Overview

In 03, the thing that worked best was teaching. But when I say teaching, I was quietly assuming something. To teach, you need a means of conveying. For a rat, that means is the voice.

So what does that voice carry? Let me say it up front: not language. A rat's voice carries mood and situation, not sentences or commands. In this article I build the rat's voice as close to the real thing as I can, and look at what it conveys and what it does not.

Bringing the Voice Closer to the Real Thing

Until now, the voice inside the colony was just a symbol. A single mark that meant danger. But a real rat's voice has more structure than that. So I rebuilt it to match the features described in the literature.

A rat's voice is ultrasound＊, almost inaudible to the human ear. It splits into two broad bands. The lower one, around 22kHz＊, is a long, flat call, made in pain or danger. The higher one, around 50kHz, is a short call whose pitch moves around, made when a rat is with others or playing. Think of the first as an "I don't like this" call, and the second as an "I like this" call.

The two bands at 22kHz and 50kHz, the syllable types, their lengths, and the statistics of their ordering
The vocal repertoire. The low 22kHz alarm, and around ten syllable types in the high 50kHz range. Aversive calls are long, pleasant calls are short. The ordering of syllables is biased, not random.

The higher calls have finer shapes. Flat, upward, downward, arched, trilled — around ten syllable types. And the calls do not come out in any order. The syllable that follows a given syllable is biased. There is something like a loose grammar. When I traced the statistics in the literature, my model showed the same features. The calls split cleanly into two bands, aversive calls are long (a median of about 900 milliseconds), pleasant calls are short (about 40 milliseconds), and the ordering of syllables is more ordered than random (the spread of transitions is 2.45, smaller than the 3.20 you get from shuffling the order).

The Voice Carries Mood, Not Thought

What matters here is what the voice does not carry.

This voice is not language. It does not carry a contentful sentence like "there is a snake to the right." What it carries is the sender's state. Whether it is afraid, or calm. The one who hears reads that mood and reacts. Hear 22kHz and flee; hear 50kHz and approach. It does not read content — it senses the other's state.

def speak(self):
    # The voice comes from internal state (affect), not a proposition.
    band = "22kHz" if self.valence < 0 else "50kHz"
    return Call(band=band, syllable=self.pick_syllable())

def hear(self, call):
    # The hearer reacts to the other's state, not to the content.
    self.flee() if call.band == "22kHz" else self.approach()

Compared to human language, this may look thin. Even so, it can do a great deal. Danger spreads through the colony in an instant, and calm sets the mood of a place.

From social 50kHz calls, to a 22kHz alarm when a predator appears, and back to social calls
The voice switches with the situation. The social 50kHz when others are near, the 22kHz alarm when a predator comes, and then back to social calls.

What the Voice Reveals and Hides

To be honest, this does not reproduce a real rat's vocalizations as they are. I only shaped the model to fit the statistical features in the literature; I am not synthesizing actual calls.

And there is an important reservation. What I modeled is the rat's mood, not its thought. What you can read from the voice is a state, not what is in the head. This distinction will matter later. The voice carries affect and context. Meaning and culture are built on top of it.

Next

The voice became the medium of culture. The voice that teaches, the voice that warns, the voice that brings the colony into step.

If so, I can ask this. How durable is what rides on that medium? When the ones who teach are gone, what happens to the knowledge the voice was carrying? Next, I look at what is really lost when the colony collapses.

Notes

  1. Ultrasonic vocalization (USV): High-frequency calls made by rats, almost inaudible to humans. They become audible when recorded and pitched down.
  2. 22kHz / 50kHz bands: 22kHz is long and flat, made in aversion or alarm; 50kHz is short with moving pitch, made in social or play contexts. Based on the synthesis by Brudzynski and Panksepp.