The Physiological Basis of Parental Feeding Behavior in the Ring Dove (Streptopelia Risoria)

Ring doves feed their squabs by regurgitation of "crop milk", a mass of desquamated cells from the crop epithelium. The reproduction and sloughing off of the cells is elicited by prolactin secreted by the bird's hypophysis. The purpose of the present study is to determine the role of prolactin in the organization of regurgitation-feeding, and to analyze the mechanism of its action. 12 adult ring doves, each of which had bred twice (in six pairs), were injected subcutaneously with 450 I. U. of prolactin, dissolved in distilled water, divided into 7 daily injections. Control birds were injected similarly with distilled water. Each bird was in an individual cage. On the day following the last injection, a squab was placed in the cage, in a normally-located nest. Of the 12 prolactin-injected birds, 10 fed the squabs; of the 12 control birds, none fed the squabs. (x 2 = 13.89, p = .001). In a second experiment, 12 adult ring doves, of the same age as those used in the first experiment, but without breeding experience, were tested by an identical procedure. The control gropp consisted of 12 birds, also with no breeding experience, injected with distilled water. None of the birds in either the experimental or the control group fed the squabs. (For a comparison of the inexperienced birds with the experienced birds of the first experiment, both injected with prolactin, x 2 = 13.89, p = .001.) In a third experiment, 12 adult ring doves, each of which had bred twice, were injected with prolactin and tested as in the first experiment, except that, starting 2 days before the test, their crops were anesthetized with Efocaine, a long-acting local anesthetic. A control group was injected with the same amount (.5 cc) of Efocaine elsewhere in the body, in order to control for possible systemic effects of the anesthetic. The crop-anesthetized birds fed the squabs significantly less often than did the birds anesthetized elsewhere (p. = .05). The results strongly suggest that prolactin elicits parental regurgitation-feeding in these circumstances (squab presented to adult when adult was not already sitting on the nest—unlike the natural situation) only if the adult bird has already had experience of feeding squabs. The prevention of the prolactin-induced response to the squab in a majority of birds by anesthetization of the crop suggests that the prolactin does not elicit the parental response primarily by its action on a brain center, but by its action on the crop. Additional data bear on the role of learning in the development of regurgitation-feeding, and on the origin of the parent's responses to visual and/or auditory stimulation by the squab: The first regurgitation of food to the squab after it emerges from the egg may follow tactual stimulation of the breast by the squab's head, or it may be at the initiative of the parent, apparently in response to non-tactual stimulation from the squab. In 41 cases of birds breeding for the first time, regurgitation was always to tactual stimulation. In 49 cases of birds breeding for the second time (this sample independent of the sample of 41 inexperienced birds), regurgitation occurred without tactual stimulation 12 times. x 2 for a comparison between birds breeding for the first and for the second time is 9.56, p = .01. By the time squabs are 6 days old, there is no difference in responsiveness to non-tactual stimulation between parents breeding for the first and for the second time (x 2 = 1.08, p = .3). Observations of the latency of regurgitation response (time from hatching of egg to first feeding of squab) of birds breeding for the first time, and of the same birds when breeding for the second time, show a reliable decrease in the latency between first and second breedings (p = .01). These data indicate that the regurgitation response of parents to the squabs, in normal breeding, is probably at first to tactual stimulation alone, but quickly becomes conditioned to visual and auditory stimuli. In addition, it appears that learning causes marked changes in the mode of first response to the squab between first and second breeding experiences. Conclusions. Prolactin acts to elicit parental regurgitation-feeding primarily because it causes engorgement of the crop and suppression of sexual behavior, rather than through an effect on central nervous mechanisms specific for parental behavior. The engorgement of the crop makes it sensitive to emetic stimuli provided by movements of the squab's head against the parent's breast. In addition, this crop-engorgement acts as a drive stimulus through which the dove learns to respond to the sight and/or sound of the squab.