Neophobia, but not perch hopping, is sensitive to long-term chronic stress intensity

To further elucidate the role that wear-and-tear plays in the transition from acute to chronic stress, we manipulated the intensity and duration of applied chronic stress to determine if behavior would respond proportionately. We brought wild house sparrows into captivity and subjected them to high-stress, medium-stress, low-stress, or captivity-only. We varied the number of stressors per day and the duration of stress periods to vary wear-and-tear, and thus the potential to exhibit chronic stress symptoms. The behaviors we assessed were neophobia (the fear of the new; assessed via food approach latency) and perch hopping (activity). We predicted that our birds would show proportionate decreases in neophobia and activity throughout a long-term chronic stress paradigm. Our results indicate that neophobia is sensitive to the intensity of chronic stress, however, the birds became more neophobic, which was the opposite of what we expected. Conversely, perch hopping did not differ across treatment groups and is thus not sensitive to the intensity of chronic stress. Together, these data show that different behavioral measurements are impacted differently by chronic stress.

Varying intensities of chronic stress induce inconsistent responses in weight and plasma metabolites in house sparrows (Passer domesticus)

One of the biggest unanswered questions in the field of stress physiology is whether variation in chronic stress intensity will produce proportional (a gradient or graded) physiological response. We were specifically interested in the timing of the entrance into homeostatic overload, or the start of chronic stress symptoms. To attempt to fill this knowledge gap we split 40 captive house sparrows (Passer domesticus) into four groups (high stress, medium stress, low stress, and a captivity-only control) and subjected them to six bouts of chronic stress over a 6-month period. We varied the number of stressors/day and the length of each individual bout with the goal of producing groups that would experience different magnitudes of wear-and-tear. To evaluate the impact of chronic stress, at the start and end of each stress bout we measured body weight and three plasma metabolites (glucose, ketones, and uric acid) in both a fasted and fed state. All metrics showed significant differences across treatment groups, with the high stress group most frequently showing the greatest changes. However, the changes did not produce a consistent profile that matched the different chronic stress intensities. We also took samples after a prolonged recovery period of 6 weeks after the chronic stressors ended. The only group difference that persisted after 6 weeks was weight-all differences across groups in metabolites recovered. The results indicate that common blood metabolites are sensitive to stressors and may show signs of wear-and-tear, but are not reliable indicators of the intensity of long-term chronic stress. Furthermore, regulatory mechanisms are robust enough to recover within 6 weeks post-stress.

Response to novel feed in dairy calves is affected by prior hay provision and presentation method

Animals raised in environments that prevent natural foraging opportunities may have difficulty adapting to novelty, such as feeding and management changes. Our objective was to evaluate how early provision and presentation of forage in dairy calves affected response to novel TMR (total mixed ration; grain and alfalfa) at weaning. Holstein heifer calves were housed individually in a covered outdoor hutch with an attached uncovered wire-fenced pen on sand bedding. Calves were fed a diet of starter grain and milk replacer (5.7-8.4L/d step-up) via a bottle (Control, n = 9) or given additional access to mountaingrass hay presented either in a bucket (Bucket, n = 9), or PVC pipe feeder (Pipe, n = 9). Treatments were applied from birth through 50 d of age, when step-down weaning began. All calves had 3 buckets and a pipe feeder provided in their uncovered pen area. On d 50, each calf was briefly blocked inside their hutch. TMR was put in the 3rd bucket that previously contained hay (Bucket) or was empty (Control, Pipe). The calf was released from the hutch and video-recorded for 30 min. Neophobia towards TMR was affected by prior experience with presentation: Bucket calves began eating TMR faster than Pipe and Control (P≤0.012) and showed the fewest number of startle responses (P = 0.004). Intake was similar across groups (P = 0.978), suggesting this apparent neophobia was transient, but Control calves took longer to eat than Bucket (P<0.001) and Pipe (P = 0.070) calves and were less likely to give up on eating to lie down instead. These results suggest that previous experience with hay improves processing ability when presented with novel TMR. Overall, response to a novel feed is affected by both early life experience, such as opportunities to process forage, and the presentation of the feed itself. Calves also appear motivated to access forage, evidenced by transient neophobia, high intake, and persistence in feeding by naïve calves.

