The Evolution of Urgency – Based and Functionally Referential Alarm Calls in Ground – Dwelling Species

Alarm calls have evolved many times in mammal and bird species which live in groups. These alarm calls generally fall into two classes; they are either urgency –based or functionally referential. Urgency – based alarm calls provide information on the proximity of a predator to the group while functionally referential alarm calls provide information on the type of predator approaching. In mammals, it has been theorized that functionally referential alarm calls evolved in species which live in complex habitats, such as forests, and urgency – based alarm calls evolved in species living in simple, homogeneous habitats, such as open grasslands. However, there are several examples of species living in simple open habitats which have evolved functionally referential alarm calls. The authors of this paper challenge the traditional view of why different types of alarm calls have evolved by observing and analyzing the alarm calls of two sympatrically living species, the Cape ground squirrel and the meerkat. Meerkats, which live in large complex social groups which often forage far from their burrows on the African savannah, have evolved different alarm calls depending on the type of predator threat (functionally referential alarm calls). Cape ground squirrels, which live in small, less complex social groups which do not venture far from the burrows they often share with meerkats, have evolved alarm calls which only convey information on the proximity of the predator threat (urgency-based). The authors of this paper hypothesized that because these two species live in the exact same habitat, the driver behind the evolution of urgency – based versus functionally referential alarm calls was the complexity of the social group rather than the habitat complexity. After recording, analyzing, and playing back alarm calls of the cape ground squirrel and observing the foraging and escape behaviors of both species, it was determined that there were significant differences between the calls and the behaviors of each species. Cape ground squirrels responded exactly the same to three different predator stimuli (terrestrial, aerial, and snake) and their calls only differed with distance and whether the predator was moving. They also foraged close to their burrows and spend most of their time in an alert position. Meerkats responded differently to different predator stimuli and their calls were different depending on predator type instead of proximity. They often travelled far from their bolt-holes while foraging and spent much of their time with their heads down or under substrate. However, meerkats also take turns at sentry duty, which cape squirrels do not. The authors determined that complexity of social groups plays a bigger role in determining alarm call type that habitat complexity in mammals.

This relates to what we talked about in class because it involves signal value and information as well signal cost. Because of the difference in their social groups, meerkats and Cape ground squirrels encode different information in their signals. Also, because meerkat signals are more complex, the sender, which is usually the sentry, is more at risk and can suffer a higher cost. Also, senders are more likely to give alarm calls when kin are near and since meerkat groups are usually comprised of related individuals, we would expect them to emit honest alarm calls.