Responsiveness to “social” releasers in cattle. I. A study of the differential and additive effects of visual and sound stimuli, with special reference to the law of heterogeneous summation

Abstract

Two experiments (Exp.1, Exp.2) were carried out to investigate the effects of social releasers, i.e. models and calls of conspecifics, on the approach responses of Friesian-Dutch heifers. In Exp.1, models consisted of either a vague representation of a conspecific's shape, or of a life-sized photograph of a conspecific. Sound (S) stimulations (calls) and movement (M) were combined to these visual (V) models. In Exp.2, V models and S stimulations were presented either separately or in combination. Models consisted of a ♂ and of a ♀ life-sized photograph and S stimulations consisted of recorded calls of ♂♂ or of ♀♀ conspecifics. In Exp.1, a significant increase in responsiveness was found after addition of sound S to the V models, especially to the photographs. Adding S to V models resulted in a much higher increase in responsiveness than adding M to V models. An additive effect was also found in Exp.2: combined presentations of V and S increased significantly the number of responses and of responding subjects as compared to either V or S separate presentations. In both Exp.1 and Exp.2, the total number of responses was a little more than twice as high for combined presentation of V and S as it was for V separate presentations, or for S separate presentations (Exp.2). In Exp.2, a complete verification of Seitz's law of heterogeneous summation of the stimuli was attempted by a further comparison, demonstrating that the releasing effect of V and S presented in combination (simultaneously) was equivalent to the number of responses found after addition of the partial effects of V and S presented separately (successively).