Laboratory-housed rats live in highly controlled environments with limited engagement opportunities, leading to predictable, sedentary behavior (Makowska and Weary, 2016). In contrast, wild rats face continuous high-risk challenges, requiring heightened vigilance to detect and respond to environmental threats such as predators, toxins, and other hazards (Koizumi et al., 2018; Lambert, 2023; Clinchy et al., 2013). By comparing selectively-bred laboratory rats to their wild Rattus norvegicus counterparts, we can gain a deeper understanding of how animals maintain vigilance in response to the unpredictable threats encountered in natural habitats.

In the mid-20^th century, neuroscience pioneer Curt Richter compared adrenal gland weights of wild rats to laboratory R. norvegicus, reporting that the wild rats’ adrenal glands were approximately 300% larger than those of laboratory populations (Richter, 1949). This finding was recently replicated when our lab reported that the adrenal glands of urban wild-trapped R. norvegicus were 320% larger than weight and sex-matched laboratory-bred rats, accompanied by a 600% increase in fecal corticosterone (CORT) metabolites (FCMs; Jacob et al., 2022). These observations suggest that heightened hypothalamic-pituitary-adrenal (HPA) axis activity in wild rats is a neurobiological correlate of enhanced vigilance, defined as sustained attention accompanied by tonic alertness (Oken et al., 2006). An understanding of the neurobiological correlates of prolonged threat assessment in wild habitats is informative, considering that survival in the wild depends on appropriate levels of vigilance (Karaer et al., 2023).

In addition to reported adrenal gland differences between wild and domestic rats, brain structures involved in survival-related behavior also appear to differ between wild and laboratory rats. For example, larger amygdala volumes have been reported in wild rats and rabbits compared to domestic species (Koizumi et al., 2018; Brusini et al., 2018); further, in animals exposed to chronic stress, neural atrophy has been observed in brain areas such as the hippocampus (McEwen et al., 2016). Despite the documented association between chronic stress and brain atrophy in specific brain structures, wild rats in our previous research exhibited relatively larger brain volumes and increased neuronal density in the cerebellum, a brain area that is important for movement adaptation (Jacob et al., 2022). Together, these findings point to the existence of significant neurobiological adaptations to heightened vigilance in wild rats (Carrilho et al., 2024; Koizumi et al., 2018). Interestingly, a recent study reported smaller cerebella and comparable numbers of Purkinje neurons in wild R. norvegicus compared to laboratory strains; however, the wild rats were F3 generations from wild-trapped rats, different than wild-trapped rats (Waner-Mariquito et al., 2025). This discrepancy suggests that, whereas domestication may have an impact on rat cerebella volume, exposure to wild habitats throughout the rats’ development is also an influential factor (Jacob et al., 2022).

Neuroendocrine adaptations to chronically stressful environments may be driven by key molecular mechanisms such as mineralocorticoid receptor (MR) activity and expression. Whereas glucocorticoid receptors (GRs) are located throughout the brain, MRs are localized in the limbic areas, especially the hippocampus and amygdala (ter Heegde et al., 2015). Within the hippocampus, the highest MR concentration is in the CA2 region, an area associated with plasticity functions, especially related to memory, social recognition, and coping appraisal (McCann et al, 2021; Joëls and de Kloet, 2017). MRs are colocalized with GRs in the hippocampus but have a higher affinity (i.e., 10 times higher) to glucocorticoids than observed in GRs (de Kloet, 2022). Additionally, higher levels of glucocorticoids have been associated with heightened behavioral vigilance (Voellmy et al., 2014), as well as neural vigilance (i.e., neural reactivity; Henckens et al., 2016), in response to biologically relevant threatening stimuli.

Further, MR expression affects the transcription of genes related to neuroplasticity, including growth factors and cell-adhesion molecules (ter Heegde et al., 2015). Thus, past research suggests that MR engagement is involved in both the initial appraisal of stressful stimuli and subsequent memory systems of the experience, likely influencing future responses to similar stressful contexts (McCann et al., 2021). In clinical investigations, lower MR activation and the putative effect on stress appraisal and coping are associated with psychiatric conditions such as depression, anxiety disorders, and schizophrenia (ter Heegde et al., 2015). In support of these observations, antidepressants such as imipramine have been shown to increase hippocampal MR mRNA expression in the rat hippocampus (Brady et al., 1991, Seckl and Fink, 1992).

