About the Gorillas and other great apes. I am not a specialist but:
https://zslpublications.onlinelibra...M8ZRZ04Wvg51fH29uGkzudO151Pl1G-0J6GbMJ-N_WcNF
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Research indicates that regurgitation and reingestion (R/R) is a relatively common behaviour in zoo-housed great apes, with most studies to date carried out on Western lowland gorillas Gorilla gorilla gorilla and Chimpanzees Pan troglodytes. R/R is an abnormal behaviour because great apes are not anatomically adapted to regurgitate their food as part of their normal feeding processes, and because this behaviour has not been observed in members of the species living freely in the wild, in conditions that would allow a full behavioural range."
It states in the abstract of this review from 2018.
https://onlinelibrary.wiley.com/doi...ww2a3eyEVOb022QRZ92kfhnxbqKD3TtwPCh-Fmo6o2ip6
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Responses indicated that 60% of the AZA gorilla population engages in R/R to some degree and 24% of the population are observed exhibiting coprophagy on a weekly basis. With this preliminary information on coprophagy, and the realization that the Zoo community has been unsuccessful at eliminating R/R over the past 30 years, the next steps should be aimed at empirically assessing the potential drivers of these behaviors. Continuing efforts to further understand and eliminate R/R and coprophagy in zoo-housed primates is important not only because these behaviors are unsightly to visitors, but more importantly, R/R and coprophagy may signal deficiencies in current practices surrounding animal care." It states in the abstract of a publication of a survey from 2021.
This is something I definitely know quite a bit about... I've observed R/R behaviors when observing the gorillas my research team works with, and on one occasion observed coprophagy. The biggest challenge with these behaviors is that in the wild gorillas typically are foraging for large portions of the day. Given that zoos often feed calorically-dense chow mixes, it becomes very difficult to ensure both a natural foraging regimen and a healthy diet. Most zoos now feed gorillas more frequently throughout the day, which does help a lot, but some zoos now also take it further and have instituted "chow-free diets", first developed at North Carolina Zoo, which allow gorillas to spend much more of their days foraging and spending less time being inactive or with abnormal behaviors. Here's a short overview of this diet:
Zoo Research: A Diet with A-peel! | North Carolina Zoo.
The unfortunate aspect of the chow-free diet is that it doesn't work in all climates, given that fresh browse is an essential component of the chow-free diet, however it has been beneficial from both physical health and activity budget perspectives in zoos which have attempted it.
"Responses indicated that 60% of the AZA gorilla population engages in R/R to some degree and 24% of the population are observed exhibiting coprophagy on a weekly basis. With this preliminary information on coprophagy, and the realization that the Zoo community has been unsuccessful at eliminating R/R over the past 30 years, the next steps should be aimed at empirically assessing the potential drivers of these behaviors. Continuing efforts to further understand and eliminate R/R and coprophagy in zoo-housed primates is important not only because these behaviors are unsightly to visitors, but more importantly, R/R and coprophagy may signal deficiencies in current practices surrounding animal care." It states in the abstract of a publication of a survey from 2021.
I would not consider this either as thriving or doing well. These behavioural disorders are just signs that they are being deprived of something in their environment. No one would say that a person who engages in regurgitation or coprophagy (those cases exist) is doing well. So why would gorillas be doing well while displaying such disorders?
Well coprophagy is not a "behavioral disorder" tied to captivity, as it has been documented in wild gorillas on a number of occasions too (e.g.,
https://onlinelibrary.wiley.com/doi/10.1111/aje.13003), and is even more common in wild chimpanzees. Coprophagy is thought to have nutritional benefits as it allows the animals to get maximum nutritional intake from their food.
R/R is a different story, however. That said, even if "60% of the AZA gorilla population engages in R/R to some degree", doesn't mean that all of them have a behavioral disorder. I've observed R/R in all five gorillas I've observed at Buffalo Zoo, and yet only one of the five displays the behavior on a frequent basis. While yes, it is concerning when it occurs frequently, just as other stereotypies like pacing, excessive grooming, etc., would be, given that there is only one possible known physiological impact (I'll get to this later) of R/R, I don't think it is overly concerning to see a single occurrence of R/R.
The added complexity of R/R behavior, at least within the context of Buffalo Zoo's gorilla troop, is we observe R/R behavior more frequently after gorillas were fed a preferred food item- for example it occurs more if fed beets or carrots than if fed zucchini or green beans. Surely there is a benefit to allowing animals to eat their preferred food items (obviously in healthy quantities and not as their exclusive diet), so removing these items as a way of decreasing R/R is not likely to improve the gorilla's welfare.
As for the physiological impact, there was work done at Cleveland Metroparks Zoo which showed a correlation between the frequency of R/R behavior and higher insulin levels. I can't seem to find the paper on it right now, however it isn't yet known whether there is a causation here (or in what direction). Certainly a good avenue for future research, however, as this could provide the key to better understanding this abnormal behavior.
but saying that they do well in zoos is a dangerous statement. It just raises the perception that we do not have anything else to improve in the lives of these animals,
See, I don't agree that it raises that perception. I don't think it's dangerous or inaccurate to say that "such-and-such species does well in zoos", while acknowledging that there are still ways to improve their overall well-being. I don't think there is a single species where there isn't the possibility of future innovations improving their welfare (yes- including species like domestic dogs that we know substantial information on), but welfare doesn't need to be 100% perfect in order to be good.
In my opinion, for a species to do well in captivity, they should not display any behavioural/health disorder that would not be found in their wild counterparts. I am not a specialist in every taxa (not even gorillas) so I am not aware of any case as such. Even our pets often suffer from disorders that are not seen in wild animals.
Yet any species will display behavioral/health disorders when housed in the wrong circumstances. Even if say, gorillas at "Zoo X" display certain behavioral disorders which may be evidence of them not thriving, that doesn't mean the gorillas at "Zoo Y" don't thrive under human care. Animal welfare should typically be analyzed at the level of the individual animal, so making sweeping generalizations that "this species does well (or does poorly) in zoos" will seldom be 100% accurate.
It's also important to question why certain behavioral or health conditions exist in captive populations but not in their wild counterparts. If a wild serval suffers a severe leg injury, there is a high chance that it won't survive, and yet under human care that serval can get a leg amputation and then continue to live a potentially high-quality life. Are various heart diseases and cancers more common in zoo animals evidence of the animals not thriving, or are they evidence that animals which don't die prematurely from disease, predation, or starvation are more likely to develop age-related illnesses? Are behavioral abnormalities a side effect of living in zoos, or do they reflect the fact wild animals with a behavioral abnormality will likely die before reaching adulthood?