Coping with urban habitats via glucocorticoid regulation: physiology, behavior, and life history in stream fishes

Hormonal dynamics of matrotrophy vs. lecithotrophy in live-bearing fish reproduction

We explored the relationship between gestational states, fecundity, and steroid hormone levels in three species of live-bearing fish with different maternal provisioning strategies. We studied two lecithotrophic species, Gambusia affinis and Xiphophorus couchianus, where embryos feed exclusively on yolk stored in the eggs, and one matrotrophic species, Heterandria formosa, which actively transfers nutrients to embryos through a follicular placenta. We measured water-borne cortisol, estradiol, and progesterone along with brood size (fecundity) and gestational stage(s). We examined the physiological costs of both maternal provisioning modes. Matrotrophy likely imposes energetic demands due to active nutrient transfer, while lecithotrophy may incur costs from carrying many large embryos. We hypothesized that fecundity, gestational stage, and hormones would covary differently in lecithotrophic vs. matrotrophic species. We found no relationships between hormones and fecundity or gestational stage in any species. However, in H. formosa, we found a positive relationship between estradiol levels and female mass, and a negative relationship between progesterone levels and female mass indicating a change in the circulating levels of both hormones as females grow. We observed differences in average hormone levels among species: the matrotrophic species had higher progesterone and lower estradiol compared to lecithotrophic species. Higher estradiol in lecithotrophic species may relate to egg yolk formation, while placental structures could play a role in progesterone production in matrotrophic species. Elevated cortisol in H. formosa suggests either higher energetic costs or a preparative role for reproduction. Our findings highlight progesterone's importance in maintaining gestation in matrotrophic species, like other placental species.

Fish with slow life-history cope better with chronic manganese exposure than fish with fast life-history

Animals with different life-history types vary in their stress-coping styles, which can affect their fitness and survival in changing environments. We studied how chronic exposure to manganese sulfate (MnSO4), a common aquatic pollutant, affects life-history traits, physiology, and behavior of zebrafish (Danio rerio) with two life-history types: fast (previously selected for fast juvenile growth, early maturation, and small adult body size) and slow life histories (selected for slow juvenile growth, late maturation, and large adult body size). We found that MnSO4 had negative effects on growth and condition factors, but the magnitude of these effects depended on the life-history type. Individuals with fast life histories were more susceptible to MnSO4 than fish with slow life histories as they had lower growth rate, condition factor and feeding probability in high MnSO4 concentrations. Our results demonstrate that MnSO4 can impair fish performance, and life-history variation can modulate the stress-coping ability of individuals.

Habitat structural complexity predicts cognitive performance and behaviour in western mosquitofish

Urban stream syndrome alters stream habitat complexity. We define habitat complexity as the degree of variation in physical habitat structure, with increasing variation equating to higher complexity. Habitat complexity affects species composition and shapes animal ecology, physiology, behaviour and cognition. We used a delayed detour test to measure whether cognitive processes (motor self-regulation) and behaviour (risk-taking) of female Western mosquitofish, Gambusia affinis, varied with habitat structural complexity (low, moderate and high) that was quantified visually for nine populations. We predicted that motor self-regulation and risk-taking behaviour would increase with increasing habitat complexity, yet we found support for the opposite. Lower complexity habitats offer less refuge potentially leading to higher predation pressure and selecting for greater risk-taking by fish with higher motor self-regulation. Our findings provide insight into how habitat complexity can shape cognitive processes and behaviour and offers a broader understanding of why some species may tolerate conditions of urbanized environments.

Lack of glucocorticoid flexibility is indicative of wear-and-tear in Hyla versicolor tadpoles from agricultural environments

In habitats where stressors are frequent or persistent, it can become increasingly difficult for wildlife to appropriately match their endocrine responses to these more challenging environments. The dynamic regulation of glucocorticoid (GC) hormones plays a crucial role in determining how well individuals cope with environmental changes. Amphibians exposed to agricultural stressors can dampen aspects of their GC profile (baseline, agitation, recovery, stress responsiveness, and negative feedback) to cope in these stressful environments, but this dampening can lead to reductions in an individual's reactive scope and a loss of endocrine flexibility. Organic agriculture could potentially limit some of these effects, however, little is known about how amphibians respond physiologically to organic agricultural environments. We compared GC profiles of Hyla versicolor tadpoles from three treatments: natural ponds (<5% agriculture within 500m), ponds near organic farms, and ponds near conventional farms. We hypothesized that tadpoles would cope with agricultural habitats by dampening stress responsiveness and exhibiting more efficient negative feedback and that the magnitude of these changes in response would differ based on agricultural method. We found that tadpoles from conventional and organic ponds were less likely to downregulate GCs via negative feedback after stressor exposure than tadpoles from natural ponds. For agricultural tadpoles that did downregulate GCs after the stressor, we found lower stress responsiveness and faster downregulation to baseline corticosterone than tadpoles from natural ponds. These results point to an accumulation of wear-and-tear, leading to an overall reduction in reactive scope and limited GC flexibility in our agricultural tadpoles. Regardless of agricultural method used, agricultural tadpoles exhibited the same patterns of GC response, indicating that current efforts to incentivize farmers to switch to organic farming methods may not be sufficient to address negative agricultural impacts on amphibians.

