Zoologger: The sea cow with super-sensing hairs

Zoologger is our weekly column highlighting extraordinary animals ? and occasionally other organisms ? from around the world

Species: Trichechus manatus latirostris
Habitat: Coastal waters of the west Atlantic and Gulf of Mexico, particularly the Florida coast

Big animals, and that includes humans, are clumsy, lumbering and noisy. Tolkien summed it up when he described hobbits hiding away "when large, stupid folk like you and me come blundering along, making a noise like elephants which they can hear a mile off".

We megafauna also tend to be oblivious to the small things around us, but the hulking Florida manatee is an honourable exception to the rule. Despite being a lot larger than the average human, it can detect tiny water currents generated by a moving object, even if those currents are at the nanoscale.

Manatees are not, at first sight, graceful or skilled. Commonly known as sea cows because of their habit of grazing on underwater seagrass, they look a bit like overstuffed seals.

Clocking in at around 3 metres long, Florida manatees live up and down the southern US coast, in Atlantic waters and the Gulf of Mexico. They are deemed endangered by the IUCN Red List: there are fewer than 2500 adults left in the wild. Changes to their habitat, and the risk of collisions with boats, mean the population is likely to decline further still.

They can't cope with temperatures below about 20??C, so tend to stick to the murky shallow waters of mangroves and seagrass meadows, migrating either southward or upriver to shelter from winter weather. Nowadays, wintertime often sends them packing to the warm outflows pumped out by power plants.

Deft movers

Their movements create something of a conundrum for the biologists who study them. For such a lumbering animal, they are remarkably adept at navigating through muddy waters, cluttered with obstacles like fallen trees. "We don't understand how they can migrate, or how they do it so deftly," says Joe Gaspard of the Mote Marine Laboratory and Aquarium in Sarasota, Florida. "They can get up to 30 kilometres an hour in short bursts, and still navigate without any trouble."

To make matters even more confusing, their sight is poor, and unlike river dolphins, they don't have electrosensing or echolocation to help them find their way. So Gaspard wondered if they might simply have a very acute sense of touch.

Gaspard tested two manatees, Buffett and Hugh, who have lived all their lives in captivity after their parents were rescued from a poisonous red tide in 1996. He trained them to approach a 6-centimetre sphere that was dangled in the water on a rod and made to vibrate gently. If the manatees sensed the vibration, they touched a paddle with their muzzles and were rewarded with pieces of their favourite foods ? apples, carrots, beets and monkey biscuits.

Gaspard found that the movements the manatees could pick up were minuscule, even as a small as a nanometre. Hugh was slightly more sensitive than Buffett, detecting movements of just 0.9 nanometres.

Super-sensors

Manatees' hairs, it turns out, are rather like whiskers, turning them into super-sensors. When Gaspard covered them with mesh netting, the manatees became much less sensitive. "The tactile sense is probably their primary sensory system," says Gaspard. "It is unique in the mammalian world."

Unfortunately, their sense of touch doesn't help the manatees escape one of the biggest threats they face: boats. Gaspard thinks their sensitive hairs can only pick up objects within a few metres, so by the time they detect a speeding boat it is practically on top of them.

Journal reference: Journal of Comparative Physiology A, DOI: 10.1007/s00359-013-0822-x

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