Potential Role of Vocalizations in West Indian Manatees

Abstract

Although vocalizations have been relatively well studied in several groups of marine mammals, very little is known regarding the context and function of vocalizations of West Indian manatees (Trichechus manatus). Manatee vocalizations have been described by Schevill and Watkins (1965), Evans and Herald (1970), Sonoda and Takemura (1973), and Hartman (1979). Most of these authors presumed that the calls were primarily used for social reasons, such as maintaining cow/calf contact, rather than for navigation. While studying the ecology and behavior of manatees at Blue Spring Run (Volusia Co., Florida) and in the St. Johns River, Florida (Bengtson, 1981), manatees were heard vocalizing in a variety of behavioral contexts other than cow/calf interactions. In order to examine the importance of vocalizations during certain behaviors, we measured call rates during different activities. Recordings of manatees vocalizations were made opportunistically in Blue Spring Run and the St. Johns River using an Inter-Oceans Systems, Inc. R-130 hydrophone (frequency response range 20 Hz to 8 kHz) and a Panasonic RQ-309AS tape recorder. This equipment was well within the range of manatee vocalizations described by Schevill and Watkins (1965). The behavioral situations in which recordings were made included: 1) Natural feeding: grazing on submerged, rooted vegetation while moving slowly along the bottom; 2) Swimming: traveling in the St. Johns River while passing our anchored boat (not recorded in Blue Spring Run); 3) Bottom resting: resting or sleeping on the bottom of the waterway, rising periodically to the surface to breathe; 4) Artificial feeding: experimental presentation of plants to manatees by lowering a tray holding various plant species into Blue Spring Run. Sometimes two or more manatees jostled each other while attempting to gain access to the tray (Bengtson, 1983); 5) Milling: manatees grouped together but not cavorting (see below) as they moved slowly about; and 6) Cavorting: rolling, pushing, nuzzling, and embracing each other, sometimes with much chasing and splashing. There were several potential sources of error in the recordings and counts of manatees. Both the number of manatees present and the classification of different behaviors may have been misidentified outside the clear waters of Blue Spring (78% of the recordings were made in Blue Spring Run). In the St. Johns River, the number of manatees was estimated by counting their noses as they breathed. At night our estimates were poorer, based on the number of radio-tagged individuals and the breathing frequency. Another possible source of error was the vocalizations of nearby manatees involved in a different behavior. For example, in some cases when recordings were being made of individuals cavorting, others nearby might have been bottom resting or feeding. However, most recordings were made when an individual was alone or when individuals in a group of manatees were behaving similarly. The presence of cow/calf pairs (present in 18% of the samples) may have raised calling rates in our recordings. Rather than attempt to differentiate between calls (whistle-squeaks and chirps), all vocalizations were lumped together and reported as number of calls per unit time. Although both sexes were present during recordings, no differentiation was made between their vocalizations. A total of 3,744 calls was tallied during 21.5 h of recording on 19 days in February through April, 1979. Of these, 2,124 calls were selected for analysis on the basis of knowing manatees' behavior. Calls recorded during periods when the manatees' behavior was uncertain were discarded. Recordings were analyzed for average number of calls (discrete pulses) per 5-min interval for each taping session within a behavioral type. Because the number of manatees per session ranged between 1 and 20 and the length of observation period varied between sampling sessions, the mean number of calls per manatee per unit time was calculated for each behavioral category. These data were normalized using a square root transformation, which stabilized the variances among the different behaviors. A one-way analysis of variance and Tukey's Q test indicated significantly different call rates among behaviors (Fig. 1). Rates of vocalizations 816 Vol. 66, No. 4