Constitutive gene expression differs in three brain regions important for cognition in neophobic and non-neophobic house sparrows (Passer domesticus)

Neophobia (aversion to new objects, food, and environments) is a personality trait that affects the ability of wildlife to adapt to new challenges and opportunities. Despite the ubiquity and importance of this trait, the molecular mechanisms underlying repeatable individual differences in neophobia in wild animals are poorly understood. We evaluated wild-caught house sparrows (Passer domesticus) for neophobia in the lab using novel object tests. We then selected a subset of neophobic and non-neophobic individuals (n = 3 of each, all females) and extracted RNA from four brain regions involved in learning, memory, threat perception, and executive function: striatum, caudal dorsomedial hippocampus, medial ventral arcopallium, and caudolateral nidopallium (NCL). Our analysis of differentially expressed genes (DEGs) used 11,889 gene regions annotated in the house sparrow reference genome for which we had an average of 25.7 million mapped reads/sample. PERMANOVA identified significant effects of brain region, phenotype (neophobic vs. non-neophobic), and a brain region by phenotype interaction. Comparing neophobic and non-neophobic birds revealed constitutive differences in DEGs in three of the four brain regions examined: hippocampus (12% of the transcriptome significantly differentially expressed), striatum (4%) and NCL (3%). DEGs included important known neuroendocrine mediators of learning, memory, executive function, and anxiety behavior, including serotonin receptor 5A, dopamine receptors 1, 2 and 5 (downregulated in neophobic birds), and estrogen receptor beta (upregulated in neophobic birds). These results suggest that some of the behavioral differences between phenotypes may be due to underlying gene expression differences in the brain. The large number of DEGs in neophobic and non-neophobic birds also implies that there are major differences in neural function between the two phenotypes that could affect a wide variety of behavioral traits beyond neophobia.

The Effect of a Combined Fast and Chronic Stress on Body Mass, Blood Metabolites, Corticosterone, and Behavior in House Sparrows (Passer domesticus)

One aspect of the Reactive Scope Model is wear-and-tear, which describes a decrease in an animal's ability to cope with a stressor, typically because of a period of chronic or repeated stressors. We investigated whether wear-and-tear due to chronic stress would accelerate a transition from phase II to phase III of fasting. We exposed house sparrows (Passer domesticus) to three weeks of daily fasts combined with daily intermittent repeated acute stressors to create chronic stress, followed by two weeks of daily fasts without stressors. We measured circulating glucose, β-hydroxybutyrate (a ketone), and uric acid in both fasted and fed states. We expected birds to be in phase II (high fat breakdown) in a fasted state, but if wear-and-tear accumulated sufficiently, we hypothesized a shift to phase III (high protein breakdown). Throughout the experiment, the birds exhibited elevated β-hydroxybutyrate when fasting but no changes in circulating uric acid, indicating that a transition to phase III did not occur. In both a fasted and fed state, the birds increased glucose mobilization throughout the experiment, suggesting wear-and-tear occurred, but was not sufficient to induce a shift to phase III. Additionally, the birds exhibited a significant decrease in weight, no change in corticosterone, and a transient decrease in neophobia with chronic stress. In conclusion, the birds appear to have experienced wear-and-tear, but our protocol did not accelerate the transition from phase II to phase III of fasting.

A transient reduction in circulating corticosterone reduces object neophobia in male house sparrows

Aversive reactions to novelty (or "neophobia") have been described in a wide variety of different animal species and can affect an individual's ability to exploit new resources and avoid potential dangers. However, despite its ecological importance, the proximate causes of neophobia are poorly understood. In this study, we tested the role of glucocorticoid hormones in neophobia in wild-caught house sparrows (Passer domesticus, n = 11 males) by giving an injection of the drug mitotane that reduced endogenous corticosterone for several days or a vehicle control, and then examined the latency to feed when the food dish was presented with or without a novel object in, on, or near the dish. Each sparrow was exposed to multiple novel object and control trials and received both vehicle control and mitotane treatments, with a week between treatments to allow the drug to wash out. As found previously, all novel objects significantly increased sparrows' latency to feed compared to no object present. Reducing corticosterone using mitotane significantly reduced the latency to feed in the presence of novel objects. In control trials without objects, mitotane had no significant effects on feeding time. Although we have shown that corticosterone affects neophobia, further studies using specific receptor agonists and antagonists will help clarify the neurobiological mechanisms involved and determine whether baseline or stress-induced corticosterone is driving this effect. These results suggest that increased glucocorticoids (e.g., due to human-induced stressors) could increase neophobia, affecting the ability of individuals to exploit novel resources, and, ultimately, to persist in human-altered environments.

