Tarsiers are among the most popular in Philippines and other Southeast Asian countries. It is regarded as one of the world's smallest primates and has a length of five inches with huge round eyes.

Tarsiers communicate like humans. They use different ways to express themselves. However, it is extremely strange that some tarsiers choose to keep silent.

Tarsiers are found worldwide in the previous years, but now have settled mostly in Philippines and Borneo. Tarsiers found in these two countries are called the "ordinarily silent" type. This means that they don't produce a sound similar to what we normally observe with all the other species.

Several scientists wondered about their being silent. Many are bewildered why they aren't the same as their counterparts using their verbal calls to express something like when they are telling their rivals to keep away or making a sound that implies danger.

They generally produce audible sounds to communicate and socialize with other tarsiers. And so, the mystery with silent tarsiers begins.

Nathaniel Dominy, a Dartmouth anthropologist, worked on this secrecy. Soon, he discovered that tarsiers were silent to our ears. They are apparently communicating through ultrasound. The silent type tarsiers are using too high frequencies for humans.

Many animals like dogs, fishes and even insects can listen to ultrasonic frequencies, but it isn't typical for mammals to communicate at this level of frequency.

Tarsiers in Borneo and Philippines are making use of ultrasonic frequencies to communicate. They're using an audible range that is absolutely beyond what other primates can listen to. The scientists say tarsiers can produce a pure ultrasonic scream at 70 kilohertz.

Given the discoveries of the research, this is completely a mysterious fact tarsiers kept for themselves. They have a secret form of communication that is associated with a shocking level of frequency.