Thought-speak is the way Andalites communicate since they have no mouths. It is a language that almost all species can understand. There are two different types of thought-speak: open thought-speak, in which one broadcasts their thoughts to everyone in the vicinity; and closed thought-speak, where you specifically send your thoughts to a specific person or people.
Early Andalites used sign language instead of thought-speak, and had an older language as well.[1]
Usage[]
Thought-speak is used by those in morph, contemporary Andalites, Leerans, Iskoort, Mercora, the Helmacron, Garatrons (who have an extremely fast thought-speak), the Ellimist (as an Andalite), and the Crayak.
In The Suspicion, Cassie recognizes Ax's thought-speak voice, "which meant he was probably in his normal body." This suggests that you may sound different in different morphs and everyone's thought-speak voice is as distinguishable or more from a sound producing voice. Many other aliens communicate by thought-speak. The Animorphs can also communicate in thought-speak, but only in a morph as shown in a few books where Ax has talked to the others via thought speak, although he was human at the time. There are also limits on thought-speak, such as distance. For example, in The Visitor, while Tobias was on the roof of Chapman's house, Rachel did not hear his Thought-speak in the basement. In The Underground, Rachel couldn't Thought-speak past an estimated 150 feet underground as a mole. In The Other, Gafinilan-Estrif-Valad states that he can use thought speak with his since-childhood friend as long as they are on the same planet, though the exact words are unclear the "sound" helps him know Mertil-Iscar-Elmand is living.