Are there any studies that show that the animals are "perpetually stressed because of their surroundings"? I am honestly curious, I have never seen any that show that to be the case.
I take no position, as the matter is complex and I haven't read-up on it, but I remembered this question.
I'm a fan of Jon Coe, and today I found Jon Coe and Sabrina Brando's paper, "Confronting Back-of-House Traditions: Primates as a Case Study" (Journal of Zoological and Botanical Gardens, published 26 July 2022), available at
https://joncoe.net/wp-content/uploads/2022/07/Confronting-Back-of-House-Traditions-2022.pdf.
Page 385 begins the discussion of the effect of
back-of-house noise on animals. It reads, in part:
4.9. Acoustic
“Many zoos are acoustically boring for species whose dominant sense is audition; that is, they lack acoustic complexity and variation (including temporal) that would be encountered in the wild. For example, no dawn bird chorus”.
(Robert Young, animal behavior and environmental enrichment specialist, personal communication 24 July 2021.)
The BOH [(Back-of-House)] acoustic environment is divided into two types: beneficial sounds and stressful noise. Table 2 gives an overview of acoustic problems and opportunities. [for Table 2, see the PDF]
Noise is defined as any sound that is undesired or interferes with one’s hearing (
Definition of NOISE) (accessed on 18 June 2022). What is considered noise is species-specific and dependent on the individual, and noise from both within and outside BOH areas can be a stressor
[75–77]. Soltis noted the following:
“A general guideline for noise exposure to animals may be difficult due to, among other things, different frequency ranges of hearing for different species and individuals; species and individuals’ reactivity; differences between chronic and acute exposure to noxious sound; and differences in characteristics of sounds” (J. Soltis, sensory environment researcher, Disney Animal Kingdom personal communication, 8 February 2019).
For detailed discussions of harmful effects of anthropogenic acoustic environments, see [77–79]. Several types of noises and their potential impacts on animals have been studied
[80–82]. BOH areas may provide a haven when animals seek to evade external noise generated by visitor crowds, entertainment, or construction or when building acoustic insulation is inadequate. External noise may be especially vexing when it coincides with the animals’ natural rest periods.
Built-in BOH internal noise sources include clanging steel gates and counterweights, mechanized gate operating systems, ventilation systems, plumbing, and pumps. The noise of the ventilation in one orangutan BOH exceeded external concert noise [77]. Available technologies exist for noise reduction. Built-in internal noise sources can be avoided with proper design and construction but are difficult to correct in existing facilities. Pressure washers are a common internal noise source. When multiple species are housed within hearing-range of each other, species-typical sounds from one species may become disruptive noise to another. Noise-cancelling technology works best for predictable, constant noise in highly localized areas (J. Soltis, personal communication, 8 February 2019) and may be effective in providing a degree of acoustic isolation in small primate nest boxes, for example.
Once noise is counteracted, it is possible to add BOH soundscapes. Anthony, noting the lack of data on noise tolerance in laboratory animals, suggested creating acoustic environments similar to those found in the species’ natural habitats [82].
Here are the citations, if anyone wants to follow-up on them. 77 to 79 seem to address the matter (I'll probably take a look as well, when I have time.):
75. Clark, F.; King, A.J. A critical review of zoo-based olfactory enrichment. In Chemical Signals in Vertebrates 11; Springer: New York, NY, USA, 2008; pp. 391–398.
76. Orban, D.A.; Soltis, J.; Perkins, L.; Mellen, J.D. Sound at the zoo: Using animal monitoring, sound measurement, and noise reduction in zoo animal management. Zoo Biol. 2017, 36, 231–236. [CrossRef] (
http://doi.org/10.1002/zoo.21366)
77. Morgan, K.N.; Tromborg, C.T. Sources of stress in captivity. Appl. Anim. Behav. Sci. 2007, 102, 262–302. [CrossRef] (Redirecting) [I found it here (p. 265 begins the noise discussion): https://www.researchgate.net/publication/228667970_Sources_of_stress_in_captivity_Appl_Anim_Behav_Sci#:~:text=These include abiotic, environmental sources,and uncomfortable temperatures or substrates.]
78. Cronin, K.A.; Bethell, E.J.; Jacobson, S.L.; Egelkamp, C.; Hopper, L.M.; Ross, S.R. Evaluating mood changes in response to anthropogenic noise with a response-slowing task in three species of zoo-housed primates. Anim. Behav. Cogn. 2018, 5, 209–221. [CrossRef] (Evaluating mood changes in response to anthropogenic noise with a response-slowing task in three species of zoo-housed primates — Animal Behavior and Cognition) [this is a direct link to the PDF of the article: https://www.animalbehaviorandcognition.org/uploads/journals/19/AB_C_2018_Vol5(2)_Cronin et al.pdf]
79. De Queiroz, B. How Does the Zoo Soundscape Affect the Zoo Experience for Animals and Visitors? Ph.D. Thesis, University of Salford, Salford, UK, June 2018. Available online [link from Coe's PDF corrected here, link to the PDF is on the right side of the page]: How does the zoo soundscape affect the zoo experience for animals and visitors?
80. Wells, D.L.; Coleman, D.; Challis, M.G. A note on the effect of auditory stimulation on the behaviour and welfare of zoo-housed gorillas. Appl. Anim. Behav. Sci. 2006, 100, 327–332. [CrossRef] (
Redirecting)
81. Anthony, A. Criteria for acoustics in animal housing. Lab. Anim. Care 1963, 2, 340. [PubMed] (
Criteria for acoustics in animal housing - PubMed)
82. Robbins, L.; Margulis, S.W. The effects of auditory enrichment on gorillas. Zoo Biol. 2014, 33, 197–203. [CrossRef] (
http://doi.org/10.1002/zoo.21127)