Marcus Clauss and João Pedro Meireles argue that society accepts different forms of animal management in different contexts, yet often judges zoos by a different standard.

Great apes in zoos have a history of receiving diets resembling those of humans in industrialized societies, with ensuing obesity and comorbidities. We analyzed adult zoo body mass (BM) data of 1290 chimpanzees, 141 bonobos, 231 Bornean orangutans, 186 Sumatran orangutans, and 719 Western gorillas in comparison to BM of free-ranging conspecifics. Females and males considered overweight or obese were 62% and 45% in chimpanzees, 14% and 3% in bonobos, 88% and 81% in Bornean and 74% and 72% in Sumatran orangutans, and 67% and 12% in Western gorillas. The higher degree of overweight in females might also be linked to a lack of reproductive activity. Historical improvements, if at all, were evident in gorillas. We hypothesize that the main cause of obesity is the use of diet items that are low in fiber, high in easily digestible carbohydrates, and therefore highly palatable (such as cultivated fruit and pelleted or extruded compound feeds high in starch), instead of the recommended use of higher-fiber items. As zoo animals do not choose diets for themselves but are provided with the items they can choose from, it is the responsibility of zoos to provide primates with diet items that can safely be used in a group setting without the incentive of overeating and the risk of monopolization by dominant individuals, that is, most plausibly, high-fiber diets. For the optimal welfare of such long-living taxa, the focus under human care should shift from sheer longevity to health span, for which the prevention of obesity is key.

Obesity is an often-stated issue in zoo animals, including primates, and has historically been particularly emphasized for lemurs. By contrast, similar reports for colobine species-which are literally called 'slim monkeys' in at least one other language-are absent. This has been hypothsized to be linked to the colobine foregut fermentation system; diets high in easily digestible carbohydrates might disturb the microbiome in the foregut, similar to acidosis in domestic ruminants, leading to reduced intake and disease rather than obesity, but empirical data is lacking. We compared body mass (BM) data for 14 lemur species (7187 individuals) and 10 colobine species (1443 individuals) with literature data on free-ranging specimens to test this concept. In contrast to expectations, a similar percentage of species had a majority of individuals above the 'healthy' BM range in at least one sex (36% of lemur and 40% of colobine species). However, colobines had a higher percentage of individuals considered 'underweight', especially in the two rarely kept species with the more complex 'quadripartite' stomachs (Nasalis larvatus, Pygathrix nemaeus). Neither sample size, sexual size dimorphism, the degree of folivory, nor indications for a seasonal physiology were clearly associated with species' propensity for zoo obesity. There were no clear historical trends in the development of obesity in these species. We conclude that no general rules about species' susceptibility to obesity appear to apply. Body mass and condition of zoo primates should be monitored continuously, and feeding regimes possibly be adapted to contain diets of higher fibre levels more similar to those reported in natural diets.

Tapirs (Tapirus spp.) are large, long-lived herbivores native to Central and South America and Southeast Asia. All species are currently classified as Vulnerable (T. terrestris) or Endangered (other species) in the IUCN SSC Red List. With populations in the wild continuing to decline, the value of ex-situ populations as insurance populations increases. To assess demographic patterns, we evaluated juvenile and adult survivorship trends among the three tapir species (Tapirus bairdii, T. indicus, and T. terrestris) frequently kept in zoos from 1870 to 2024 using Cox proportional hazard models. Survivorship of Baird's tapirs was better than that of the two other species. Overall, both juvenile and adult survivorship has increased significantly. This was expected, given the developments in tapir husbandry over the past decades. Despite significant progress in husbandry practices, important areas for improvement remain, particularly in nutrition. Not all institutions provide optimal diets, which can affect health and longevity. In all three species, we document an increase in the proportion of old age-individuals and a decline in that of juveniles. As tapir survivorship increases and more individuals reach advanced ages, new challenges will arise in using controlled mortality to ensure viable populations.

