On the incongruity between developmental plasticity and methodological rigidity

LIVING ORGANISMS ADJUST THEIR PHENOTYPES ACCORDING TO ENVIRONMENTAL INFLUENCES One thousand nine hundred thirty six: this is the number of citations retrieved in “pubmed” using the search terms: “developmental plasticity rodent. ” The “results by year” graphical trend, automatically plotted, indicates that this number has been linearly increasing over the last 30 years. Therefore, developmental plasticity, the range of different phenotypes potentially branching from an identical genotype (West-Eberard, 2003), is not a novel concept (Weininger et al., 1954). The notion that environmental factors modulate individual maturation, specifically during highly plastic developmental stages, is also not novel. Disciplines like psychology and evolutionary ecology devised theoretical and practical tools to understand the link between experiential factors and phenotypic adjustments. Whilst Freud proposed that adult neuroses build upon infantile experiences (Freud, 1918), contemporary authors demonstrated that several psychiatric disorders often root in early childhood, when abuse and/or neglect may increase individual vulnerability to depression (Heim and Nemeroff, 1999, 2001). These studies attempted to define the potentially disruptive nature of severe developmental stress. Complementary to them, other studies demonstrated that stress during development may represent a constructive force: specifically, moderate precocious environmental challenges have been proposed to favor resilience (Lyons and Macri, 2011), i.e., program the organism to handle repeated stressors in a more efficient way. Studies conducted in rodents (Macri et al., 2011), birds (Henriksen et al., 2011), primates (Parker et al., 2006; Parker and Maestripieri, 2011), and humans (DiCorcia and Tronick, 2011; Flinn et al., 2011; Seery, 2011), demonstrate that precocious exposure to mild stress (being briefly separated from dams during lactation, exposed to low doses of stress hormones, or reared to mothers requested to seek for food instead of being allowed unlimited foraging) promotes resilience. At the same time that behavioral neuroscientists started disclosing the inextricable link between developing organisms and their environments, ethologists and evolutionary ecologists attempted to understand the functional meaning of these processes. Bateson and colleagues discussed the representative example of the freshwater crustacean Daphnia (Bateson et al., 2004). Offspring of this species develop a protective “helmet,” reducing the odds of being predated, if their mothers were exposed to a predator odor. Yet, the energetic costs associated with helmet patterning reduce individual competitive success in a predatorfree environment. Thus, the success of each phenotype is dictated by the presence or absence of predators and, ultimately, by the correspondence (match) between neonatal forecasting and adult life conditions. A directional phenotypic adjustment in conformity with developmental cues has also been observed in rodents (Sachser, 1993; Sachser et al., 1994; Liu et al., 1997) and humans (Hales and Barker, 2001; Wells, 2007, 2011). The concept of resilience should be integrated within this theoretical framework, whereby precocious challenges may forecast an adult environment characterized by the presence of multiple stressors, to which the individual phenotype is accordingly tuned. Maladaptive or pathological outcomes may occur under several circumstances, among which the following two are of particular interest: (1) external challenges are too elevated to permit adaptive processes thereby exceeding individual adaptive capacities (Sultan, 2003); (2) developmental experiences do not provide a reliable indication of the challenges to be encountered later in life (phenotypic mismatch). Along with the observation that experiential factors adjust individual development, so also the fundamental underlying mechanisms started being detailed. To investigate these mechanisms, laboratory rodents have often constituted the methodology of choice. For example, several studies demonstrated that being reared to a careful rat mother favored adult resilience through a non-genomic mother-offspring transfer mechanism (Francis et al., 1999). Specifically, increased adult resilience was shown to depend on maternally mediated epigenetic regulations at the level of DNA methylation (Weaver et al., 2004). Ultimately, laboratory animals constitute the cornerstone against which developmental plasticity is demonstrated and dissected. I therefore find it quite ironic that such plasticity tends to be contrasted when it comes to using laboratory rodents as experimental subjects. Such contrast becomes particularly evident when current housing and breeding standards are considered. Thus, either in the case of conventional or enriched housing there exists a strong strive to equate living conditions across different facilities. Such strive is theoretically justified by the need to minimize and equalize environmental sources