Genetics of initial imprinting responses: selection, and heterosis
Domestic chicks were tested for initial response, approach, and stay-near tendencies during one 5-minute test exposure to a distant audio-visual 1mprinting apparatus. Significant interline differences demonstrated genetic variation.
A bidirectional selection program was initiated with time to respond as the selected trait. Approach and stay-near tendencies were measured as associated traits. Results through the F4 generation disclosed that in the fast response line the realized heritability of the selected trait was .32 and the correlated realized heritabilities for time to approach and for time spent near the apparatus were .36 and .32, respect1vely. In the slow response line selection was ineffective, and heritabilities for the three traits were zero. Genetic and phenotypic relationships among the traits were high in the fast response line, but genetic relationships in the slow response line could not be determined because of the zero heritabilities. The asymmetrical response to selection could not be explained by the usual causes, and it is hypothesized that responsiveness is a threshold trait.
Comparisons of 290 purebred chicks with 379 crossbred chicks for response, approach, and stay-near tendencies provided evidence of heterosis. This demonstration of non-additive effects implies fitness roles for these traits and supports the idea that they indicate imprinting tendencies.
Time of day tested, sex, hatch, mating type, developmental age, and 24-hr body we1ght were considered independent variables influencing the behavior traits. Multiple regressions showed that the initiation of responsiveness is not predictable. However, once a chick responded, its behavior was highly predictable; this supports the hypothesis from the selection experiment that responsiveness is a threshold trait.
Sensory modalities involved in satiating the behavior traits were differentiated by comparisons of non-handled chicks with those exposed to visual, tactile, and visual - plus-tacti1e stimuli just prior to testing. Visual stimulation alone increased responses and approaches, whereas tactile stimulation alone had no effect on these behaviors. Tactile-plus-visual stimulation just prior to testing inhibited response and approach tendencies. Eva1uation of the effects of prior socialization and handling indicated that isolation enhanced approach responses, and that prior handling had the opposite effect. An age and/or memory factor was introduced when chicks were handled in the light at times other than just prior to testing. Handling at 5 and 9 hours had no effect on the approach response, whereas handling at 13, 17. or 23 hours post-hatching decreased such responses. Results were explained on a drive satiation hypothesis.