Even without considering genetic diversity, managing zoo populations sustainably without applying controlled mortality faces a predictable dilemma: Holding space is limited, and zoos worth their salt will inevitably become better at reducing accidental (uncontrolled) mortality. For most species, longevity attainable under good husbandry conditions is considerably longer than generation time, and even though most animals can reproduce until death, there is reproductive senescence, where the probability of successful reproduction decreases with age. Thus, over time, available holding space will inevitably be filled by older animals of lower fecundity. Breeding recommendations that aim at exact numerical replacement to avoid surplus at all cost rather than accounting for average success rates (so that the number of offspring will overshoot the target by up to 50% of the time) will stochastically lead to declines in population size. For small populations, extinction in zoos thus predictably becomes a matter of time. Only large populations will be able to exist at high proportions of old animals, with waves of replacement cycles. Therefore, gaining public acceptance for respectful and humane controlled mortality is essential for the long-term sustainability of small zoo populations. Concepts of continuously increasing holding capacities, using frozen gametes to generate offspring when needed, or nonspecific hopes that compassionate conservation will align individual longevity and conservation goals, detract from the necessity to act. Details of welfare and education can support the call for controlled mortality, but it is the inevitability of the outlined process that obliges us to choose honestly between being empathetic observers of extinction or taking responsibility to conserve animal species.

Under conditions of dietary sodium (Na+) excess, the kidneys may fail to adequately excrete Na+, potentially compromising blood pressure homeostasis. Body tissues, such as skin, can offer sites of short-term extrarenal Na+ storage and previous research has shown that this can help guard against hypertension in small mammals (e.g., rodents). Large mammals have relatively greater Na+ storage potential, but whether extrarenal Na+ storage occurs for this group is unknown. Here, we report preliminary evidence of extrarenal Na+ storage in cattle. We provided a large pulse-dose of NaCl to four cattle (body mass: 720 kg) and measured excretion of Na+ and potassium (K+) in urine and faeces for a period of 7-days. Following NaCl administration, Na+ excretion spiked in both urine and faeces for 48 h before returning to baseline measurements. After 96 h, however, Na+ excretion increased again; a consistent physiological phenomenon across all individuals studied. We did not observe a pattern in urinary K+ excretion, indicating that the mechanism of Na+ storage does not appear to involve exchange for K+. However, faecal K+ excretion was reciprocal to that of Na+, presumably reflecting exchange of Na+/K+ across the walls of the large intestine. We infer that during the initial period of Na+ stress, short-term extrarenal Na+ storage occurred and the stored Na+ was later released only when the body had returned to Na+ homeostasis. Additional experiments are required to understand how patterns of Na+ regulation changes across body sizes and the specific body compartments involved. Cattle may be a useful model system for examining the impact of high Na+ intake in mammals larger than humans.

Improvements in wildlife husbandry mean that many zoo animals are living longer. This has put pressure on the finite holding capacity of zoos, which has often been addressed through a curtailing of reproduction to reduce population growth rates. Here, we explore how such actions have impacted the demographic trends of 774 mammal populations in European and North American zoo populations between 1970 and 2023. Irrespective of whether the data are clustered by region, taxonomic group, conservation status, or breeding program type, the proportion of old individuals has increased continuously, mirrored by a decrease in juveniles and actively reproducing adults. This aging demographic trend compromises the long-term sustainability of zoo populations and thus the ability of zoos to meet ex situ conservation goals. As the observed trends do not show signs of abating, reflection on current zoo population management is required.

Feeding practices are critical for the welfare of zoo-housed carnivores, yet zoo diets often lack the complexity of natural prey, potentially reducing stimulation and increasing stereotypic behaviours. This study examined how different food items combined with a fasting regime influenced the behaviour and activity budgets of three jaguars Panthera onca over eight weeks. Four feeding treatments were tested: featherless chicken, feathered chicken, beef meat without skin on bone (all with three feeding, four fasting days per week), and whole sheep or goat carcasses (two feeding, five fasting days per week). Behaviours were recorded continuously based on an ethogram, and beta regression mixedeffects models were used for analysis. Whole carcasses elicited the longest feeding times (88 +/- 21 minutes), compared to meat on bone (58 +/- 23 minutes) and chicken (32 +/- 10 minutes feathered, 12 +/- 4 minutes featherless) on feeding days. Large carcasses also encouraged unique behaviours such as food carrying, guarding, and manipulation, while providing intermittent feeding opportunities and possibly prolonged satiation. Although these differences were evident in feeding-related behaviours, overall activity budgets remained largely stable, and stereotypic pacing was not substantially affected by diet composition or fasting schedules. These findings underscore the enrichment potential of whole carcasses in promoting natural behaviours and enhancing feeding engagement. Zoos may benefit from incorporating more varied and naturalistic feeding regimes to improve animal welfare.

