Denver Zoo

Anthropogenic activities cause the accumulation of biologically reactive nitrogen in ecosystems worldwide, leading to substantial changes in plant community structure and function, particularly in nitrogen-limited grasslands. Responses of plant communities and primary productivity vary depending on the magnitude of eutrophication and climate of the ecosystem, yet the exact form of these relationships is largely unknown. Here, we report results from the first 5years of an experiment in which nitrogen was added at eight levels, ranging from 0 to 30gm-2 at two grassland sites bookending the broad precipitation gradient of the US Central Plains: (1) semi-arid shortgrass steppe and (2) mesic tallgrass prairie. This allowed us to examine the mediating effects of climate on short-term aboveground net primary productivity (ANPP) and plant community responses to nitrogen addition. Although nitrogen addition caused a decrease in plant species richness at both grassland sites, the two sites differed in their responses in ANPP and plant composition. At the shortgrass site, we found no effect of nitrogen addition at any level on ANPP, but compositional change occurred starting at 5gm-2. In contrast, ANPP at the tallgrass site increased at 5gm-2 then saturated, but no significant compositional change was observed. Collectively, these results provide two key insights: (1) ANPP and plant community responses can be decoupled with short-term nitrogen addition and (2) site-level water limitation can result in contrasting responses of grasslands to 5years of nitrogen addition, but with these effects manifesting at the same critical load of addition.

Abstract Adult male elephants (‘bulls’) have traditionally been housed alone in managed care (e.g., zoos, sanctuaries) due to their large size and increased aggression during musth, as well as the fact that they were long thought to generally lead solitary lives in the wild. We now know, however, that wild bull elephants regularly socialise both with females and other bulls, and they benefit from social interactions. This case study of socialisation of bull Asian elephants at Denver Zoo Conservation Alliance demonstrates that housing bulls socially enhances their welfare. Bulls that were housed with other bulls engaged in significantly less stereotypic behaviour than when housed alone. This reduction in stereotypy when housed socially was especially pronounced when bulls were in musth. Housing bulls together also reduced aggressive behaviour, increased affiliative (i.e., friendly) behaviour and provided opportunities for social learning. These findings highlight the benefits of socially housing bull elephants for enhancing their welfare in managed care. Information © The Authors 2025

In late 2019, SARS-CoV-2 spilled over from an animal host into humans, where it efficiently spread, resulting in the COVID-19 pandemic. Through both natural and experimental infections, we learned that many animal species are susceptible to SARS-CoV-2. Importantly, animals in close proximity to humans, including companion, farmed, and those at zoos and aquariums, became infected, and many studies demonstrated transmission to/from humans in these settings. In this study, we first review the literature of SARS-CoV-2 infections in tigers and lions and compare species, sex, age, virus and antibody detection assay, and types, frequency, and length of clinical signs, demonstrating broad heterogeneity among infections. We then describe a SARS-CoV-2 outbreak in lions, tigers, and hyenas at Denver Zoo in late 2021. Animals were tested for viral RNA (vRNA) for 4 months. Lions had significantly more vRNA in nasal swabs than both tigers and hyenas, and many individual lions experienced viral recrudescence after weeks of undetectable vRNA. Infectious virus was correlated with high levels of vRNA and was more likely to be detected earlier during infection. Four months post-infection, all tested animals generated robust neutralizing antibody titers. Animals were infected with Delta lineage AY.20 identical to a variant circulating at less than 1% in Colorado humans at that time, suggesting a single spillover event from an infected human spread within and between species housed at the zoo. Better understanding of epidemiology and susceptibility of SARS-CoV-2 infections in animals is critical to limit the current and future spread and protect animal and human health.IMPORTANCESurveillance and experimental testing have shown many animal species, including companion, wildlife, and conservatory, are susceptible to SARS-CoV-2. Early in the COVID-19 pandemic, big cats at zoological institutions were among the first documented cases of naturally infected animals; however, challenges in the ability to collect longitudinal samples in zoo animals have limited our understanding of SARS-CoV-2 kinetics and clearance in these settings. We measured SARS-CoV-2 infections over 4 months in lions, tigers, and hyenas at Denver Zoo and detected viral RNA, infectious virus, neutralizing antibodies, and recrudescence after initial clearance. We found lions had longer and higher levels of virus compared to the other species. All animals were infected by a rare viral lineage circulating in the human population, suggesting a single spillover followed by interspecies transmission. These data are important in better understanding natural SARS-CoV-2 spillover, spread, and infection kinetics within multiple species of zoo animals.