Physiological stress response to urbanisation differs between native and invasive squirrel species

Novel pressures derived from urbanisation can alter native habitats and ultimately impact wildlife. Coping with such human-driven changes might induce shifts in species phenotypic traits, such as physiological responses to anthropogenic stressors. Preadaptation to face those challenges has been suggested to favour settlement and spread of invasive alien species in urbanised areas which, consequently, might respond differently than ecologically similar native species to stressors posed by urbanisation. The activation of the hypothalamic-pituitary-adrenal (HPA) axis and the subsequent release of glucocorticoids (GCs) has been suggested to mediate responses to anthropogenic disturbance in vertebrates. Furthermore, intraspecific competition, in conjunction with stressors related to urbanisation, might affect invasive and native species physiological stress responses differently. Using a parallel pseudo-experimental study system we measured faecal glucocorticoid metabolite (FGM) concentrations of the native Eurasian red squirrel and the invasive alien Eastern grey squirrel along a rural-urban gradient and in relation to conspecific density. The two species responded differently to challenges posed by the synergic effect of urbanisation and intraspecific competition. Association of FGMs and conspecific density in native red squirrels varied between rural and suburban sites, potentially depending on differential HPA axis responses. In urban sites, this relationship did not differ significantly from that in rural and suburban ones. Conversely, invasive grey squirrels' FGMs did not vary in relation to conspecific density, nor differed along the rural-urban gradient. Improving knowledge about native and competing invasive species' physiological responses to anthropogenic stressors can support conservation strategies in habitats altered by man. Our findings suggested that the invasive squirrels might be preadapted to cope with these challenges in urbanised areas, potentially increasing their success under the future global change scenario.

Linking microbiome and stress hormone responses in wild tropical treefrogs across continuous and fragmented forests

The amphibian skin microbiome is an important component of anti-pathogen defense, but the impact of environmental change on the link between microbiome composition and host stress remains unclear. In this study, we used radiotelemetry and host translocation to track microbiome composition and function, pathogen infection, and host stress over time across natural movement paths for the forest-associated treefrog, Boana faber. We found a negative correlation between cortisol levels and putative microbiome function for frogs translocated to forest fragments, indicating strong integration of host stress response and anti-pathogen potential of the microbiome. Additionally, we observed a capacity for resilience (resistance to structural change and functional loss) in the amphibian skin microbiome, with maintenance of putative pathogen-inhibitory function despite major temporal shifts in microbiome composition. Although microbiome community composition did not return to baseline during the study period, the rate of microbiome change indicated that forest fragmentation had more pronounced effects on microbiome composition than translocation alone. Our findings reveal associations between stress hormones and host microbiome defenses, with implications for resilience of amphibians and their associated microbes facing accelerated tropical deforestation.

Elevated water temperature initially affects reproduction and behavior but not cognitive performance or physiology in Gambusia affinis

Warming temperatures associated with climate change and urbanization affect both terrestrial and aquatic populations with freshwater fish being especially vulnerable. As fish rely on water temperature to regulate their body temperature, elevated temperatures can alter physiology and in turn behavioral and cognitive skills. We examined whether reproduction, physiology, behavior, and cognitive skills were altered by exposure to elevated water temperatures during one reproductive cycle in the live-bearing fish, Gambusia affinis. We found that within four days of exposure to a higher temperature (31°C), females were more likely to drop underdeveloped offspring than females maintained at 25°C. However, females did not show a change in cortisol release rates over time or altered fecundity and reproductive allotment, despite increased growth at the higher temperature. But in the heat treatment fish that started the experiment with higher baseline cortisol dropped their offspring sooner than fish with lower cortisol release rates. We used a detour test to explore behavior and cognitive skills at three time points after exposure to the heat treatments: early, midway, and at the end (day 7, 20 and 34). We found that on day 7, females were less likely to exit the starting chamber when maintained at 31°C but did not differ in their time to exit the starting chamber or in their motivation (reach the clear barrier). Similarly, females did not differ in their time to swim around the barrier to reach a female fish reward (solving skill). Nonetheless, we found a link between behavior and cognition, where females who were slower to exit the start chamber got around the barrier faster, indicating that they learned from prior experience. Together our results indicate that G. affinis is initially affected by elevated water temperatures but may partially cope with higher temperatures by not altering their hypothalamus-interrenal axis (baseline cortisol), and at the same time this might act to buffer their young. Acclimation may reduce costs for this species and potentially explain why they are successful invaders and tolerant species despite climate change.

