Abstract and Summary
The general aim of this study was to examine ontogenetic and situational variation in the use and structure of the jump-yip display of black-tailed prairie dogs (Cynomys ludovicianus). Treatment of the data was guided by two proposals: (1) that the usual dichotomy between “signaller” and “situational” referents of signals be replaced by the idea that signal structure is constrained by the relationship between signaller and situation, and (2) that adult-focussed explanations of age differences in communicatory behavior be complemented by accounts based on age-specific considerations.
To explore the utility of a relational view of signal referents, we attempted to influence the structure of jump-yipping by “manipulating” both sides of the prairie dog-snake relationship to yield approximately comparable changes in prairie dog vulnerability to snakes. On the snake side of the relationship, we manipulated snake venomousness (non-venomous bullsnakes and venomous rattlesnakes) and snake size (large and small snakes of both species). On the prairie dog side of the relationship we “manipulated” prairie dog age (pups and adults). Thus, feral pup and adult black-tailed prairie dogs were video and audio taped during exposure in separate trials to each of 4 partially anesthetized snakes — a large and a small bullsnake (Pituophis melanoleucus sayi), and a large and small Western Diamondback rattlesnake (Crotalus atrox). Quantitative analysis of sonagrams of snake-elicited calls demonstrated that approximately comparable manipulations on both sides of the relationship of prairie dog vulnerability to snakes yielded approximately comparable changes in vocal structure. Adults only jump-yipped, whereas pups both jump-yipped and barked, a call usually associated with predators to whom prairie dogs are more vulnerable, such as coyotes. Since pup jump-yips were also more bark-like than adult jump-yips, we concluded that pup call structure reflected greater signaller vulnerability to snakes than adult call structure did. Further quantitative analysis revealed that in general adults jump-yipped as though they were more vulnerable to larger and to venomous snakes, i.e., by using more bark-like jump-yips. Pups exhibited similar apparent vocal adjustments, but these were ambiguous because vocal idiosyncrasies may have been confounded with the influence of snakes on call structure.
Age-specific considerations seemed particularly applicable to these results. Whereas barking by pups at snakes might have seemed inappropriate by adult standards, the use of this call seemed appropriate, given the pups' probable greater vulnerability to snakes. The pups' more bark-like yips were likewise consistent with this age-specific interpretation. Although rendered ambiguous by potential confounding, possible age differences in vocal discrimination among snakes were also tentatively interpreted on the basis of age-specific vulnerability to snakes.
We concluded that consideration of signaller-situation relationships, and their age-related changes might be widely applicable in explanations of communicatory behavior. Since these concepts were derived from a “management” view of communication, this view appears to have substantial explanatory potential.