Invisible but detectable: listening for
wild Amazonian manatees
Renata S. Sousa-Lima1,2, Rodney A. Rountree3,
Marcos Roberto M. Brito1, Isadora M. Carletti1, Vera M. F. da Silva4
1Universidade Federal do Rio Grande do Norte, Departamento de Fisiologia, Laboratório de Bioacústica (LaB), Natal, RN, Brazil.
2Bioacoustics Research Program, Lab of Ornithology, Cornell University, Ithaca, NY, USA.
3Marine Ecology and Technology Applications, Inc, Waquoit, MA, USA.
4Instituto Nacional de Pesquisas da Amazônia, Laboratório de Mamíferos Aquáticos, Manaus, AM, Brazil.
We thank Fundação O Boticário de Proteção à Natureza, CNPq,Instituto de Desenvolvimento Sustentável Mamirauá, INPA/MCT and Petrobrás Ambiental for the logistical and financial
support in Brazil and Operation Wallacea (http://opwall.com/) in Peru.
Introduction Direct observation of wild Amazonian manatees (Trichechus inunguis) is almost impossible due to low water visibility
(<1m) and the cryptic behavior of the animals that became extremely shy after centuries of hunting (Best 1982, 1984). Alternative
non-invasive methods are needed to monitor populations of this endangered species, which is both endemic to the Amazon region
and the only exclusively freshwater manatee (Rosas 1994). T. inunguis is known to emit sounds in captivity (Evans & Herald 1970;
Sonoda & Takemura 1973; Sousa-Lima et al. 2002) that can be attributed to specific individuals and to different sexes and age classes
(Sousa-Lima et al. 2002). Here we provide the first published recordings of Amazonian manatee sounds in the wild and evidence that
PAM can be used for detection and monitoring of these elusive mammals in the wild.
1) to obtain recordings of manatees in the wild, and compare them with recordings from captivity,
2) to verify if Amazonian manatees can be detected in the wild using PAM.
Methods (See Figure 1) Manual identification of calls was done in Raven Pro 1.4 (Cornell Lab of Ornithology) using following parameters: Hann window; 256 points window size. Clear
calls with high signal-to-noise ratio were selected for extraction of the acoustic parameters. The acoustic parameter values obtained in the wild were used to compare with values from
captive animals (Sousa-Lima et al. 2002; Dantas 2009).
Results We recorded 114 calls in the Urini Lake, 166 calls in the Purus River (Rosas et al., 2003), and 35 calls in Pacaya Samiria National Reserve. Eighty seven (87) calls were selected
from the Brazilian sample of calls; 13 from the Peruvian data set and 94 calls from captive individuals (Figure 2). We found no significant differences for peak fundamental frequency (t-
test; p = 0.88206) between wild and captive calls. Principal component analysis grouped wild and captive calls from Brazil together and separated calls obtained in the wild from Peru
(Figure 3). Additionally, the frequency modulation pattern and overall aspect of the signals recorded in the wild were recognized by experts in the field as manatee calls (Figure 2).
Discussion The calls recorded in the wild matched information and calling rates for Amazonian manatee
calls in captivity. T. inunguis has seasonal reproduction with births peaking between February and May (Best
1982, 1984). Our recordings were made shortly after the birth period; therefore we probably recorded the
most vocal individuals: lactating females (3-5.5calls/min: Dantas 2009) and their calves (2-8 calls/min: Dantas
2009; Sousa-Lima et al. 2008). The use of playback of calls to acquire the Brazilian data might have elicited
calls back from less vocal individuals (other adults call less than once per minute, Sousa-Lima et al. 2002;
BEST, R. C. 1982. Seasonal Breeding in the Amazonian Manatee, Trichechus inunguis (Mammalia: Sirenia). Biotropica, 14 (1): 76-78.
BEST, R. C. 1984. Trichechus inunguis vulgo peixe-boi. Ciência Hoje, 2 (10): 66-73.
