Thomas's Langur

Thomas's langurs produce alarm calls where rate of calling encodes threat urgency more than call type. A single call indicates low-level alert; rapid repeated calls indicate immediate danger. This rate-based encoding system complements or substitutes for the categorical systems seen in vervets and other species.

The urgency gradient is continuous. Rather than discrete categories (dangerous vs safe), calling rate varies continuously with threat proximity and behavior. This analog encoding captures more information than digital categories could, at the cost of requiring receivers to process graded variation.

Group spread affects calling behavior. When groups are dispersed, callers produce longer call sequences—apparently compensating for the difficulty of ensuring all group members receive the message. The signaler adjusts to receiver distribution, demonstrating awareness of audience.

Males and females differ in calling patterns. Males call more frequently, possibly because male-male competition creates selection pressure for conspicuous protection displays. Calling functions both as genuine warning and as status advertisement. The dual function is common in alarm systems.

For organizations, Thomas's langurs illustrate that urgency encoding matters as much as categorical information. How often and how urgently messages are sent conveys information beyond message content.