Metamorphosis and adaptation in the behavior of developing organisms

Introduction to special issue: 'Life history and learning: how childhood, caregiving and old age shape cognition and culture in humans and other animals'

This special issue focuses on the relationship between life history and learning, especially during human evolution. 'Life history' refers to the developmental programme of an organism, including its period of immaturity, reproductive rate and timing, caregiving investment and longevity. Across many species an extended childhood and high caregiving investment appear to be correlated with particular kinds of plasticity and learning. Human life history is particularly distinctive; humans evolved an exceptionally long childhood and old age, and an unusually high level of caregiving investment, at the same time that they evolved distinctive capacities for cognition and culture. The contributors explore the relations between life history, plasticity and learning across a wide range of methods and populations, including theoretical and empirical work in biology, anthropology and developmental psychology. This article is part of the theme issue 'Life history and learning: how childhood, caregiving and old age shape cognition and culture in humans and other animals'.

Verbal Development, Behavioral Metamorphosis, and the Evolution of Language

Building on Skinner's theory of verbal behavior, research over the last few decades confirmed verbal speaker operants, added the role of the listener, added the identification of speaker and listener interaction between and within individuals, and identified verbal behavior developmental cusps. Meanwhile, comparative biology focused on how and why language evolved in Homo sapiens. Findings about differences in behavior that neurotypical children demonstrated in their verbal development, and even more so in research that identified and established missing verbal behavior cusps, suggested changes analogous to metamorphosis. These striking changes in stimulus control found in the onset of cusps from the preverbal to the fully verbal child led us to an expansion of the concept of metamorphosis from morphology to the domain of behavior. The major findings of this comparative perspective are presented here as they have led us from experimental analyses of verbal development to metamorphosis as complex verbal behavior transformation and finally to a novel hypothesis about the evolution of language based on the concepts and research described here. To our knowledge, this is the first formulation of verbal development as behavioral metamorphosis in the context of evolutionary developmental biology.

Early Adversity and the Neotenous Human Brain

Human brain development is optimized to learn from environmental cues. The protracted development of the cortex and its connections with subcortical targets has been argued to permit more opportunity for acquiring complex behaviors. This review uses the example of amygdala-medial prefrontal cortex circuitry development to illustrate a principle of human development-namely, that the extension of the brain's developmental timeline allows for the (species-expected) collaboration between child and parent in co-construction of the human brain. The neurobiology underlying affective learning capitalizes on this protracted timeline to develop a rich affective repertoire in adulthood. Humans are afforded this luxuriously slow development in part by the extended period of caregiving provided by parents, and parents aid in scaffolding the process of maturation during childhood. Just as adequate caregiving is a potent effector of brain development, so is adverse caregiving, which is the largest environmental risk factor for adult mental illness. There are large individual differences in neurobiological outcomes following caregiving adversity, indicating that these pathways are probabilistic, rather than deterministic, and prolonged plasticity in human brain development may also allow for subsequent amelioration by positive experiences. The extant research indicates that the development of mental health cannot be considered without consideration of children in the context of their families.

Development of behavioral responses to thermal challenges

Body temperature regulation involves the development of responses to cold and warm challenges. Matching our understanding of the development of body temperature regulation to warm challenges with that of cold challenges will enhance our understanding of the ontogeny of thermoregulation and reveal different adaptive specializations. Warm and cold thermoregulation are important processes, and they include direct thermal effects on offspring, as well as indirect effects on them, such as those imposed by thermally associated alterations of maternal behavior. The present paper is a selective review of the existing literature and a report of some new empirical data, aimed at processes of mammalian development, especially those affecting behavior. We briefly discuss the development of body temperature regulation in rats and mice, and thermal aspects of maternal behavior with emphasis on responses to high temperatures. The new data extend previous analyses of individual and group responses in developing rodents to warm and cool ambient temperatures. This literature not only reveals a variety of adaptive specializations during development, but it points to the earlier appearance in young mammals of abilities to combat heat loss, relative to protections from hyperthermia. These relative developmental delays in compensatory defenses to heating appear to render young mammals especially vulnerable to environmental warming. We describe cascading consequences of warming-effects that illustrate interactions across levels of physiological, neural, and behavioral development.

