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.

You have probably noticed it if you’ve ever tried to catch a fish using your bare hands or a small net: the uncanny ability of these creatures to escape, sometimes even before you make a move. Most fish species are incredibly fast and seem to be virtual mind-readers when it comes to predicting when and where you will make your next attempt.

The reason behind this remarkable talent is a special circuit present in the brains of many species of fish. Fish ears constantly sense the sound pressure on each side of the body and if the ear on one side detects a disturbance, the muscles of the fish will automatically bend the body into a c-shape facing the opposite direction. This involuntary reaction makes it possible for the fish to start swimming way from harms way as quickly as possible. Scientists call it C-start and it is highly advantageous when escaping from predators. That is, until you venture upon the Tentacled snake (Erpeton tentaculatum) of South-East Asia.

While studying the Tentacled snake, Kenneth Catania, associate professor of biological sciences at Vanderbilt University, realized that this snake has found a way of exploiting the C-start reflex to its advantage.

Using video recordings of snake (see below) and prey Catania was able to slow down the chain of events enough to make them noticeable for a human eye, and what he saw amazed him. Instead of fleeing from the snake, fish would swim right into the mouth of the predator nearly four times out of five. How could this be?

When hunting, the Tentacled snake forms its body into a peculiar J-shape with its head at the bottom of the “J”. It then remains absolutely still until suitable prey ventures close enough to the “hook”-area of the J. When it finally strikes, it rarely misses since the fish seem to be magically drawn to the jaws of their attacker. In 120 attacks carried out by four different snakes, Catania observed no less than 78 percent of the fish turning toward the snake’s head instead of swimming away from it.

Catania also noticed something else: before the snakes moved their head to strike, they always flexed a point midway down the body. A hydrophone placed in the aquarium unveiled that by flexing its body, the snake produces sound waves intense enough to trigger the fish’s C-start reflex, and since the sound comes from a spot opposite the head of the hungry snake, the C-start reflex forces the fish to turn and swim directly towards the snake’s mouth.

“Once the C-start begins, the fish can’t turn back,” Catania explained. “The snake has found a way to use the fish’s escape reflex to its advantage. I haven’t been able to find reports of any other predators that exhibit a similar ability to influence and predict the future behavior of their prey,”

The C-start behaviour is actually so predictable that the snake doesn’t even bother to aim for the initial position of its prey and then adjust its direction as most predators would. Instead, it goes directly for the spot where it knows the fish will be heading.

“The best evidence for this is the cases when the snake misses,” says Catania. “Not all the targeted fish react with a C-start and the snake almost always misses those that don’t react reflexively.”

Kenneth Catania studies the brains and behaviour of species with extreme specializations. His new snake study is published this week in the online early edition of the journal Proceedings of the National Academy of Sciences.

Researchers at Tokyo Institute of Technology have undertaken what is believed to be the very first CT scan of eggs inside a coelacanth fish.

“I was surprised to see that all the eggs were the same size,” said Dr Norihiro Okada, a bioscience professor at the university and a member of the research team. “I hope to do research into why this is.”

Each coelacanth fish was roughly 170 cm (67 in) long and weighed about 70 kg (154 lbs). After being captured off the coast of Tanzania, both fishes were frozen and send to Japan where the CT scan showed how each fish contained roughly 40 eggs; each egg being about 7 cm (almost 2 ¾ in) in diameter.

The eggs of a coelacanth are never released into the water because the offspring hatch while still inside their mother. The young fish sometimes reach a length of 30 cm (12 in) before leaving their mother’s body.

Coelacanths were long believed to have gone extinct around the same time as the dinosaurs, until scientists realized that these fishes actually turn up in the nets of African and Asian fishermen now and then. The first confirmed finding is from 1938 when a specimen was captured in the Indian Ocean.

Coelacanths are of special interest to evolutionary biologists since they are thought to represent an early step in the evolution of fish to amphibians. You can read more about this in our coelacanth article.