The review, conducted by a single investigator using PubMed and Google Scholar, began with 1,608 publications and, after rigorous screening against modified PRISMA 2020 guidelines, distilled the evidence down to 34 peer-reviewed studies, comprising 31 research papers and three case studies. Nearly all of the included work was published between 2014 and 2025, underscoring how recently the topic has captured scientific attention. The United Kingdom and the United States together produced almost two-thirds of the studies, with additional contributions from Australia, Brazil, New Zealand, China, Pakistan, Portugal and Singapore. Mammals dominated the species roster, with non-human primates studied most often, but birds were also well represented, including flamingos, penguins, hornbills, kiwi and free-flight aviary collections, alongside a handful of reptiles such as an American alligator, a Siamese crocodilian and semi-aquatic turtles. Notably, no study examined teleost fish, and half of all investigations evaluated two or fewer individual animals, a small sample size that limits how broadly the results can be generalized.

Behavioural observation was by far the most common assessment tool, deployed in 28 of the studies through species-specific ethograms that catalogue activities such as resting, foraging, pacing and social interaction. The single most frequently documented response was increased vigilance, reported in ten studies across macaques, gibbons, capuchins, peccaries, orangutans, giraffes, hornbills, cheetahs and mixed aviary species. Vigilance, the scanning behaviour animals use to monitor their environment for threats, is widely interpreted as a sign of heightened arousal or anxiety. Other documented responses included retreating away from visitors or out of sight, decreased rest, increased self- or social grooming, increased pacing, reduced appetite, increased swimming, altered circadian activity patterns, greater time spent underwater in a Siamese crocodile, and even increased protectiveness of infants in orangutans. Importantly, six studies found no statistically significant behavioural change linked to noise, and no publication reported overt abnormalities such as self-inflicted trauma or serious aggression.

Physiological measurement, by contrast, was rare. Only six studies assessed glucocorticoid concentrations, the hormonal fingerprints of activity in the hypothalamic-pituitary-adrenal axis, with five pairing faecal or urinary hormone analysis with behavioural observation. Three of the six reported elevated glucocorticoids: one in okapis attributed to visitor presence, one in orangutans linked to ventilation system noise, and one in a giant panda later attributed to normal physiological variation unrelated to noise. The remaining studies found no elevation or no correlation with behaviour. The review’s authors note that the broader literature increasingly cautions against interpreting glucocorticoids as stand-alone welfare indicators, since levels can fluctuate with reproductive status, circadian rhythms and sampling methods, and argue that pairing hormone data with complementary measures, such as the cognitive bias testing used in one primate study, would produce far more robust welfare conclusions.

Perhaps the most surprising finding concerns the sources of the noise itself. Visitor noise was the focus of 27 studies, but construction noise appeared in five and concerts in two. Yet in two publications, researchers suspected that ventilation systems, not visitors, were the real acoustic culprit, because decibel levels did not rise appreciably when crowds grew. A third study found that behavioural changes tracked a habitat’s waterfall feature more closely than visitor noise. One investigation concluded that reducing visitor numbers or limiting opening hours did not guarantee a quieter environment, highlighting the persistent hum of mechanical infrastructure in zoo soundscapes. At the same time, enclosure acoustics can work in animals’ favour: one study showed that a habitat’s soundscape substantially buffered short-term fluctuations in external noise, suggesting that thoughtful acoustic design can be a protective factor rather than a hazard.

The objective acoustic data gathered across the studies spanned an extraordinary range, from 51.5 decibels to a peak of 122.2 decibels recorded near a sloth habitat in a walk-through enclosure. Concert noise reached a maximum of 88 decibels and construction noise 76 decibels. One study measured construction and non-construction days at different frequencies in hertz, while another compared how A-weighted and C-weighted decibel filters affected penguin species, finding that only southern rockhoppers responded positively to pool use under elevated C-weighted levels. These technical details matter because sound meters positioned on the public-facing side of a habitat may not accurately represent what an animal actually perceives, a limitation the review flags as a significant obstacle to drawing welfare conclusions from sound pressure levels alone.

When it came to characterizing welfare outcomes, the literature revealed a striking inconsistency. Eleven studies suspected a negative welfare impact from anthropogenic noise based on observed behavioural changes, nine could not determine whether welfare was affected, seven reported no substantial impact, and seven did not address welfare as an outcome at all. Welfare assessment protocols were rarely described in a standardized way, and many studies acknowledged confounding factors, including the visual presence of visitors, the sight of construction workers, species-level and individual differences in noise sensitivity, and even the acoustic interference of precipitation. Under the Five Domains model of animal welfare, which integrates nutrition, physical environment, health, behavioural interaction and mental state, these complications underscore that noise cannot be evaluated in isolation. The ventilation system example illustrates the dilemma: mechanical noise may be stressful, yet reducing ventilation could compromise thermal comfort and air quality, producing a net welfare loss across multiple domains.

Fourteen publications offered mitigation recommendations, clustering around three practical strategies. Public education, delivered through signage or trained staff, was the most frequently cited, and one study found that ‘watching eyes’ signage requesting quietness measurably reduced noise levels. The second strategy centred on giving animals choice and control over their location, an established contributor to positive welfare, with several studies documenting animals retreating to indoor or off-exhibit spaces to escape noise. Additional suggestions included controlling visitor numbers and timing, habituation training, adding soothing background sound, increasing vegetation, redesigning enclosures to dampen sound and continuing behavioural monitoring during disruptive periods. Operational planning, such as relocating noise-sensitive species during extended construction, could preserve visitor engagement while protecting animal welfare.

The review concludes that anthropogenic noise alone may not harm welfare in every case, since some studies could not disentangle acoustic from visual disturbances, and that species and individuals differ markedly in their responses. Individual-level welfare assessments, the authors argue, allow institutions to plan more appropriately and may prevent costly habitat reconstruction by identifying minor, targeted modifications before welfare is seriously compromised. As zoo accreditation standards evolve, the authors suggest that noise deserves more formal inclusion in welfare assessment frameworks, and that future research should explore how animals respond to different human-generated frequencies, whether reactions differ between chronic and acute exposure, and which documented behaviours genuinely reflect negative welfare states. For a field barely a decade old, the message is clear: the soundscape of the modern zoo is not just background noise, but a variable that zoological institutions can measure, design around and ultimately manage to the benefit of the animals in their care.

Subject of Research: The effects of anthropogenic noise on animal welfare in zoological settings

Article Title: A Scoping Review: The Effect of Anthropogenic Noise on Animal Welfare in Zoological Settings

Article References: Kruse, T. N., & Flint, M. (2026). A Scoping Review: The Effect of Anthropogenic Noise on Animal Welfare in Zoological Settings. Veterinary Medicine and Science, 12(5), Article e71197. https://doi.org/10.1002/vms3.71197

Image Credits: AI Generated

DOI: 10.1002/vms3.71197

Keywords: anthropogenic noise, zoo animal welfare, animal behaviour, soundscape, glucocorticoids, visitor effects, Five Domains model, noise mitigation, scoping review, captive animals, vigilance, zoo acoustics