Chronic stress reverses enhanced neophobia following an acute stressor in European starlings

Neophobia is an animal's avoidance of novelty. Animals tend to respond to novel objects by increasing their latency to approach the objects, and they eventually habituate after repeated exposure by attenuating this increased approach latency. Interestingly, the physiological stress response does not appear to have a causal link to neophobia, although acute stress can prevent animals from habituating to novel objects, possibly through a permissive effect. Chronic stress can induce an anxiety-like state in animals, while often disrupting the ability to respond to acute stress. We thus hypothesized that chronic stress may increase neophobia and tested this by inducing chronic stress in wild-caught European starlings (Sturnus vulgaris). Four distinct anthropogenic stressors were administered daily for 30 min each in a randomized order for 21 days. We then evaluated whether exposure to chronic stress altered the latency to approach a novel object placed on or near a food dish presented after overnight fasting. Chronically stressed birds and nonstressed controls exhibited similar initial neophobic responses to novel objects and showed similar habituation in response to repeated exposure. However, when birds were exposed to 15 min of restraint before repeated exposure to the same object, habituation was eliminated in control birds (i.e., they continued to respond with neophobia), whereas chronically stressed birds continued to show habituation as measured by attenuated approach latencies. These results demonstrate that an acute stress response (restraint) has a different impact on neophobia depending upon whether the bird is or is not concurrently exposed to chronic stress.

No, you go first: phenotype and social context affect house sparrow neophobia

Novel object trials are commonly used to assess aversion to novelty (neophobia), and previous work has shown neophobia can be influenced by the social environment, but whether the altered behaviour persists afterwards (social learning) is largely unknown in wild animals. We assessed house sparrow (Passer domesticus) novel object responses before, during and after being paired with a conspecific of either similar or different behavioural phenotype. During paired trials, animals housed with a similar or more neophobic partner demonstrated an increased aversion to novel objects. This change did not persist a week after unpairing, but neophobia decreased after unpairing in birds previously housed with a less neophobic partner. We also compared novel object responses to non-object control trials to validate our experimental procedure. Our results provide evidence of social learning in a highly successful invasive species, and an interesting asymmetry in the effects of social environment on neophobia behaviour depending on the animal's initial behavioural phenotype.

Baseline corticosterone is associated with parental care in virgin Japanese quail (Coturnix japonica)

Glucocorticoids (GCs) are thought to impact reproductive success, and ultimately fitness. In this study we focus specifically on the relationship between GCs and parental care. Captive bred Japanese quail (Coturnix japonica) do not show spontaneous parental care behavior, however this behavior can be induced through a sensitization procedure. We investigated how the GC status of Japanese quail relates to parental care in animals of both sexes exposed or not to a chick sensitization treatment. To assess GC status, we obtained baseline plasma and feather cort samples, and used the HPA-axis function test to assess stress responsiveness by examining the response to a standardized stressor as well as negative feedback efficacy through dexamethasone injection. Next, birds were either exposed to chicks overnight in a small enclosure (sensitization treatment) or were enclosed but not exposed to chicks (control). The following morning, adult behaviors were filmed in the presence of a fresh set of chicks for 20 min. A final serum GC sample was obtained to assess if exposure to novel chicks was perceived as stressful. In control animals, baseline GCs were associated with increased total parental care duration and decreased latency to first parental care event. Interestingly, the opposite relationship was found in the sensitization group. Finally, exposure to novel chicks was not associated with an increase in corticosterone in either group. Overall it appears that baseline GCs are correlated with parental care in captive bred Japanese quail, and that the relationship changes direction depending on whether or not sensitization has occurred.