**Background:** Habitat complexity refers to the physical geometry of an enclosure, including abiotic components (size, rocks, waterways, lighting) and biotic components (plant diversity, presence of conspecifics or other species). The authors argue that complexity is not static—it varies temporally (daily, seasonally, annually) and spatially, and its perception depends on the species' size, sensory capabilities, activity period, social structure, and life history. The paper aims to review how enclosure complexity influences zoo animal behaviour and physiology, and to provide practical guidance for exhibit design and management.
**Methods:** This is a narrative review. The authors synthesize existing literature on habitat complexity, environmental enrichment, animal welfare, and exhibit design. They do not conduct a systematic search or meta-analysis. The paper draws on examples from a wide range of taxa, including primates, carnivores, birds, reptiles, and marine species, and references studies on behavioural diversity, stress physiology (corticosterone, catecholamines), space use indices, and the Five Domains of Welfare framework.
**Key Results:** The authors report that increased enclosure complexity is generally associated with higher diversity of natural behaviours, reduced abnormal behaviours (e.g., pacing in red pandas), and improved physiological markers of welfare (e.g., decreased corticosterone and catecholamine release). Specific examples include: Amur tigers increased locomotion and decreased inactivity when given access to two different enclosures; laying hens showed lower blood glucose and heterophil:lymphocyte ratios when able to choose preferred habitats; Malayan sun bears in enriched enclosures showed better welfare based on behavioural measures; and chimpanzees used more enclosure areas and displayed more natural behaviours after moving to naturalistic, enriched enclosures. The authors also note that complexity can mitigate negative visitor effects—for example, five captive felid species housed in enclosures with hiding places showed fewer abnormal behaviours in the presence of visitors. However, they caution that excessive stimulation can be stressful, and that very complex enclosures may hinder animal visibility for visitors and complicate husbandry (e.g., difficulty locating sick animals). The paper emphasizes that complexity must be species-specific: a rainforest enclosure may be highly complex for a lizard but not for a monkey, and sensory capabilities (e.g., UV vision in birds, echolocation in bats) mean that human perception of complexity often differs from that of the animals.
**Clinical Implications:** The authors advocate for exhibit planning that integrates four key aspects: (1) sufficient space to allow appropriate complexity and stimulation; (2) naturalistic design and environmental enrichment to increase complexity; (3) alignment with educational goals to enhance visitor experience; and (4) dynamic architecture or manipulable structures that allow animals to control their level of complexity. They recommend using space use indices to evaluate enclosure zones and identify avoided or overused areas, and stress the importance of providing choice and control (e.g., access to indoor/outdoor areas, varied microclimates). The paper also highlights the potential role of epigenetics—specifically DNA methylation patterns—as biomarkers of environmental stress, suggesting that reduced stress from complex enclosures could improve the conservation value of captive populations. The authors conclude that science-based enclosure design benefits animal welfare, conservation, and education, and that ongoing evaluation and adaptation of complexity is essential.