White Ceratotherium simum and black Diceros bicornis rhinoceroses differ in ecology and sociality, which is reflected in their ease of care in zoos. Zoo-housed black rhinos are susceptible to unusual diseases, like iron overload disorder and hypophosphatemia, that have hypothetically been linked to obesity. We applied published body condition scores (BCS; 1-5) to 84 adult white and 57 adult black rhinos living in 41 European zoos, established our own BCS protocol with overview scores and scores for individual body regions, based on our ability to distinguish scores on photographs, and related these to husbandry conditions. Exact body mass was available for 17 and 20 white and black rhinos, and was estimated for the rest. Scoring was conducted by one single observer, twice in a blinded manner. White rhinos were kept in larger enclosures and received a lower proportion of non-forage feeds than black rhinos. Contrary to previously published BCS protocols, the protocol presented here achieved a distinctively higher number of identical scores at repeated scoring from photographs for all evaluated body regions. BCS were generally positively correlated to body mass, but were not related to the percentage of non-forage feed in the individual diets. White rhinos had higher scores than black rhinos (e.g., BCS overview side 3.83 +/- 0.76 vs. 3.39 +/- 0.85), suggesting that obesity, if at all, is more common in white rhinos. Additionally, BCS were relatively independent of age in white rhinos but declined in black rhinos with age, supporting concepts of chronic, accumulating health problems in black rhinos. In black rhinos, BCS were lower in animals with impaired dental status compared to animals without such alterations. In both species, males tended towards lower BCS compared to females. BCS had no effect on female black rhinos breeding status, but tentative evidence suggested that non-breeding white rhinos show higher BCS. Our findings do not support concepts that associate black rhino health problems with obesity, but emphasize the relevance of strategies for maintaining long-term health. In white rhinos, the provision of ad libitum forage is considered beneficial from a behavioural point of view, but forages of sufficiently low nutritional (but appropriate) quality should be chosen to prevent overconditioning.

Free-ranging animals often ingest markedly different diets than their counterparts under human care, whether companion animals, production animals, or zoo animals. A nutrient with a prominent lack in the diet under human care across species is fiber. Dietary fibers are more difficult to digest than other nutrients and require microbial fermentation, typically resulting in an overall reduction of digestibility and metabolic efficiency. Nevertheless, or therefore, dietary fiber supports health, as it fosters a healthy gut microbiome and limits the risk of obesity and its comorbidities. And due to their lower energy density, high-fiber diets typically increase feeding time, and therefore reduce time available for undesired behaviors. As an adaptation to natural environments replete with high-fiber feeds, humans and animals alike have evolved gustatory preferences for low-fiber, energy-dense feeds. In zoo animal nutrition, the occurrence of both, potentially involuntary feeding of low-fiber diets due to uncritically following feeding traditions, and the deliberate deviation in fiber levels from natural diets, can be documented. Zoo staff appear to transfer their own evolved preference for high-energy feeds to the animals in their care, making zoo animal nutrition a showcase of cultural traditions clashing with biological knowledge. The fiber levels of zoo animal diets can therefore serve as a scale bar for how much zoo management is guided by biological knowledge as opposed to other factors (e.g., anthropomorphism). Increasing fiber levels, possibly to levels observed in the wild, holds great potential for further improvements in the health and behavioral management of zoo animals.