An integrated physiological perspective on anthropogenic stressors in the Gulf coast toad (Incilius nebulifer)

Anthropogenic environmental change, including climate change and urbanization, results in warmer temperatures in both terrestrial and aquatic habitats and changes in community assemblages including invasive species introductions, among many other alterations. Anurans are particularly susceptible to these changes because generally they have a biphasic lifecycle and rely on aquatic and terrestrial habitats for survival. Changes such as warmer water temperature can result in direct and carryover effects, after metamorphosis that decrease fitness. However, Gulf Coast toads (Incilius (Bufo) nebulifer) are expanding their range, including into anthropogenically disturbed areas. We hypothesize that I. nebulifer copes with warmer water, reduced water levels, and invasive species by altering their physiology and/or behavior. Corticosterone is the primary glucocorticoid in amphibians, and it modulates many aspects of physiology and behavior, potentially including lipid storage and hop performance, during unpredictable (stressful) events. As a true toad, I. nebulifer also produces bufadienolide toxins that aid in its antipredator defense and may have tradeoffs with corticosterone. In a fully factorial design, we measured baseline corticosterone levels in tadpoles in response to two treatments: decreased water levels and increased water temperatures. After metamorphosis, we measured the corticosterone profile and other associated responses to exposure to the predatory red imported fire ant (Solenopsis invicta; RIFA). We found that tadpoles had elevated baseline corticosterone release rates when reared in warmer water and reduced water levels. Toadlets also had elevated baseline corticosterone release rates when exposed to any combination of two of the three treatments but when exposed to all three treatments toadlets instead showed elevated magnitude of their stress response. Predator avoidance (as measured by hop performance) was reduced after exposure to RIFA. Tadpoles from warmer water developed more quickly and were smaller in mass after metamorphosis. Toadlets had reduced production of two of the three detected bufadienolides and increased energy storage (lipids) after exposure to warmer water and reduced growth after exposure to reduced water levels. We found direct and carryover effects of common anthropogenic changes in I. nebulifer that may aid in their ability to persist despite these changes.

Gut Bacterial Communities Vary across Habitats and Their Diversity Increases with Increasing Glucocorticoids in Toad Tadpoles

The gut microbiome is important for host health and can be influenced by environmental and hormonal changes. We studied the interactions between anthropogenic land use, glucocorticoid hormones, and gut bacterial communities in common toads (Bufo bufo). We sampled tadpoles from ponds of three habitat types (natural, agricultural, and urban ponds), examined gut microbiome composition using amplicon sequencing of the 16S rRNA gene, and measured the associated stress physiology using water-borne hormones. Tadpoles from different habitat types significantly differed in bacterial composition. However, bacterial richness, Shannon diversity, and Firmicutes to Bacteroidota ratio did not vary with habitat type. In contrast with other studies, we found a positive correlation between baseline corticosterone release rate and bacterial diversity. Stress response and negative feedback were not significantly correlated with bacterial diversity. These results suggest that, despite alterations in the composition of intestinal bacterial communities due to land-use change, common toad tadpoles in anthropogenic habitats may maintain their physiological health in terms of the “gut-brain axis”.

An evolutionary perspective on stress responses, damage and repair

Variation in stress responses has been investigated in relation to environmental factors, species ecology, life history and fitness. Moreover, mechanistic studies have unravelled molecular mechanisms of how acute and chronic stress responses cause physiological impacts ('damage'), and how this damage can be repaired. However, it is not yet understood how the fitness effects of damage and repair influence stress response evolution. Here we study the evolution of hormone levels as a function of stressor occurrence, damage and the efficiency of repair. We hypothesise that the evolution of stress responses depends on the fitness consequences of damage and the ability to repair that damage. To obtain some general insights, we model a simplified scenario in which an organism repeatedly encounters a stressor with a certain frequency and predictability (temporal autocorrelation). The organism can defend itself by mounting a stress response (elevated hormone level), but this causes damage that takes time to repair. We identify optimal strategies in this scenario and then investigate how those strategies respond to acute and chronic exposures to the stressor. We find that for higher repair rates, baseline and peak hormone levels are higher. This typically means that the organism experiences higher levels of damage, which it can afford because that damage is repaired more quickly, but for very high repair rates the damage does not build up. With increasing predictability of the stressor, stress responses are sustained for longer, because the animal expects the stressor to persist, and thus damage builds up. This can result in very high (and potentially fatal) levels of damage when organisms are exposed to chronic stressors to which they are not evolutionarily adapted. Overall, our results highlight that at least three factors need to be considered jointly to advance our understanding of how stress physiology has evolved: (i) temporal dynamics of stressor occurrence; (ii) relative mortality risk imposed by the stressor itself versus damage caused by the stress response; and (iii) the efficiency of repair mechanisms.