Impaired fear extinction in adolescent rodents: Behavioural and neural analyses

Despite adolescence being a developmental window of vulnerability, up until very recently there were surprisingly few studies on fear extinction during this period. Here we summarise the recent work in this area, focusing on the unique behavioural and neural characteristics of fear extinction in adolescent rodents, and humans where relevant. A prominent hypothesis posits that anxiety disorders peak during late childhood/adolescence due to the non-linear maturation of the fear inhibition neural circuitry. We discuss evidence that impaired extinction retention in adolescence is due to subregions of the medial prefrontal cortex and amygdala mediating fear inhibition being underactive while other subregions that mediate fear expression are overactive. We also review work on various interventions and surprising circumstances which enhance fear extinction in adolescence. This latter work revealed that the neural correlates of extinction in adolescence are different to that in younger and older animals even when extinction retention is not impaired. This growing body of work highlights that adolescence is a unique period of development for fear inhibition.

Forming competing fear learning and extinction memories in adolescence makes fear difficult to inhibit

Fear inhibition is markedly impaired in adolescent rodents and humans. The present experiments investigated whether this impairment is critically determined by the animal's age at the time of fear learning or their age at fear extinction. Male rats (n = 170) were tested for extinction retention after conditioning and extinction at different ages. We examined neural correlates of impaired extinction retention by detection of phosphorylated mitogen-activated protein kinase immunoreactivity (pMAPK-IR) in several brain regions. Unexpectedly, adolescent rats exhibited good extinction retention if fear was acquired before adolescence. Further, fear acquired in adolescence could be successfully extinguished in adulthood but not within adolescence. Adolescent rats did not show extinction-induced increases in pMAPK-IR in the medial prefrontal cortex or the basolateral amygdala, or a pattern of reduced caudal central amygdala pMAPK-IR, as was observed in juveniles. This dampened prefrontal and basolateral amygdala MAPK activation following extinction in adolescence occurred even when there was no impairment in extinction retention. In contrast, only adolescent animals that exhibited impaired extinction retention showed elevated pMAPK-IR in the posterior paraventricular thalamus. These data suggest that neither the animal's age at the time of fear acquisition or extinction determines whether impaired extinction retention is exhibited. Rather, it appears that forming competing fear conditioning and extinction memories in adolescence renders this a vulnerable developmental period in which fear is difficult to inhibit. Furthermore, even under conditions that promote good extinction, the neural correlates of extinction in adolescence are different than those recruited in animals of other ages.

Influence of ontogenetic age on the role of dentate granule neurons

New neurons are continuously produced in the adult dentate gyrus of the hippocampus, a key structure in learning and memory. It has been shown that adult neurogenesis is crucial for normal memory processing. However, it is not known whether neurons born during the developmental period and during adulthood support the same functions. Here, we demonstrate that neurons born in neonates (first postnatal week) are activated in different memory processes when they are mature compared to neurons born in adults. By imaging the activation of these two different neuron generations in the same rat and using the IEG Zif268 and Fos, we show that these neurons are involved in discriminating dissimilar contexts and spatial problem solving, respectively. These findings demonstrate that the ontogenetic stage during which neurons are generated is crucial for their function within the memory network.

Development and evolution of hidden regulators: Selective breeding for an infantile phenotype

Mother-infant separation in the rat has been used as an analytical tool to reveal biosocial processes underlying infant physiology and behavior. The same strategy has guided a project in which selective breeding for an infantile behavior has provided insights into how biological systems become recruited and integrated as expressions of temperamental affective responses. Two lines of rats (High and Low USV lines) were selectively bred based on rates of USV emission to maternal separation and isolation at postnatal day (P) 10. After many generations of breeding, the High and Low lines show widespread and distinctly different profiles of physiology and behavior in the first 3 weeks of life. Insights gained from longitudinal studies suggest that selection may work by reorganizing developmental processes, not just a given trait, over the postnatal period. As animal models, the lines have the potential to provide valuable tools for understanding developmental mechanisms underlying genetic and developmental risk for depression/anxiety syndromes in children and adults.

Análise motivacional, causal e funcional da brincadeira em duas espécies de roedores

A brincadeira é um fenômeno bastante comum em indivíduos jovens de diferentes espécies, principalmente mamíferos. O objetivo do presente artigo foi fazer uma revisão sobre pesquisas realizadas com roedores, mais especificamente hamsters dourados (Mesocricetus auratus) e ratos albinos (Rattus norvegicus). Esses animais apresentam os mais altos índices de complexidade de brincadeira e são os mais estudados em laboratório nessa área, entre os roedores. Com base nos artigos da literatura pesquisada, conclui-se que a brincadeira: a) é um sistema motivacional próprio que apresenta características que o diferencia de outros sistemas motivacionais; b) é modulada pelo sexo e idade do indivíduo e dos parceiros envolvidos na interação social; e c) o significado funcional pode estar relacionado com a preparação do indivíduo para viver em ambientes sociais e treinamento físico. Nesse sentido, a brincadeira pode ser relevante para se compreender aspectos do desenvolvimento comportamental e social.