Food preference and selection of free-ranging giraffes have been well recorded, but limited data concerning the chemical composition of their diet and faeces are currently available. This pilot study added information on natural diet, nutritional quality and a description of faecal chemical composition of free-ranging giraffe in the Free State Province, South Africa. Measured crude protein levels in leaves were lower compared to previous reports. The leaf calcium (Ca) content was higher than the phosphorus (P) content in all samples, supporting concepts that giraffe may be P but not Ca limited. In the faecal samples of the present study, nitrogen and P concentrations were above the thresholds considered indicative of nutritional deficiency.

Estimates of energy intake are important for modelling population dynamics and assessing habitat quality of killer whales (Orcinus orca) and for husbandry under managed care. We estimated the energy requirements of four individual killer whales at Marineland Côte d’Azur in Antibes, France, using records of daily food intake, fish nutrient composition, and animal body mass over eight years for each animal. Relative metabolizable energy intake for maintenance (zero body mass change) and for average winter growth (0.07 to 0.09% body mass/d) were estimated at 667 to 892 and 1,054 to 1,237 MJ ME/kg0.75/d, respectively, representing two to four times the average mammalian basal metabolic rate. Seasonal fluctuations were observed in body mass as well as in food (kg) and energy (J) intake. Body mass, age, water temperature, season, and day length were all significantly correlated to energy intake, but models that accounted for day length fit the data distinctly better than models that accounted for water temperature, suggesting a photoperiodic control of intake. Intake generally increased from June to December and decreased from December to June. The turning points corresponded to winter and summer solstices, some two to three months prior to distinct seasonal changes in water temperature. We propose that changes in ambient temperature are not a strong stimulus for seasonal adjustment of food intake in killer whales. Given that turning points in body mass—presumably due to changes in blubber reserves—occur at a delay of two to three months after turning points in food intake, instantaneous adjustment of intake based on water temperature would often happen “too late” to adequately adjust the blubber layer for the thermoregulatory challenge. By using photoperiod to anticipate imminent temperature developments, killer whales and possibly other delphinid or even other marine mammal species adjust intake so that resulting extremes in body mass and blubber thickness parallel extremes in ambient temperature.

Iron overload disorder (IOD) is a common condition in ex-situ black rhinoceroses (Diceros bicornis), although it has not been reported in the wild. This study aimed to gain a deeper understanding of the relationship between 25-hydroxy vitamin D [25(OH)D], inflammatory markers, insulin levels, the gut microbiome, dietary components, and transferrin saturation (TS) in ex-situ black rhinoceroses. Blood and fecal samples from 11 black rhinoceroses at five different European zoological institutions were monitored over a 1-year period. Inflammatory markers such as interleukin 6 (IL-6), serum amyloid A (SAA), interferon γ (IFN-γ), and tumor necrosis factor α (TNF-α) were analyzed. Our study corroborates the findings of previous research, which demonstrated that insulin, inflammatory markers, and TS% are higher in ex-situ black rhinoceroses compared to published wild ranges. Our data show no correlations between insulin, 25(OH)D, TS%, inflammatory markers, or short-chain fatty acids (SFCAs). Serum 25(OH)D exhibited significantly higher levels in summer than in winter. Transferrin saturation was influenced by age, which is consistent with previous studies. The microbiome did not differ significantly among individuals, institutions, sex, or season, unlike the mycobiome, which exhibited significant differences across institutions. The impact of the mycobiome differences on the physiology of the animals could not be determined from this study.

In various domestic mammals, smaller breeds tend to have proportionally larger teeth, whereas this is not a universal trend across mammals. This suggests that body size can evolve faster than tooth size, leading to the prediction that tooth-body size scaling differs among closely related versus distant taxa. Here, we test this pattern in a new computed tomography (CT)Skriffer Utgitt au det Norske Vidensk-Akad scan dataset on 302 adult domestic rabbits of various breeds (maxilla and mandible) and compare this to 198 literature data from 20 nondomestic lagomorph species (maxilla only). Skull or mandible length served as body size proxies. The allometric scaling of the length of the cheek tooth row and the diastema, as well as the jaw width, with these proxies and with each other was investigated. In domestic rabbits, there is a negative allometric scaling between tooth row length and body size, that is, smaller animals have relatively larger teeth. Similar patterns appear within some wild lagomorph species, while across species, tooth row length and body size scale isometrically. These findings add to reports on allometric growth in domestic species. As the seeming advantage of disproportionately more chewing surface in smaller species is apparently lost over time, the postulate of an optimal tooth row size for an organismal design emerges, for which there is currently no theory. Possibly, veterinary experiences that dental problems are particularly frequent in small domestic breeds with relatively larger teeth might provide leverage for such a concept. Studies on the physiological effects of differences in relative tooth size between breeds are warranted.

