If you startled a deer, you might not expect it to jump into the nearest pond and submerge itself for minutes.
But that is exactly what two species of mouse-deer in Asia do when confronted by predators, scientists have found.
One other African mouse-deer species is known to do the same thing, but the new discovery suggests all ruminants may once have had an affinity with water.
It also lends support to the idea that whales evolved from water-loving creatures that looked like small deer.
There are around 10 species of mouse-deer, which are also called 'chevrotains'.
All belong to the ancient ruminant family Tragulidae, which split some 50 million years ago from other ruminants, the group that went on to evolve into cattle, goats, sheep, deer and antelope.
Each are small, deer-like creatures that unusually don't have antlers or horns. Instead they have large upper canine teeth, which in the males project down either side of the lower jaw.
The largest species, which stands no more than 80cm tall, lives in Africa and is thought to be the most primitive of all mouse-deer. Known as the water-chevrotain, this animal likes to live in swampy habitats. When alarmed, it dashes for the nearest river where it submerges and swims underwater to safety.
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All of the other species of mouse-deer, which live in southeast Asia and India and Sri Lanka were thought to be dry-land animals.
Diving deer
That was until researchers witnessed some remarkable behaviour during two separate incidents.
The first occurred in June 2008 during a biodiversity survey in northern Central Kalimantan Province in Borneo, Indonesia.
During the survey, observers saw a mouse-deer swimming in a forest stream. When the animal noticed the observers it submerged. Over the next hour, they saw it come to the surface four or five times, and maybe more unseen. But it often remained submerged for more than five minutes at a time.
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Eventually the observers caught the animal, which they identified as a pregnant female, then released it unharmed. 
Among the survey team was the wife of Erik Meijaard, a senior ecologist working with the Nature Conservancy in Balikpapan, Indonesia.
Meijaard knew of anecdotal reports by local people who described deer hiding in creeks and rivers when chased by their dogs. When he saw photos of the deer he identified it as a greater mouse-deer (Tragulus napu).
Three observers saw a mountain mouse-deer (Moschiola spp) run into a pond and start to swim, hotly pursued by a brown mongoose. The mouse-deer submerged itself, and eventually the mongoose retreated. The deer left the water only to be chased straight back into it by the mongoose.
"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. 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," he says.
Origins of whales
Meijaard, Wijeyeratne and Umilaela, who saw the submerged Bornean mouse-deer, describe both incidents 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."
The behaviour bolsters one leading theory regarding the origin of whales.
In 2007, scientists led by Hans Thewissen of the Northeastern Ohio Universities College of Medicine in Ohio published details of a remarkable fossil called Indohyus.
This fossil was of a ruminant animal that looked like a small deer, but also had morphological features that showed it could be an ancestor of early whales.
The discovery that two Asian species of mouse-deer are comfortable underwater shows that at least three species of modern tragulid share an aquatic escape behaviour.
Because these species diverged at least 35 million years ago, their ancestor also likely behaved in the same way, again bolstering the the idea that a deer-like ruminant may have evolved to produce the modern cetacean group of whales and dolphins.
Hippos, the closest modern relative of whales, also dive for water when threatened, a behaviour that may have been lost over time by other modern species such as sheep and antelope.
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