As place-based conservation organizations, zoos are in a central position to support individuals in making small changes in their lives that will support the protection of wildlife and their habitats. This paper describes the secondary analysis of data collected from multi-phase front-end, exploratory evaluation that informed the development of a conservation action campaign in association with a non-profit, urban zoo. In phase one, internal organization staff were invited to attend workshops during which they brainstormed potential conservation actions that they felt were important for the zoo to promote. They identified and ranked 164 unique actions. In phase two, the ranking was used to narrow down the 164 actions to 20 actions which were used to develop a survey administered to visitors who opted in to receiving online surveys from the zoo. The survey asked participants to state their interest in each of the 20 conservation actions. The Transtheoretical Model of Behavior Change informed the analysis of responses. Through this approach we identified actions that people were already doing, interested in doing, and not interested in doing. The responses from this survey were used to narrow down the list further to 10 actions used in a survey in phase three. This second survey administered to zoo visitors on grounds asked participants which of the 10 actions they would be most interested in doing, and the perceived barriers and benefits of doing them. This process allowed us to use evidence-based decision making to choose which conservation actions would resonate most with the community for our conservation action campaign. We also were able to identify values visitors held that might influence environmentally friendly behaviors. Visitors who responded to this survey tended to respond in ways that aligned with self-transcendent values. The research suggests that the campaign should focus on habitat restoration and remediation and purchasing wildlife friendly coffee and other products.

Cownose rays (Rhinoptera bonasus) are susceptible to ocular disease with their prominent globes, but despite being popular animals housed in aquaria, there is little published information about their normal ocular anatomy and common pathologic ocular findings. A total of 63 live cownose rays (CNR) from three unrelated, separately housed groups had ocular examinations, and 5 adult rays were selected for ocular ultrasound. All examinations were performed out of the water, and most without anesthesia. Clinical findings were described, categorized, and scored by severity. Sixty-two of 63 rays (123 eyes) had clinical abnormalities, including 110 eyes with corneal pathology (mild = 76, moderate/severe = 34) and 74 eyes with intraocular pathology (mild = 44, moderate/severe = 30). Grey-to-white corneal opacities were the most common pathology (n = 58 rays/100 eyes) followed by cataracts (n = 41 rays/58 eyes), then persistent (or dysplastic) pupillary membranes (n = 14 rays/15 eyes). Most pathologic findings appeared inactive, but one aquarium had several CNR with active ocular pathology. There was a significant association between the diagnosis of moderate/severe corneal and intraocular pathology with age (P = 0.008 and P = 0.014, respectively) and weight (P = 0.001 and P = 0.039, respectively), as well as moderate/severe corneal pathology and group sampled (P = 0.03). There were no other significant variables identified. Additionally, histopathology of 14 eyes (11 rays) from two different facilities were examined, with keratitis (n = 8) and uveitis (n = 2) as the most common lesions. This study shows a high prevalence of pathologic ocular findings in cownose ray eyes with heavier adults more likely to be affected than lighter juveniles. Comprehensive ocular evaluation is important in this species and serial ocular exams and future studies should be pursued to monitor ocular disease progression and better understand possible etiologies.