Ontogenetic adaptation and learning: a developmental constraint in learning for a thermal reinforcer

Hoffman, Flory, and Alberts (1999) demonstrated that 1-, 5-, and 11-day-old rats in a cool environment (25 degrees C) acquired an operant head-turning response when rewarded with a 20-s warming of the platform on which they lay. In the current experiment 5- and 11-day-old rats in a hot environment (40 degrees C) acquired the head-turning response when rewarded with a 20-s cooling of the platform on which they lay, but 1-day-olds did not. The concept of ontogenetic adaptation helps us interpret these results: Neonatal thermotaxis constrains the 1-day-olds from learning a novel operant response for a cool reinforcer in a hot environment. Because the thermotaxis wanes from birth, it is not as strong in 5- and 11-day-old pups that are thus able to learn the operant for a cool reinforcer.

Neonatal thermotaxis improves reversal of a thermally reinforced operant response

One-, 5-, and 11-day-old rats in a cool environment (25 degrees C) acquired an operant response when rewarded with a 20-s-long warming of the platform (from 25 to 36 degrees C) on which they lay. In Experiment 1, the head-turning response was learned by pups at all ages. When the contingency was reversed so that pups were reinforced for turning to the side opposite that correct during training, the original response extinguished for 1-day-olds, but not for 5- or 11-day-olds. In Experiment 2, the rewarded side was randomly selected for each trial. One-day-olds perseverated in turning to the side correct on that trial while the reinforcer remained on, but 5- and 11-day-old rat pups did not. We conclude that 1-day-old pups were more responsive to the change in experimental contingency in Experiment 1 due to this thermotaxic behavior.

Experience during suckling alters later spatial learning

These experiments explore the role of preweaning experience in learning during the juvenile period. Pups that had been reared with many nipples available reached criterion on an 8-arm radial maze in a few trials; conversely, pups reared with only a few nipples required 3 times the number of trials to reach criterion (Experiment 1). Pups that had been reared with relatively few nipples available rarely nipple-shifted, while those that had been reared with a particularly high density of nipples shifted more frequently (Expt 2). A rearing procedure was devised that allowed precise experimental control of all phases of the suckling experience (Expt 3). Allowing or preventing a single behavior, nipple-shifting, while holding all other variables constant, was sufficient to affect acquisition of the maze task. In Experiment 4, the specificity of the early experience for later tasks was explored using a variety of nonspatial, lever-pressing operants. Rearing condition did not affect acquisition of a lever-pressing operant or of a visual discrimination task. However, pups reared with a high density of nipples responded at higher rates to a variable interval schedule and were more resistant to extinction. The possibility that strategy, rather than learning ability, was affected by rearing condition was assessed using a 2-arm maze task that was structured to present an optimal strategy of either win-shift or win-stay (Expt 5). The ease with which rats acquired the win-stay task was unaffected by rearing condition; all groups performed at about chance levels. However, pups reared with many nipples more readily acquired the win-shift task.

Development of maternal behaviors in prepubertal rats at three ages: age-characteristic patterns of responses

Individual 18-, 24-, or 30-day-old male and female Wistar prepubertal rats (juveniles) were continuously exposed to 3-8-day old pups in order to determine developmental differences in the expression of maternal and other behaviors during the process of sensitization. The results that emerged from this study are as follows: Maternal behaviors are neither consistent nor integrated in their expression prepubertally; exposure to pups facilitates the emergence of some maternal behaviors in juveniles more easily than others, e.g., retrieving and anogenital licking more than ventral crouching or nestbuilding; there are age-related differences in the expression of different maternal behaviors, e.g., 24-day-olds retrieve more than at other ages; in our strain, few, if any, gender differences are apparent in the expression of maternal behaviors prepubertally; juvenile play behaviors (charge, pounce) are related to retrieving behavior in the context of exposure to pups. Overall, the results suggest that maternal behaviors are distinct developmentally and probably involve different mechanisms in prepubertal animals.

A specialization of taste aversion learning during suckling and its weaning-associated transformation

During ontogenesis, altricial mammals advance through different ecological niches which require correspondingly different adaptive strategies. We view learning as an important feature of ontogenetic adaptations and consider the acquisition and expression of taste aversion learning during nursing within this framework. Fifteen-day-old preweanling rat pups fail to acquire conditioned taste aversions while nursing, whereas 20-day-old weanlings readily learn flavor aversions while suckling (Martin & Alberts, 1979). The developmental emergence of taste aversion learning while nursing coincides with reduction in maternal milk supply and the rat pup's transition to a solid food diet. We considered the ontogenetic transformation of learning while nursing as a feeding-related adaptive strategy of the weanling rat and tested two predictions derived from this view: (1) delayed weaning might postpone acquisition or expression of suckling-related taste aversions and (2) conditions that permit weaning (e.g., access to alternate food sources) might induce acquisition or expression of such taste aversions. Both predictions were confirmed.

