High in the Filipino rainforest, speeches are being made and trysts announced. Predators and prey may well be limited in their understanding, but the tiniest primate, the tarsier has been letting us turn a deaf ear. "Tarsiers are among only a handful of mammals known to communicate in the pure ultrasound," quotes the lead author in the Royal Society's Biology Letters, Marissa Ramsier.

At 20kHz, adult human ears turn off and we call the higher frequencies, "ultrasound." These graphs simply illustrate the extreme 91kHz abilities of Tarsier Syrichta and possibly other tarsiers. Other clever hearers include whales, the domestic cat (which you can frighten away with ultrasound), bats, rodents and the moths which are hunted by bats. This is the first primate, apart from owl monkeys at 45kHz, to be proved to use, "silent communication," just as if it were trying to avoid being detected. This is of course one of the most likely reasons for its use of ultrasound. You can't hear it. Other predators certainly can't and many prey items can't either! A private and also very efficient chat line, helping to eliminate low frequency interruptions.

This tarsier from Mindanao has a higher vocal range than other tarsiers too. Having 15 distinct calls below 16kHz, other tarsiers have alarm, rival-deterring and social calls, but T. syrichta seemed to "talk" less. With such rare, endangered species as the tarsiers, experimentation was impossible till recent sub-dermal electrodes with minimum invasive effect were invented. Anaesthetised and placed in sound chambers, it is difficult to justify the animals' use, but if we can save several species by understanding their audio-environment, then this clean use of wild animals is justified. All of the six used were totally unharmed by the procedure and quite probably these researchers were the most care-conscious available, using at all times the protocol of the Institutional Animal Care and Use Committee of Dartmouth College.

Vocal recordings from Bohol and Leyte islands were a much simpler kettle of tarsiers. 35 tarsiers to be exact. When handled (possibly indicating alarm) and within temporary enclosures, ultrasound was emitted. The dominant call frequency was defined as the most energetic, while the lowest frequency showed up on spectrograms like that shown in fig 1. Distinct vocal recordings between 67 and 79kHz were like other tarsiers (which use 34kHz maximum), but at much higher frequency.

The big eyes of tarsiers lack a reflective retina (tapetum) like those of other nocturnal animals, despite being visual predators of Arthropods. In dark clouded conditions, Ramsier, Cunningham et al from various Philippine and US universities suppose an auditory aid. Moths and katydid crickets use ultrasound and such intense hearing could simply detect rustling noises in trees. A very well adapted little primate has just shocked us again with unique abilities, possibly keeping it extant while other species in the Philippine jungles approach extinction.