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The western fence lizard occupies an instructive position along the urbanization spectrum. Unlike species that either vanish entirely from developed landscapes or thrive in the densest urban cores, Sceloporus occidentalis occupies a middle ground, being largely absent from heavily built-up centers yet commonly abundant in suburban parks, gardens, and yards. This intermediate pattern makes the species especially valuable for urban evolutionary ecology, because populations can be sampled across a genuine gradient of human influence rather than being compared as simple urban-versus-rural dichotomies. The Santa Barbara study region offers a particularly suitable natural laboratory in this respect, as residential development, commercial zones, and relatively undeveloped chaparral and grassland habitats interleave across a compact area, allowing researchers to capture fine-grained variation in urban intensity while holding regional climate broadly constant.

The sex-specific pattern in natural antibody-mediated agglutination deserves particular attention. In the sampled populations, females showed rising agglutination scores with increasing urbanization while males showed the opposite trend. Such divergent responses between the sexes are not commonly reported in urban wildlife studies, which often pool sexes or lack the statistical power to test for interactions. Yet sex-specific immune modulation is biologically plausible for several reasons. Female lizards in spring are approaching or undergoing vitellogenesis, the process of yolk production that places substantial energetic and nutritional demands on the body. Immune proteins and the energetic costs of immune maintenance interact with reproductive investment in ways that can differ sharply between males and females. If urban habitats alter food availability, thermal environments, or parasite exposure differently for each sex—for instance, if females forage in microhabitats that differ from those used by males—then the immune-reproduction trade-off could shift in opposite directions for the two sexes along the gradient. The finding underscores the importance of analyzing male and female physiology separately in urban adaptation studies, a practice that remains inconsistent across the literature.

The distinction between the two branches of innate immunity measured in the study also merits elaboration. Natural antibodies are immunoglobulins produced without prior antigen exposure, forming a first line of defense that recognizes broad molecular patterns on pathogens and damaged cells. They facilitate opsonization, in which pathogens are tagged for destruction by phagocytic cells, and they can initiate the complement cascade, a chain reaction of plasma proteins that punches holes in microbial membranes, resulting in lysis. Because these two arms are related but mechanistically distinct, they can respond differently to environmental stressors. In this study, agglutination varied with urbanization in a sex-dependent manner, whereas complement-mediated lysis showed no gradient-wide differences. This dissociation suggests that the urban environment may be reshaping specific components of constitutive immunity rather than uniformly suppressing or enhancing the entire innate immune system. Such specificity is consistent with the emerging view in ecoimmunology that immune defenses are not a single currency but a portfolio of traits with different costs, benefits, and sensitivities to ecological conditions.

The corticosterone results add an important seasonal dimension. Corticosterone is the primary glucocorticoid hormone in reptiles, secreted by the adrenal glands in response to stressors and playing a central role in mobilizing energy reserves. Baseline corticosterone, measured when an animal is not acutely stressed, reflects the allostatic load an individual carries—the cumulative energetic demands of its environment and life stage. The finding that spring baseline corticosterone rose with urbanization while summer levels did not suggests that the challenges of urban living may be concentrated in the breeding season. Spring is when lizards emerge from winter dormancy, begin foraging intensively, and initiate reproduction. If urban environments impose heightened energetic demands during this window—through altered prey availability, increased disturbance from human activity, thermoregulatory costs on artificial substrates, or noise and light pollution—the hormonal signature might appear in spring and then dissipate once the breeding season concludes. Alternatively, the seasonal pattern could reflect changes in the composition of the sampled population, since reproductive condition varies across the activity season.

Interpreting elevated corticosterone requires caution, a point emphasized in recent endocrine literature. Chronically elevated baseline glucocorticoids are often assumed to indicate chronic stress and impaired welfare, but hormone levels can also reflect successful physiological adjustment to demanding conditions. An animal coping effectively with a high-energy urban lifestyle might maintain moderately elevated corticosterone as part of normal energy mobilization, without suffering the pathological consequences associated with chronic stress, such as immune suppression or reproductive failure. The simultaneous observation in this study that summer corticosterone was unaffected, and that body condition did not vary across the gradient, complicates any simple narrative of urban lizards being chronically stressed. Instead, the spring hormone pattern may represent a seasonally specific physiological response whose fitness consequences remain to be determined through longitudinal or experimental work.

The morphological and reproductive findings—smaller body size, reduced egg volume, and reduced clutch volume in more urbanized sites, with body condition and clutch size unchanged—paint a nuanced picture of life-history adjustment. Body condition, typically calculated as mass relative to body length, integrates energetic stores; its stability across the gradient suggests that urban lizards are not simply malnourished. Yet smaller adult body size and smaller eggs indicate that something about urban development constrains growth or shifts investment patterns. Egg volume is a key life-history trait because larger eggs generally produce larger hatchlings with greater reserves, which can enhance survival in the critical weeks after emergence. Reduced clutch volume, the total volume of eggs in a clutch, could arise from the combination of smaller eggs with unchanged clutch numbers, implying that urban females maintain their reproductive output in terms of offspring number while reducing per-offspring provisioning. This pattern is consistent with pace-of-life theory, which predicts that animals in environments with higher extrinsic mortality or greater unpredictability may shift toward producing more, smaller offspring at the expense of individual offspring quality, or may mature at smaller sizes after faster growth schedules.

