Early Imprinting Research Articles

Maternal Behavior

Parturition in mares is rapid and is followed by a brief period of sensitivity to imprinting on a foal. There is large individual variation in normal maternal style, but normal mothers actively defend their foal, remain near the foal when it is sleeping, tolerate or assist nursing, and do not injure their own foal. Disturbance of a mare and foal during the early imprinting period can predispose a mare to rejection of her foal; therefore, it should be avoided. There are a variety of forms of foal rejection and numerous etiologies. Therefore, each case should be evaluated individually.

Imprinting of hepatic estrogen-binding proteins by neonatal androgens.

Previous studies discovered a second class of estrogen-binding proteins distinct from estrogen receptor which exhibited higher capacity, lower affinity (HCLA) binding properties. HCLA sites underwent postpubertal sex differentiation, such that adult male levels were at least 10-fold higher than adult female levels. Neonatal castration of male rats prevented the subsequent sex differentiation of HCLA binding sites; adult male rats that were castrated neonatally exhibited typically female concentrations of these binding sites. If male rats were castrated at 19 days of age or later, postpubertal sex differentiation of HCLA binding sites proceeded as observed for intact males. Administration of testosterone propionate (TP) to castrated (neonatally) males during a critical period (days 6-13) imprinted for the subsequent sex differentiation of HCLA sites, whereas TP administration at other times did not. The expression of these imprinted sites was not manifested until puberty, as neonatal manipulation or TP treatment had no effect on HCLA sites in immature rats. The imprinted effect on HCLA binding sites appeared to be permanent and irreversible. Treatment of castrate males with diethylstilbestrol (DES) or zearalenol (P-1496) during the critical period was incapable of restoring development of normal male levels of HCLA sites. Competitive binding studies using several steroid hormones revealed similar data for intact males and castrate (neonatal) male rats that had received TP during the critical period. These studies demonstrate an early imprinting period during which androgen exposure programs for postpubertal development of sex differentiation of HCLA binding sites.

Human assortative mating and genetic equilibrium: An evolutionary perspective

Assortative mating in human and other animal species is reviewed. Assortive mating is most often positive (between individuals sharing phenotypic similarity), occurs on a variety of normal and abnormal traits, strengthens the mating bond, and increases fertility. These results can best be understood if it is assumed that natural selection operates to increase genetic homology between mates and between parents and off-spring. The outcome of selection for positive assortment is to increase the genetic relatedness among family members, thus facilitating communication and altruism and increasing inclusive fitness without an additional reproductive effort. The opposing evolutionary vector is to restrict excessive homozygosity and consequent inbreeding deppression by minimizing matings between individuals of greatest similarity (e.g., members of nuclear families). The result of these opposing evolutionary vectors is a negative relationship between the degree of positive assortative mating and heritability ( h 2): individuals gravitate toward those of similar but not identical phenotypic (and genetic) similarity. Individuals may assess their own and their potential mates' homozygosity and mate so as to maximize genetic homogamy and still avoid excessive inbreeding. Early imprinting and learning within the family unit act as the proximate mechanisms to establish the criteria for optimal mate selection. Predictions that follow from this model are advanced, and several problems are discussed. Laboratory and field work with animals will be especially valuable in advancing our understanding of assortative mating.

Early imprinting in wild and game-farm mallards (Anas platyrhynchos): Genotype and arousal.

Early imprinting in wild and game-farm mallards (Anas platyrhynchos): Genotype and arousal.

Embryonic Pituitary Adrenal Axis, Behavior Development and Domestication in Birds

ACTH and corticosterone exert opposite effects on the approach and imprinting behavior of newly hatched ducklings. Wild mallard and domesticated Pekin ducklings differ in the early posthatch period in both plasma corticosterone levels and approach/avoidance behaviors. Injection of Pekin duckling embryos with pituitary—adrenocortical hormones alters both later adrenal function and certain aspects of posthatch behavior. These birds have behavioral and hormonal characteristics which resemble those of wild mallards. The hypothesis that behavioral differences in wild and domesticated ducklings result from a higher level of pituitary adrenal function in the wild embryo is explored. Although adrenocortical function changes during domestication in many species, evidence that the hormonal changes mediate the concomitant changes in approach and avoidance behavior remains inconclusive. Factors which cause adrenal function and early behaviors to differ in wild and domesticated genotypes must be sought in the gene action during embryonic development. Since imprinting behavior is modulated by pituitary—adrenal hormones, any factor which affects post—hatch adrenal function may potentially affect imprinting. Later behavior development in the adult is strongly dependent on neonatal experiences; and, therefore, hormonal modulation of early imprinting behavior may constitute an important determinant of adult social behavior.

II. REMARKS ON RECEIVING THE C. ANDERSON ALDRICH AWARD

The C. Anderson Aldrich Award, when given to a man so late in life for what he has contributed so much earlier, unavoidably awakes reminiscences. Let me, therefore, first devote a few words to the most important pediatrician in my life. He looked after my health when I was 3 years old and then entered my life more decisively by marrying my widowed mother. We moved into his house. In those days, of course, a pediatrician's office was attached to his home, and for years when I came home from school in the afternoon, I had to make my way through groups of worried mothers and sick children who sat in the entrance hall as well as in the waiting room. I did not become a pediatrician proper, so to speak, but I eventually was trained in child analysis. Beyond this, in all my thinking, the world for me has always been full of mothers and children which seemed to appeal for responsible help in saving not only lives, but also the vitality and integrity of living. If your award attests to the fact that we share this responsibility, it is also, in my case, a tribute to the power of early professional imprinting. Now it so happens that to my surprise and joy the second most important pediatrician in my life is right here today and is, in fact, last year's recipient of the Aldrich Award. Our acquaintance goes way back, to what now seem prehistoric days. But I felt closest to Ben Spock during some years in Pittsburgh when he permitted me, a regular visitor to the staff, to share on occasion his well baby clinic at the Arsenal Health Center.

Early Arousal and Imprinting in Chicks

Arousal outside the test situation and prior to response acquisition can affect strength of imprinting, if such treatment occurs early enough in life. Young chicks handled in darkness either at 5 or 9 hours of age and exposed to a moving surrogate a number of hours later were compated with nonhandled controls by means of following tests given at 30 and 54 hours of age. Handling at 5 hours resulted in a significant increment in the later following response. Handling at 9 hours produced no effect.