Two species of Asian mouse-deer have been observed utilizing a very interesting technique to get away from predators; they jump into the water and stay there until its safe to come up. By carefully swimming up to the surface to breathe now and then they can stay submerged for long periods of time.

People living in the Indonesian country side have always claimed that deer hide in the water when chased by their dogs, but it wasn’t until the behaviour was observed by a team of scientists doing a biodiversity survey that it caught the attention of the larger scientific community.

In June 2008, the team visited the northern Central Kalimantan Province in Borneo, Indonesia where they suddenly spotted a mouse-deer swimming in a forest stream. When the deer understood that it was being watched by humans, it went below the surface and remained hidden. Over the next hour, team members could see it come to the surface four or five times. Although it probably went up for air a few more times without being noticed, it could clearly remain submerged for more than five minutes at a time.

Eventually, the researchers caught the animal and photographed it before releasing it back into the wild unharmed. It was a pregnant female deer.

One of the members of the team is the wife of Erik Meijaard, a senior ecologist working with the Nature Conservancy in Balikpapan, Indonesia. When she showed her husband the photograph, he identified it as a Greater mouse-deer (Tragulus napu).

That same years, another group of observers witnessed a Mountain mouse-deer (Moschiola spp) throwing itself into pond and swimming under water to get a way from a hungry mongoose in Sri Lanka. The mongoose followed it into the pond, but eventually retreated as the deer continued to stay submerged.

“It came running again and dived into the water and swam underwater. I photographed this clearly and it became clear to me at this stage that swimming was an established part of its escape repertoire,” says Gehan de Silva Wijeyeratne, who saw the incident.

“Seeing it swim underwater was a shock”, he says. “Many mammals can swim in water. But other than those which are adapted for an aquatic existence, swimming is clumsy. The mouse-deer seemed comfortable, it seemed adapted.”

Both incidents have now been described in the journal “Mammalian Biology”.

“This is the first time that this behaviour has been described for Asian mouse-deer species,” says Meijaard. “I was very excited when I heard the mouse-deer stories because it resolved one of those mysteries that local people had told me about but that had remained hidden to science.”

What is a mouse deer?

Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Family: Tragulidae

Mouse deer are small deer-like animals with large upper canine teeth. In male specimens you can even see the teeth project down either side of the lower jaw. Ten different species of mouse-deer have been described by science and all except one live in South-East Asia. The Water Chevrotain (Hyemoschus aquaticus) is the only mouse deer native to the African continent and it is also the largest member of the family.

The Water Chevrotain (Hyemoschus aquaticus) lives in swampy habitats and is known to dash into the nearest river as soon as it is spooked by something. Until recently, this was the only mouse deer in which the habit of swimming under water and staying submerged for long periods of time had been described and all the Asian members of the family Tragulidae were thought to be strictly dry-land animals.