DANTAS, G. A. 2009. Ontogenia do Padrão Vocal Individual do Peixe-Boi da Amazônia Trichechus inunguis (Sirenia, Trichechidae). 71 f. Dissertação (Mestrado em Biologia de Água Doce e Pesca
Interior) – Biologia Tropical e Recursos Naturais, Instituto Nacional de Pesquisas da Amazônia.
EVANS, W. E. & HERALD, E. S. 1970. Underwater calls of a captive Amazon manatee, Trichechus inunguis. Journal of Mammalogy, 51: 820-823.
ROSAS, F. C. W. 1994. Biology, conservation and status of the Amazonian manatee, Trichechus inunguis. Mammal Review, 24: 49-59.
ROSAS, F. C. W., SOUSA-LIMA, R. S. & DA SILVA, V. M. F. 2003. Avaliação Preliminar dos mamíferos do baixo Rio Purus. In: DEUS, C. P., DA SILVEIRA, R. & PY-DANIEL, L. H. (Eds.). Piagaçu-Purus: Bases
Científicas para Criação de uma Reserva de Desenvolvimetno Sustentável. Manaus, IDSM, p. 49-59.
ROUNTREE, R. A. 2012. Listening to Fish Sounds in the Amazon. Biodiversity Science Issue 5. (http://www.biodiversityscience.com/2012/01/31/fish-sounds-in-the-amazon/).
SONODA, S. & TAKEMURA, A. 1973. Underwater sounds of the manatees, Trichechus manatus manatus and Trichechus inunguis (Trichechidae). Reports of the Institute for Breeding Research,
Tokyo University of Agriculture, 4: 19-24.
SOUSA-LIMA, R. S., PAGLIA, A. P. & DA FONSECA, G. A. B. 2002. Signature information and individual recognition in the isolation calls of Amazonian manatees, Trichechus inunguis (Mammalia:
Sirenia). Animal Behaviour, 63: 301-310.
SOUSA-LIMA, R. S., PAGLIA, A. P. & DA FONSECA, G. A. B. 2008. Gender, Age and Identity Information in the Isolation Calls of Antillean Manatees, Trichechus manatus manatus. Aquatic Mammals,
34 (1): 109-122.
Conclusion It is possible to detect and monitor Amazonian manatee populations in the wild by means of
passive acoustics especially during and following the peak of birth for the species.
Figure 1 Recordings between June and July in the Amazon River basin in Brazil during manatee
acoustic surveys in the Urini Lake (Yellow Triangle, Amanã Sustainable Development Reserve)
and in the Purus River (Green Triangle; Rosas et al. 2003) were done using a Sony Walkman Pro
(WM-D6C; flat audio frequency response 0.04-15kHz ±3 dB) and an omnidirectional
hydrophones (model 50Ca of the Cetacean Research Technology – CRT or HTI 94 SSQ) with a
sensitivity of -161dB re 1µPa and a frequency range response of 0.01-310kH in response to
captive manatee calf calls played back using an Oceaneer underwater speaker (DRS-8, frequency
range 0.1-16kHz with peak resonance frequency at 4.5kHz, 158dB re 1µPa). In Peru, recordings
were made opportunistically in July at Pacaya Samiria National Reserve (Red Triangle) during
fish surveys conducted by Operation Wallacea (Rountree 2012) with a SQ26-H1 recorder system
from CRT (frequency response of up to 40kHz). The animals were never seen during the
-8 -6 -4 -2 0 2 4 6 8
Figure 3 Plot of first and second PCA axes showing 2 main groups, one circled in
red showing calls from wild individuals from Peru and a second larger group circled
in yellow and green showing calls from wild and captive individuals from Brazil.
Figure 2 Spectrograms of : A) wild manatee call recorded
in Brazil and B) captive manatee call recorded at INPA.
Photo: José Anselmo D´Affonseca Neto