Functional reference refers to a type of communication used by some animals in which information can be incorporated about an object or event external to the animal. With alarm calls, for example, encoded information may refer to the species of predator, the urgency of the response required, or the type of evasive action to be taken in response to a particular type of predator. Many species have different alarm calls for aerial versus terrestrial predators; including many species of ground squirrels (Spermophilus sp.), tree squirrels (Tamiasciurus hudsonicus), chickens (Gallus gallus domesticus), dwarf mongooses (Helogale undulata), suricates (Suricata suricatta), and tamarins (Saguinus fuscicollis and Saguinus mystax). Other species have alarm calls that differ for different predators. Gunnison’s prairie dog’s (Cynomys gunnisoni) have a different call for red-tailed hawks (Buteo jamaicensis), humans (Homo sapiens), coyotes (Canis latrans), and domestic dogs (Canis familiaris).
“Another level of information that can potentially be encoded into alarm calls involves descriptive features of the predators.” In prior experiments with Gunnison’s prairie dogs, significant differences in alarm calls for four humans wearing different colored shirts were reported; however, the four humans also differed in height and sex, and each human only wore one color of shirt throughout the experiment. In the current paper, the researchers controlled for these confounding variables by using three similarly sized human females, each of which wore all three colors of shirt when approaching the prairie dog colony.
The three humans walked through the colony on one of three paths wearing a yellow, green, or blue t-shirt, blue jeans, and sunglasses. While they walked through the colony, an observer sat on a hill 30 m away from the edge of the colony to record the alarm calls and caller identity for each of the walk-throughs. Overall 48 individually marked prairie dogs were recorded resulting in 82 calls. Nine of the prairie dogs called to all three shirts (27 calls), sixteen called to two shirts (32 calls), and twenty three called to one of the shirts (23 calls).
The researchers found that the prairie dogs use aucostically different alarm calls for yellow versus blue shirts and for green versus blue shirts but no difference was found between the alarm calls for yellow versus green shirts. This evidence goes in line with the fact that prairie dogs have dichromatic color vision and can see wavelengths in the blue and yelow part of the perceptual visual spectrum but have a more difficult time discriminating between wavelengths that are close to one another in the green-yellow part of the color spectrum. No difference was found in the calls for the three individual humans or in the calls for the three different paths walked.
Referential communication, however, is split into two parts:
1. Productional specificity => the callers’ production of signals that refer to external objects or events.
2. Perceptual specificity => the receivers of signals being able to interpret the signals as meaningful information
The present study has succeeded in demonstrating productional specificity but have not yet demonstrated perceptual specificity. Perceptual specificity is normally tested by playing back the alarm calls to a colony and seeing how the colony reacts to each different call. Demonstrating perceptual specificity in this case has proven difficult due to the fact that the prairie dogs responded to all different shirt colors in the same way that they would respond to any human (by running to their burrows and diving inside). However, it seems unlikely that the animals would encode this information into their alarm calls without the ability to recognize and translate this information into a useful reaction at the perceptual end. “Labeling the characteristics of individual predators may allow the prairie dogs to become familiar with the hunting styles of individual predators and take evasive action that matches a particular hunting style.”
Source
Slobodchikoff, C., Paseka, A., & Verdolin, J. (2008). Prairie dog alarm calls encode labels about predator colors Animal Cognition, 12 (3), 435-439 DOI: 10.1007/s10071-008-0203-y