Periadolescence: age-dependent behavior and psychopharmacological responsivity in rats

The behavior and psychopharmacological sensitivity of periadolescent rats are examined in this review. Periadolescent rats are hyperactive and engage in more conspecific play behavior than younger or older rats. When compared with other-aged rats, periadolescents exhibit enhanced performance in simple active-avoidance learning tasks, but perform poorly in more complex appetitive and avoidance learning tasks in which increases in locomotor activity do not improve performance, perhaps as a result of age-specific alterations in selective attention or stimulus processing. Such behavioral "anomalies" of periadolescent animals observed in traditional laboratory situations may be in some way adaptive when considered in the context of the animals' natural habitat. In terms of psychopharmacological responsiveness, periadolescent rats, when compared with younger or older animals, are less sensitive to catecholaminergic agonists but are more responsive to the catecholaminergic antagonist haloperidol. This pattern of psychopharmacological sensitivity suggests that the catecholaminergic systems may be temporarily hyposensitive during the periadolescent period. Evidence is presented that a negative feedback system in the form of dopamine autoreceptors may become functionally mature in mesolimbic brain regions during the periadolescent period. The possibility is presented that maturation of these self-inhibitory autoreceptors might result in a temporary decrease in the efficacy of mesolimbic dopamine projections, perhaps contributing to the psychopharmacological and behavioral characteristics of periadolescent animals. In support of this suggestion, evidence is reviewed indicating that the behavior of adult animals with lesions of the ventral tegmental area, a region containing cell bodies from which these mesolimbic dopaminergic projections originate, resembles that of periadolescent rats.

The development of early non-verbal communication: an evaluation of its meaning

Communication and social preadaptation have interpretative problems in early interaction. Both have strong teleological commitment and are not therefore wholly appropriate. The mother-infant system gradually achieves the hallmark of true teleology largely because the mother acts as if the newborn's foetal-like behaviour had communicative significance. Viewing newborn behaviour as such is compatible with the biological principle of preadaptation which avoids vague assumptions about early behaviour being socially 'prewired'. It is recommended that the mother be given an observer's role as it may lead to a useful methodology for understanding early communication. Studying deviancy in early relationships is also considered.

Environmental effects on the ontogeny of exploratory and escape behaviors of Mongolian gerbils

Mongolian gerbils reared in standard laboratory cages and gerbils reared in cages containing a burrowlike shelter did not differ in their rate of development of escape responses to sudden visual stimulation. The presence of shelter in the rearing environment did, however, markedly slow the development of adult-like patterns of exploration. Reduced exposure to illumination, experienced by shelter-reared subjects, proved responsible for this retardation of development. The effect of shelter-rearing on the development of exploratory behavior is interpreted as demonstrating an environmentally induced maintenance of an adaptive juvenile pattern of behavior.

The lactation-blocking drug bromocriptine and its application to studies of weaning and behavioral development

An experimental method for blocking maternal lactation is reviewed and the possible application of this technique for experimentally manipulating weaning is considered. Maternal milk production can be inhibited using the prolactin-suppressing drug bromocriptine. The suitability of bromocriptine for use in behavioural experiments is considered. The pharmacology of bromocriptine (CB 154) is briefly outlined and a compilation of the reported lactation-inhibiting doses for various species is presented. The possible endocrine and behavioral side-effects and the toxicity of the drug are discussed. It is concluded that, in most species studied so far, the drug is relatively free from significant side-effects at the low doses needed to suppress lactation. Guidelines for the practical use of bromocriptine are suggested. Finally, some ideas about the possible application of the drug to the study of behavioral development and parent-offspring relationships are discussed.

A fit mouse is a hoppy mouse: jumping behavior in 15-day-old Mus musculus

An analysis of a juvenile hopping response from an 8 X 8 diallel cross is used to demonstrate the experimental genetic approach for testing presumed adaptive fitness of behaviors in developing organisms. In accordance with predictions, the explosive jumping behavior exhibited by 15-day mice is characterized by a pattern of genetic dominance toward high expression of the trait. Wild mice show even more vigorous responses, indicating that selection pressures maintaining high responding have been relaxed during domestication. These data suggest some applications and limitations of genetic methods for the study of behavioral evolution as related to development.