Precocious adult behaviour in the young chick

Abstract

Summary If a domestic fowl chick is excited by repeated thrusts towards it of a solid object, it will usually attempt to leap on top of the object. If then the object is suddenly lowered so that it is easy to mount, a male chick will often mount and go through a full copulatory sequence. This may be observed as early as the second day; on the average 20–30 per cent males copulated during the first 16 days of life in each experimental group. Female chicks copulated but very much less frequently. A second diet, used to control for possible exogenous steroids, had no effect on incidence of copulation, and comb size showed no correlation with copulation. A second strain of fowl (gamecock), and two species of pheasant, also showed juvenile copulation, whilst guinea fowl gave intense juvenile titbitting. The juvenile copulation of the main experimental strain is thus almost certainly a normal, though usually latent, part of the repertoire of chicks of the domestic fowl and other game birds. The frequency of male copulators was not significantly affected by differences in early experience chosen as likely to affect imprinting on a moving object during the ‘critical period’. The main treatments were exposure to a moving hand for 20 or 40 min during the first 2 days of life, exposure to other chicks for similar periods, isolation from all moving objects for the first 2, 4 or 8 days of life, and living with other chicks for the first or the first 2 days of life. The chief significant effect was an increase in the percentage of copulators, particularly in the first days of testing, due to the last treatment. A common mechanism for juvenile titbitting and social facilitation of feeding is discussed. Early experience with a moving object is thus not necessary for the later evocation of copulation, nor does the imprinting of following at once cause the attachment of copulation. However, some feature of prolonged experience of other chicks in the first 2 days of life does facilitate copulation over the next 2 weeks. The discovery of juvenile copulation suggests a mechanism for sexual imprinting. Early experience of an imprinting object could affect the adult choice of an effective stimulus object for the evocation of copulation, because the early experience involved partial or complete activation of the neural substrate for copulation. Testosterone greatly increased the percentage of copulators in the present test situation, in marked contrast to its relative lack of effect in situations in which only components of copulation and aggressive behaviour are elicited. It had no effect on the incidence of copulation in female chicks. Nor did it affect the incidence of ‘juvenile titbitting’ (excited pecking and carrying of elongated objects) in either sex. Possible deductions from these results are discussed.