of burst pulses.