The parasite findings provide a possible mechanistic thread connecting several of these observations. Lower prevalence of both ticks and mites in more urbanized lizards aligns with a growing body of evidence that urban environments can disrupt parasite life cycles. Ectoparasites such as ticks often depend on specific host communities and environmental conditions—humidity, vegetation structure, and the presence of additional host species such as rodents and deer—that may be reduced or fragmented in developed areas. Mites, likewise, may be sensitive to microclimatic conditions and host density. If urban lizards genuinely carry fewer parasites, several downstream consequences follow. Reduced parasitism could lower the energetic and immunological costs of infection, potentially allowing reallocation of resources toward growth or reproduction. It could also explain why some immune measures did not show the depressed function that anthropogenic-stressor hypotheses might predict: with fewer parasites to contend with, urban lizards may not need to mount the same level of immune activity as their rural counterparts. The sex-specific agglutination pattern could even interact with parasite dynamics if males and females differ in exposure or in the immunological resources allocated to parasite resistance.

It is worth situating these results within the broader challenge of studying urbanization effects in ectotherms. Reptiles depend on external heat sources for thermoregulation, and urban landscapes alter thermal environments profoundly. Impervious surfaces, buildings, and reduced vegetation create heat islands that can extend activity seasons, shift daily activity windows, and change the thermal quality of basking sites. For a lizard like the western fence lizard, which relies on precise behavioral thermoregulation to optimize physiological performance, these changes could influence everything from digestive efficiency to immune function, since ectothermic immune responses are temperature-dependent. Urban structures such as walls, fences, and rockeries may also provide abundant basking and refuge opportunities, potentially benefiting lizards in ways that offset other urban costs. The net effect of these competing influences is difficult to predict a priori, which helps explain why reptile studies to date have reported such heterogeneous outcomes.

The methodological achievement of the study is also notable. Measuring natural antibodies, complement activity, glucose, corticosterone, morphology, reproduction, and ectoparasite prevalence in free-living lizards requires small blood volumes and careful field logistics, challenges that have historically limited multi-trait studies in reptiles. By integrating immunological, endocrinological, parasitological, and life-history endpoints within a single sampling framework across a real urban gradient, the study exemplifies the kind of multi-dimensional approach that urban ecologists have repeatedly called for. Single-trait studies risk missing the trade-offs and compensatory mechanisms that characterize physiological adjustment, whereas integrated profiles can reveal, for example, how hormonal changes might mediate the relationship between environmental stressors and reproductive output.

Several avenues for future research follow naturally from these findings. Experimental approaches—such as common-garden rearing of lizards from urban and rural sites—could distinguish between plastic physiological responses and inherited adaptation. Longitudinal monitoring of marked individuals could reveal whether smaller body size in urban lizards reflects slower growth, earlier maturation at smaller size, or selective disappearance of large individuals. Seasonal sampling at finer resolution could pinpoint when corticosterone diverges between urban and rural animals and whether the spring elevation carries over into reproductive success. Quantifying urban environmental variables directly—temperature profiles, prey abundance, noise, light at night, pollutant loads—would help identify which specific factors drive the observed patterns. Genetic analyses could test whether urban populations show signatures of selection on immune or endocrine pathways. And extending the sampling to multiple cities across the species’ broad range would test whether the Santa Barbara patterns generalize or reflect local circumstances.

For conservation and management, the study carries a cautiously optimistic message. The persistence and apparent physiological adjustment of western fence lizards in suburban environments suggest that moderately urbanized landscapes can sustain reptile populations, provided that structural habitat features such as basking sites, refuges, and foraging opportunities remain available. At the same time, the reductions in body size and reproductive investment hint that urban populations may not be demographically equivalent to rural ones, even when they appear abundant. Subtle life-history shifts can influence population dynamics over time, particularly if hatchling survival is compromised by smaller egg size. Urban planners and land managers interested in reptile conservation might therefore prioritize habitat heterogeneity, native vegetation, and connectivity between green spaces, which could simultaneously support prey communities, moderate thermal extremes, and maintain the parasite communities that shape immune dynamics. As urbanization continues to expand globally, studies of this kind—careful, multi-trait, and grounded in the natural history of the species involved—will be essential for understanding which species can adapt to human-dominated landscapes and under what conditions.

Subject of Research: Urbanization is associated with alterations in immunity, stress physiology, morphology and reproduction in western fence lizards

Article Title: Urbanization is associated with alterations in immunity, stress physiology, morphology and reproduction in western fence lizards

Article References: Sparkman, A. M., Muthiah, C. A., Reyes, E., Rurik, A. J., & Hareb, N. (2026). Urbanization is associated with alterations in immunity, stress physiology, morphology and reproduction in western fence lizards. Frontiers in Amphibian and Reptile Science, 4, Article 1733610. https://doi.org/10.3389/famrs.2026.1733610

Image Credits: AI Generated

DOI: 10.3389/famrs.2026.1733610

Keywords: urbanization, western fence lizard, Sceloporus occidentalis, innate immunity, corticosterone, reproduction, urban ecology, reptiles, Santa Barbara, stress physiology, associated, alterations