Also of interest in understanding stress adaptation in chronically stressful wild habitats is the steroid hormone dehydroepiandrosterone (DHEA). DHEA and its sulfate (DHEA-S) are produced in the adrenal glands of humans and the gonads of rats and mice (Maninger et al., 2009). Additionally, DHEA is produced in the brain and has been found to modulate glutamate [i.e., N-methyl-D-aspartate (NMDA)] and Gamma-Aminobutyric Acid (GABA) neurochemical systems (Maninger et al., 2009). Generally, DHEA is known as a marker of acute stress with additional anti-glucocorticoid properties (Dutheil et al., 2021). Further, DHEA levels are inversely related to the intensity of depressive episodes and may have clinical relevance for other psychiatric and neurologic conditions such as anxiety disorders and dementia (Peixoto et al., 2020; Sripada et al., 2013; Rasmusson et al., 2004; Souza-Teodoro et al., 2024), as well as providing neuroregenerative effects following brain injuries (Rahmani et al., 2013). Consequently, both MR and DHEA functions have been associated with emotional resilience in the face of prolonged stress exposure.

Enriched laboratory environments, consisting of larger physical spaces, social housing, and the introduction of novel stimuli, result in neural and behavioral adaptations that appear consistent with rodents housed in natural habitats (Lambert et al., 2015; Lambert, 2023). For example, rats housed in complex environments have thicker cortices, increased hippocampal neuroplasticity markers, and more complex neuronal structures than laboratory control animals (Mohammed et al., 2002; Han et al., 2022). Additionally, in models of cerebral stroke, animals housed in enriched environments exhibit increased angiogenesis and faster recovery rates (Gresita et al., 2022; Shen et al., 2020). Thus, microvasculature changes related to varying experiences or environments can lead to impactful outcomes such as altered cerebrovascular architecture and vascular pruning (Bogorad et al., 2019).

Although enriched laboratory environments mimic some aspects of natural habitats, they lack the persistent exposure to unpredictable stressors that wild rats face, which may drive more robust neurobiological adaptations. Even natural-enriched laboratory environments that contain elements of species-relevant environmental stimuli fall short of replicating the continuous presence of threats that characterize authentic natural habitats (Bardi et al., 2016). Additionally, in contrast to the constant provision of food, water, shelter, and temperature regulation in the laboratory, these variables are scarce and unpredictable for wild rats. To survive such threats and challenges, wild rats must accurately assess risk and employ appropriate coping and decision-making strategies to respond in a manner that maximizes the probability of survival. Accordingly, heightened sensitivity in sensory and motor cortical areas likely facilitates responsiveness to such real-time threats in wild habitats, leading to enhanced survival (Krubitzer et al., 2011). Also of putative relevance, prior research with varying experiential training suggests that the lateral habenula (LHb), an epithalamic brain structure, is involved in coping appraisal as it facilitates freeze responses in threatening contexts (Hones and Mizumori, 2022). Serving as an interface between internal emotional processes and external context, the LHb facilitates rapid and efficient responses to threatening stimuli (Baker et al., 2022). Thus, in addition to limbic structures such as the hippocampus and amygdala (Koizumi et al., 2018; Zanette et al., 2019), several brain areas are involved in decision-making and stress responses—responses that shape vigilance systems and, ultimately, survival, in wild animals.

Due to their apparent tolerance of high corticosterone levels, wild rats provide a potentially valuable model for the investigation of stress adaptation mechanisms during long-term vigilant states. As previously stated, although high HPA activity is generally related to negative brain and health outcomes (Bremner, 2006; McEwen et al., 2016; Bremner and Wittbrodt, 2020), previous research in our lab indicates that wild rats have healthy brain weights and cerebellar neuronal density, with no evidence of compromised tissue thus far (Jacob et al., 2022). Given these findings, the current study further investigated the neural substrates of vigilance and resilience in wild rats. Specifically, male and female wild and laboratory-matched rats were evaluated to determine the impact of their respective habitats on the following targeted mechanisms of neural vigilance and resilience: (1) hippocampal MR and GR immunoreactivity, (2) limbic and cortical microvasculature density, and (3) neuronal/glial ratios in cortical areas important for responsivity to stressful stimuli. Additionally, markers of HPA activity were assessed, including CORT and DHEA fecal metabolites, as well as relative adrenal weights. Due to their role in both stress and immune functions, spleen weights were also collected. Based on prior findings, it was hypothesized that the wild rats would have higher levels of MR-immunoreactive (MR-ir), region-specific modifications of GR-immunoreactive (GR-ir), increased microvasculature coverage (MVC), and evidence of altered neuronal and glial cell density in targeted cortical areas associated with responses to external threats (e.g., motor, somatosensory, pyriform, anterior cingulate, and auditory cortical areas), as well as modifications in the bed nucleus of the stria terminalis (BNST), LHb, and basolateral amygdala (BLA). Spleens and adrenal glands were hypothesized to be larger in wild rats due to increased antigen and threat exposure. An enhanced understanding of these mechanisms in wild rats offers critical insights into stress adaptation that laboratory models alone cannot provide.