Hydrogen and oxygen isotope ratios in proteinaceous tissues have been used for some time in migratory, ecological, and archaeological studies. While the result of isotopic variation in drinking water and diet has been investigated with controlled feeding experiments and studies in the wild, there are few controlled feeding studies that manipulate the diet components and diet type, and this across different taxa. In this experiment, the diet fed to rats, guinea pigs, and quail varied from plant-based to insect-based and meat-based pelleted diets. We report the diet to tissue offsets for δ2H (denoted Δδ2H) and δ18O (Δδ18O) of tissue-bound organic matter in two tissue types: muscle and dentine collagen. The diet to tissue offset varies by diet type in muscle of all three species, by up to 16 ‰ (Δδ2H) and 2 ‰ (Δδ18O). In dentine collagen, a range of ~20 ‰ in Δδ2H and ~1.5 ‰ in Δδ18O are observed across diets, though in a smaller number of samples. Additionally, we note large variation in Δδ2H and Δδ18O by tissue type (δ2H = ~60 ‰, δ18O = ~3–4 ‰) and more moderate differences by species (up to δ2H = 7.4 ‰, δ18O = 1.5 ‰). The difference in consumer tissue Δδ2H and Δδ18O by diet type is important to consider as a source of isotopic variability for some studies such as migratory research or diet or drinking water reconstructions and (palaeo-)climate inferences drawn from them, particularly in species that may vary their dietary habits.

Roughage can be a significant source of vitamin D in herbivore diets. The vitamin D content of forages can vary considerably and depends, among others, on plant species, plant maturity and climate fluctuations. Ultraviolet B (UVB) exposure causes conversion of provitamin D to vitamin D and thus represents the most important influence. To characterise this effect, two batches of a young-cut roughage of high hygienic and nutritional quality were exposed to constant artificial UVB-irradiation up to 192 h and were sampled at regular intervals. In previous studies, both ergocalciferol (vitamin D2), cholecalciferol (vitamin D3) and/or their metabolites have been detected in plants. Therefore, the present roughage was analysed for both D2 and D3. Whereas cholecalciferol never exceeded the detection limit of 0.25 µg/100 g, ergocalciferol increased from 2.22 µg to 19.15 µg/100 g dry matter in a time-dependent manner. Additionally, the values indicate an interplay with other factors not investigated in this study, for example, leaf-stem-ratio in the different samples. The high amounts of ergocalciferol in the experimental roughage after UVB-exposure confirm the value of hay as a vitamin D source for managed herbivores. The quick rise with time cautions against neglecting UVB-irradiation's effect on feed, both in practical as well as in experimental settings.

Rabbits have hypselodont (ever-growing) teeth; dental growth constantly replaces dental tissue lost due to abrasion and attrition. It has been suggested that rabbits must have high mineral requirements, in particular for calcium, to fuel this constant tooth growth. However, this assumption ignores the fact that tooth wear represents finely ground tissue that is not lost to the body but swallowed during food processing. No studies exist regarding the possible recycling of dental enamel and dentin calcium. In this cross-over study, eight-months old intact female rabbits were fed with a complete pelleted diet containing either calcium carbonate/dicalcium phosphate (control) or ground rabbit teeth as calcium sources for 15 days each. After 11 days of adaptation, intake, urinary and faecal losses were quantified in a 4-day period in metabolism cages, and blood mineral levels were determined in a final blood sample. There were few relevant, significant differences between the treatments, with the exception of calcium digestibility (20 +8 % on the control vs. 33 +6 % on the tooth meal diet, P = 0.002) and total calcium retention (0.10 +0.13 vs. 0.33 +0.10 g/d, P = 0.004). The results suggest that rabbits are well able to recycle the minerals from dental tissue lost during dental wear.