Four female Japanese macaques (Macaca fuscata) from the same group as well as Wolf's guenon (Cercopithecus wolfi) and a Western lowland gorilla (Gorilla gorilla gorilla) from a second institution presented with vaginal foreign bodies in parallel with diseases of the urogenital tract or with endocrine disorders. These foreign bodies were associated with a mild to marked, diffuse vaginitis in all cases. Underlying pathological conditions consisted of a cavernous uterine hemangioma in a 20-year-old macaque, diffuse endometritis in a 21-year-old macaque, an in situ endometrial carcinoma in a 24-year-old macaque, endometritis and an ovarian cyst-like structure in the 27-year-old Western lowland gorilla, chronic cystitis and chronic renal disease in a 24-year-old macaque, and a history of hypothyroidism with irregular reproductive cycles in the 12-year-old Wolf's guenon. Vaginal foreign bodies have been reported in nonhuman primates used in biomedical research, but their concurrence with underlying conditions has not been explored. In women, vaginal foreign bodies have been linked to serious underlying medical conditions. This case series emphasizes the recommendation to investigate this abnormal behavior for underlying medical or adverse psychosocial conditions in primates under human care.

The Lake Titicaca frog is endangered due to threats such as water pollution, introduced species, and overharversting for markets, where people consume them as frog juice. This study, conducted June to November 2012, aimed to determinate the bacteria microflora living on the skin of frogs confiscated from the La Parada market, Lima, Peru, and housed individually in the Laboratory of Wildlife at the Faculty of Veterinary Medicine and Zootechnic of the Universidad Peruana Cayetano Heredia in Lima, Peru. Samples collected with sterile swabs and cultured on blood, tryptic soy, and MacConkey agars were investigated using commercially available test kits, to investigate the commonly encountered bacterial and potentially zoonotic microorganisms associated with their consumption. We found three species of zoonotic concern in the genus Vibro: Vibrio alginolyticus, Vibro cholerae, and Vibro fluvialis. Other Gram-negative species cultured included two different colonies of Aeromonas hydrophila, or Aeromonas caviae or Aeromonas sobria; Pseudomona luteola; one example of Weeksella virosa or Empedobacter brevis; and Citrobacter freundii. Gram-positive bacteria detected were Staphylococcus spp., Micrococcus spp., and Erysipelothrix rhusiopathiae. We recommend against the consumption of this frog due to the pathogens it may carry that could cause serious illness among consumers and in vendors who handle animals.

Abstract BackgroundEnteritis is a common cause of morbidity and mortality in lorikeets that can be challenging to diagnose and treat. In this study, we examine gut microbiota in two lorikeet flocks with enteritis (Columbus Zoo and Aquarium – CZA; Denver Zoo - DZ). Since 2012, the CZA flock has experienced repeated outbreaks of enteritis despite extensive diet, husbandry, and clinical modifications. In 2018, both CZA and DZ observed a spike in enteritis. Recent research has revealed that the gut microbiota can influence susceptibility to enteropathogens. We hypothesized that a dysbiosis, or alteration in the gut microbial community, was making some lorikeets more susceptible to enteritis, and our goal was to characterize this dysbiosis and determine the features that predicted susceptibility.ResultsWe employed 16S rRNA sequencing to characterize the cloacal microbiota in lorikeets (CZA n = 67, DZ n = 24) over time. We compared the microbiota of healthy lorikeets, to lorikeets with enteritis, and lorikeets susceptible to enteritis, with “susceptible” being defined as healthy birds that subsequently developed enteritis. Based on sequencing data, culture, and toxin gene detection in intestinal contents, we identified Clostridium perfringens type A (CZA and DZ) and C. colinum (CZA only) at increased relative abundances in birds with enteritis. Histopathology and immunohistochemistry further identified the presence of gram-positive bacilli and C. perfringens, respectively, in the necrotizing intestinal lesions. Finally, using Random Forests and LASSO models, we identified several features (young age and the presence of Rhodococcus fascians and Pseudomonas umsongensis) associated with susceptibility to clostridial enteritis.ConclusionsWe identified C. perfringens type A and C. colinum associated with lorikeet necrohemorrhagic enteritis at CZA and DZ. Susceptibility testing of isolates lead to an updated clinical treatment plan which ultimately resolved the outbreaks at both institutions. This work provides a foundation for understanding gut microbiota features that are permissive to clostridial colonization and host factors (e.g. age, prior infection) that shape responses to infection.