The Posthatching Consequences of Prehatching Stimulation: Changes With Amount of Prehatching and Posthatching Exposure

Abstract

AbstractAn experiment was designed to assess posthatching effects on the chick's response to the parental alarm call, of different amounts of prehatching experience of the same sound stimulus. Four groups of embryos were given 10, 30, 60 or 120 stimulation trials, a trial consisting of a 51/2 second burst of alarm calls and 25 seconds of silence. After hatching, a group given no prehatching stimulation and all stimulated groups were given 30 test trials. All chicks were tested separately. Response indices used differed; before hatching, heart rate changes were used for this purpose,and after hatching, the number of trials on which the eyes were opened and also vocalisations. Vocalisations were divided into three categories: trills, twitters and cheeps. Prehatching results showed that the heart rate change diminished in successive stimulation trials, but after about the fortieth trial it remained fairly stable, at a low level. Posthatching results showed the effect of prehatching habituation. According to the "eyes open" index the response was maximal at the onset of testing and fell off gradually. It differed very little between stimulated groups, but chicks which heard the sound stimulus for the first time after hatching responded more than those which had heard it while still in the egg. According to the number of calls, the posthatching response was affected by different amounts of pre- as well as by post-hatching experience. In naive chicks it fell at first from the control level and then rose and fell with repetition; in chicks with pre-hatching experience of the sound the rise came earlier. In all cases there was a rise to a peak and then a fall in the number of vocalisations with increased experience, whether this experience was before or after hatching. The increase in calling was attributed to a waning of the suppressing effect of the sound stimulus and the fall to waning of the effect of the sound stimulus itself. This post-hatching waning of the response occurred most rapidly in birds given the most prehatching experience and most slowly in birds which had no prehatching experience of the sound, a difference which was manifested only after the first five minutes of post-hatching testing.