Erinaceus europaeus, the European hedgehog, was formerly very common but has recently suffered a decline across several European countries. There is a long-standing tradition of hedgehog rehabilitation centers across Europe with reported high rewilding success. Surprisingly, however, few data exist on the behavior of the animals while in these centers. To gain a better understanding of hedgehog behavior under husbandry conditions rehabilitation, we documented the behavioral repertoires of four hedgehogs over a 48-hour period, including behavioral observations and quantification of food consumed and distances covered per day. Most of the hedgehogs' time was spent resting, comprising about 20 h in a 24-hour period. The majority of activity occurred at night and was spent feeding, followed by locomotion. Daily food consumption (of a dry cat kibble) ranged from 31 to 71 g of dry matter in 16-41 feeding bouts (1.1-2.6 g/bout, 2.4-3.8 min/bout). The hedgehogs covered a mean distance of 34-335 m per day, with a mean speed ranging from 0.12 to 0.25 km/h. Compared to data from free-ranging specimens, the hedgehogs spent less time active and covered less distance per day. No stereotypical behavior was observed. These findings suggest that for the aim of rehabilitation, husbandry conditions are adequate; for long-term husbandry, modifications that facilitate more locomotion and more foraging behavior would be required.

The assimilation, retention, and release of nutrients by animals fundamentally shapes their physiology and contributions to ecological processes (e.g., zoogeochemistry). Yet, information on the transit of nutrients through the bodies of large mammals remains scarce. Here, we examined how sodium (Na), a key element for animal health and ecosystem functioning, travels differently through fecal and urinary systems of cows (Bos taurus) and horses (Equus ferus caballus). We provided a large dose of Na and compared its timing of release in feces and urine to that of nonabsorbable markers. Na excretion by urine occurred approximately twice as fast as excretion by feces, yet both were shorter than indigestible particle markers. These differences correspond to rapid absorption of Na in the upper gastrointestinal tract and transport by blood to the kidneys (urine Na excretion) or resecretion of Na into the lower intestinal tract (fecal Na excretion). Interestingly, for cows, we found a second peak of Na excretion in urine and feces > 96 h after dosage. This result may indicate that surplus Na can be rapidly absorbed and stored in specific body cells (e.g., skin), from which it is later released. Using a propagule dispersal model, we found that the distance of cattle- and horse-driven nutrient dispersal by urine was 31% and 36% less than the fecal pathway and 60% and 41% less than the particle marker pathway, which is commonly used to estimate nutrient dispersal. Future physiological and zoogeochemical studies should resolve different pathways of nutrient retention and release from large mammals.

Killing animals is a ubiquitous human activity consistent with our predatory and competitive ecological roles within the global food web. However, this reality does not automatically justify the moral permissibility of the various ways and reasons why humans kill animals – additional ethical arguments are required. Multiple ethical theories or frameworks provide guidance on this subject, and here we explore the permissibility of intentional animal killing within (1) consequentialism, (2) natural law or deontology, (3) religious ethics or divine command theory, (4) virtue ethics, (5) care ethics, (6) contractarianism or social contract theory, (7) ethical particularism, and (8) environmental ethics. These frameworks are most often used to argue that intentional animal killing is morally impermissible, bad, incorrect, or wrong, yet here we show that these same ethical frameworks can be used to argue that many forms of intentional animal killing are morally permissible, good, correct, or right. Each of these ethical frameworks support constrained positions where intentional animal killing is morally permissible in a variety of common contexts, and we further address and dispel typical ethical objections to this view. Given the demonstrably widespread and consistent ways that intentional animal killing can be ethically supported across multiple frameworks, we show that it is incorrect to label such killing as categorically unethical. We encourage deeper consideration of the many ethical arguments that support intentional animal killing and the contexts in which they apply.