Does Brants' whistling rat (Parotomys brantsii) use an urgency-based alarmsystem in reaction to aerial and terrestrial predators?

It is predicted that differences in mammalian alarm call systems may be exp lained relative to the complexity of their habitat, with species inhabiting three-dimensional habitats classifying predator types (externally referent ial), and those living in two-dimensional environments indicating the level of risk (urgency-based). We tested this prediction in a two-dimensional en vironment for a small African rodent, Brants' whistling rat, Parotomys bran tsii. Colony members were presented with predator models of a raptor and pu ff adder, as well as a human observer, to investigate their alarm call repe rtoire. Alarm calls consisted of simple, single-note vocalizations, often r epeated in non-rhythmic bouts. Virtually no variation was detected in the s tructure of alarm whistles between the calls elicited by the two model pred ators and humans, indicating that P. brantsii did not identify different pr edator types by means of vocalizations. However, note duration was dependen t on the reaction of the caller: when the caller bolted towards safety, the whistle was significantly shorter than when it remained stationary. A snak e and far-off human elicited relatively long-duration calls and the caller would remain above ground, signifying a low-risk situation. High-risk encou nters with a nearby raptor or human provoked short calls before the whistli ng rat bolted underground. We conclude that P. brantsii's alarm call repert oire represents a graded 'urgency-based' system, indicating perceived threa t level rather than predator type. Our study supports the prediction that m ammalian alarm call systems in two-dimensional environments primarily provi de information indicating the level of risk and not predator type.