The endangered status of Florida’s manatees is highly publicized and their protection is widely supported. As with most endangered species, human encroachment upon habitats is arguably the most detrimental factor to the species’ wellbeing.
Perhaps the best-known fact about manatees in our state is that they don’t get along well with boats. According to the Florida Fish and Wildlife Conservation Commission (FFWCC), 63 manatees died over the past year as a result of injuries inflicted by watercraft. This number is more relative when considering that the FFWCC estimates the total manatee population of Florida is approximately 3,100 animals in the wild.
USF marine science professor David Mann and other marine scientists from around the state have been researching the hearing capabilities of manatees in hopes of gaining a better understanding of this endangered animal.
“The research was done with two captive manatees that are housed at the Mote Marine Laboratory in Sarasota,” Mann said. “What we’re looking at is the sounds that (manatees) are able to hear and how they process sounds with their brains.”
According to Mann, a technique used to screen for deafness in infants called “auditory evoked potential” was employed to acquire data about the hearing capability of manatees.
“The manatees were trained to a station, which basically means they stay in one spot,” Mann said. “Then we play a sound to them and measure the brain’s response to those sounds with sensors. By doing that we can tell what frequency range of sounds they can detect; we also looked at something that tells us how fast their auditory system is.”
According to Mann, manatees can hear a much greater spectrum of sound frequencies than humans.
“Our results show that they can detect really high frequency sounds, into the ultrasonic range,” Mann said. “Humans hear up to about 20 KHz, and dogs can hear above that. It turns out, so can manatees. It looks like their hearing range is shifted up towards a little higher frequency range than ours is.”
According to Joe Gaspard, care and training coordinator for Mote Marine Lab, the manatees were exposed to a range of sounds from a head on direction using four separate speakers.
“They have fairly decent localization abilities when stimuli is presented directly in front of them,” Gaspard said.
Hearing a noise is one thing, being able to tell where it is coming from can often be another issue.
“One question is whether they can detect boats or not given their hearing range,” Mann said. “It looks like they probably can, to some extent. The main question is if they can hear a boat, can they tell which direction it is coming from?”
Mann used the common difficulty of determining which direction the sound of an approaching fire truck is coming from as a human example. According to Mann, humans can hear pulses of sound up to a rate of 100 per second. Above this rate the pulses mesh together and are heard more like a constant tone. Mann’s research shows that manatees have the ability to hear pulses of sound up to 600 times per second.
“The primary cue that people use to tell where a sound is coming from is the time of arrival of the sound in a different ear,” Mann said. “When you go underwater sound is moving five times faster, so that cue is five times shorter. So one of the things we’re hypothesizing is that the ability of the manatee to follow this pulsing up to 600 times a second suggests that they may have evolved some mechanism, for doing sound localization underwater.”
According to Mann, this research was the beginning of a larger project in which more specific sound localization testing will be done.