The goal of this study was to explore neurobiological variables associated with vigilance and emotional resilience in wild R. norvegicus to provide insights into survival-related responses that laboratory models alone cannot fully capture. To differentiate these two concepts, we viewed vigilance as an index of sensory, cortical, and habenular neural adaptations, while resilience was assessed via stress hormone profiles and receptor distributions. The findings replicate and extend earlier reports from our laboratory indicating heightened stress reactivity in wild rats compared to laboratory-bred rodents (Jacob et al., 2022). Specifically, our previous findings indicated that wild rats exhibit a marked increase in the fecal metabolites of CORT and DHEA, alongside heavier adrenal and spleen weights. In the present study, wild rats likewise exhibited significantly higher FCMs than laboratory rats, as well as heavier adrenal and spleen weights. Due to the modest sample sizes (a challenge often associated with field studies utilizing trapped animals), findings that did not meet adjusted significance thresholds should be interpreted with caution, as it was our intent to minimize both Type I and Type II errors. Accordingly, effect size estimates and consistent patterns across multiple variables point to potential biologically relevant trends deserving further investigation (as described below). These current findings, combined with previous research outcomes, suggest that wild rats exhibit adaptive stress responses that support sustained vigilance, likely contributing to their survival in high-risk environments.

In the current study, increased glia cell density and microvascularization (MVC) were observed in the dentate gyrus with no effects observed in the anterior cingulate cortex (it is important to point out that the MVC effect was not observed in the CD-31 data, perhaps a function of the nonperfused but postfixed tissue). While not statistically significant, the observed increase in GR-ir cells in CA2 suggests that the distribution and responsivity of these receptors deserve further investigation. These observations in the hippocampus point to potential neurobiological adaptations related to threat, resilience, and memory processing in this brain area—responses critical for survival in a wild habitat. Additionally, wild rats had a higher neuron-to-glial cell ratio and neuronal density in the LHb, increased neuron density in the auditory cortex, and increased glia in the piriform cortex. Altogether, these results point to altered sensory processing and stress adaptation in wild rats; however, considering that the brain data were collected in female rats, caution should be taken when generalizing to males.

The neurobiological findings in the current study reflect potential mechanisms underlying vigilance and threat responsiveness, traits critical for survival in natural environments. For example, in the current dataset, elevated glucocorticoid levels may lead to increased arousal and sensitivity to environmental threats (Sapolsky et al., 2000; Sapolsky, 2021). The variability in stress hormone responses observed in wild rats may also reflect more fine-tuned regulation of stress hormones during specific environmental challenges (Boyle et al., 2021). However, a limitation of this study is the lack of longitudinal hormone measurements, which makes it unclear whether stress markers decrease in non-threatening contexts, such as relatively safer nesting areas. Interestingly, when wild wood mice were captured in a previous study and relocated to a laboratory habitat, their CORT levels decreased in a dramatic fashion over the course of four weeks, suggesting considerable flexibility in the wild mouse HPA-axis responsivity (Carrilho et al., 2024).

Contrary to our hypothesis, we observed lower DHEA/CORT ratios in wild rats—a finding typically seen in less resilient laboratory rodents, as well as less resilient humans (Morgan et al., 2009; Bardi et al., 2012; Lambert et al, 2020). Although lower DHEA: CORT ratios in wild rats may initially appear inconsistent with prior resilience models derived from laboratory rodents, several biological factors may contribute to this discrepancy. First, differences in reproductive status among wild females—despite the absence of overt pregnancy signs—could affect DHEA production, given gonadal influences on this hormone. Additionally, fecal hormone sampling reflects integrated hormonal activity over time and may not capture transient DHEA peaks or regulatory feedback cycles. It is also plausible that wild rats, through evolutionary adaptations, may have developed altered receptor sensitivity, enhanced local synthesis, or alternative coping mechanisms—variables that require further exploration. These findings underscore the importance of considering ecological and physiological context when interpreting hormonal indicators of resilience.

