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.

Effect of estradiol and predator cues on behavior and brain responses of captive female house sparrows (Passer domesticus).

The presence of predators can cause major changes in animal behavior, but how this interacts with hormonal state and brain activity is poorly understood. We gave female house sparrows (Passer domesticus) in post-molt condition an estradiol (n = 17) or empty implant (n = 16) for 1 week. Four weeks after implant removal, a time when female sparrows show large differences in neuronal activity to conspecific vs. heterospecific song, we exposed birds to either 30 min of conspecific song or predator calls, and video recorded their behavior. Females were then euthanized, and we examined neuronal activity using the expression of the immediate early gene (IEG) ZENK to identify how the acoustic stimuli affected neuronal activation. We predicted that if female sparrows with estradiol implants reduce neuronal activity in response to predator calls as they do to neutral tones and non-predatory heterospecifics, they would show less fear behavior and a decreased ZENK response in brain regions involved in auditory (e.g., caudomedial mesopallium) and threat perception functions (e.g., medial ventral arcopallium) compared to controls. Conversely, we predicted that if females maintain auditory and/or brain sensitivity towards predator calls, then female sparrows exposed to estradiol would not show any differences in ZENK response regardless of playback type. We found that female sparrows were less active during predator playbacks independent of hormone treatment and spent more time feeding during conspecific playback if they had previously been exposed to estradiol. We observed no effect of hormone or sound treatment on ZENK response in any region of interest. Our results suggest that female songbirds maintain vigilance towards predators even when in breeding condition.

Novel objects alter immediate early gene expression globally for ZENK and regionally for c-Fos in neophobic and non-neophobic house sparrows.

Neophobia - an animal's reluctance to approach novel objects, try new foods, or explore unfamiliar environments - affects whether animals can adapt to new environments and exploit novel resources. However, despite its importance, the neurobiological mechanisms underlying this personality trait are poorly understood. In this study, we examined regional brain activity using the expression of two immediate early genes (IEGs), ZENK and c-Fos, in response to novel objects or control conditions in captive house sparrows (Passer domesticus, n = 22). When exposed to novel objects, we predicted that we would see differential IEG activity in brain regions involved in regulating stress and emotion (hippocampus, medial ventral arcopallium, lateral septum), reward and learning (striatum), and executive function (NCL) between neophobic and non-neophobic individuals. To classify birds by phenotype, we used behavior trials that tested willingness to approach a food dish in the presence of several different novel objects, habituation to one novel object, and willingness to try several different novel foods. We then exposed birds to a new novel object or a control condition and assessed protein expression of two IEGs in neophobic vs non-neophobic individuals after this final exposure. An analysis of average sparrow feeding times in the presence of novel objects showed a bimodal distribution of neophobia behavior. There was also high repeatability of individual novel object responses, and average responses to all three trial types (novel object, novel food, and habituation to a novel object) were significantly correlated. Although we saw no differences between neophobic and non-neophobic birds in IEG expression in response to novel objects in any of the 6 brain regions examined, there was a significant global decrease in ZENK expression and a significant increase in c-Fos expression in the medial ventral arcopallium and the caudal hippocampus in response to novel objects compared to controls, suggesting that these two regions may be important in novelty detection and threat perception. Additionally, there was no object effect in the rostral hippocampus, which supports the hypothesis that the avian hippocampus may have a rostrocaudal functional gradient similar to the septotemporal gradient in mammals.