Extending from neurobiological elements of the HPA system, the LHb, a key region for processing aversive and rewarding stimuli (Mondoloni et al., 2022), exhibited increased glial immunoreactivity in wild rats. This region integrates multimodal sensory inputs and contributes to decision-making processes crucial for responding to environmental threats (Baker et al., 2022). Although not significant, GR- and MR-ir modifications in the CA2 region of the hippocampus may also influence spatial memory and navigation, functions critical for identifying and avoiding potential threats in the wild (Bhasin and Nair, 2022). This area is in need of further investigation, but altered CA2 activity could help explain the well-documented phenomenon of bait shyness in wild rats (Rzóska, 1953). Finally, neuronal and glial modifications were observed in several cortical areas relevant to real-time responses to threats in wild habitats. Specifically, altered cellular profiles in the piriform and auditory cortical areas are noteworthy given their contributions to olfactory and auditory detection of threatening stimuli. Together, these sensory systems provide valuable information about the presence of predators or other threats in an animal’s environment (Bao et al., 2016).

Evidence of increased cerebral MVC in wild rats, particularly in the dentate gyrus, was observed in the current study. Increased MVC may enhance nutrient transport and blood flow essential for real-time responsiveness to environmental challenges (Graff et al., 2021). Additionally, sympathetic-induced peripheral vasoconstriction and increased blood flow facilitate successful escape from predators (Charkoudian and Rabbitts, 2009), a response influenced by MR-mediated changes in blood pressure and vascular remodeling (DuPont and Jaffe, 2017). Although the microvasculature in the anterior cingulate cortex of the wild rats was not statistically significantly different from that of their laboratory counterparts, the values of the wild rats were also higher than those of their laboratory counterparts. Providing support for the role of MVC profiles in vigilant responses, modified cerebral blood flow has been associated with resilient responses to natural predator stimuli in laboratory rats (Kondashevskaya et al., 2022). Consequently, future research should explore the role of microvasculature in facilitating rapid escape behaviors and threat responsiveness. Additionally, although it wasn’t possible to perfuse the animals in the current study due to limitations of working in a field site, the use of perfused wild-trapped brain tissue would be informative for future investigations.

An additional area of interest for future exploration is the potential role of transposable elements (TEs), such as Long Interspersed Nuclear Elements (LINE1), in genomic plasticity and immune function in wild rats adapting to dynamic environments (Zhang et al., 2020; DeRosa et al., 2022; Lapp and Hunter, 2019). Preliminary data from our laboratories, obtained from a separate cohort, suggest possible differences in LINE1 expression between wild and laboratory rats, warranting further targeted investigation.

As previously mentioned, wild animals face persistent stressors such as predation, seasonal changes, and anthropogenic habitat destruction, making the wild rat a valuable model for investigating adaptive responses essential for survival in unpredictable environments (Sopinka et al., 2015). The distinct stress responses observed between wild and laboratory animals underscore the importance of investigating wild models in addition to traditional laboratory models to capture the full spectrum of neurobiological adaptations to chronic stress. Whereas laboratory animals often exhibit altered neural structure (e.g., hippocampal atrophy) and suppressed immune function when exposed to chronic stress (Kim et al., 2015; Selye, 1973), wild animals appear to have evolved unique coping mechanisms that mitigate the adverse health effects associated with high corticosterone levels.

While the current findings are specific to urban wild R norvegicus populations, they offer valuable insights into coping mechanisms shaped by natural ecological conditions. Additional investigations across diverse species and habitats are needed to determine the generalizability of these neurobiological adaptations. Although wild habitats differ markedly from standard laboratory settings—where animals experience limited sensory input and minimal cognitive demands—it is important to integrate species-relevant and ecologically meaningful features into laboratory environments when possible (Lambert, 2023). Ultimately, validating laboratory-based findings with data from wild populations across varied naturalistic contexts is essential for ensuring the translational relevance of preclinical models.

Findings from the current study emphasize the importance of neuroethological approaches—examining wild animals in natural habitats—as a means to uncover adaptive stress responses with potential relevance for human health. While the underlying mechanisms remain to be determined, such resilience may point to biological processes that, if better understood, could eventually inform novel strategies for managing chronic stress in humans. Neuroendocrine and vascular adaptations, in particular, represent promising avenues for future investigations, including modulating glucocorticoid receptor (GR) activity or enhancing MVC support in stress-sensitive brain areas. Additionally, information about corticotropin globulin protein (CBG; also known as transcortin) that binds to circulating corticosterone and prevents its entry into the central nervous system (CNS) will provide further clarification about the wild rats’ management of elevated peripheral CORT levels (Meyer et al., 2016). Thus, while translational applications are speculative at this stage, the wild rat model underscores the importance of investigating animals in naturalistic contexts to reveal resilience and vigilance pathways that may not be detectable in laboratory settings.