Predation risk-induced stress in vertebrates: Are ungulates equally susceptible?

Predation risk is a pervasive non-consumptive effect that can elicit acute and chronic physiological responses in prey species. While glucocorticoids (GCs) are widely used as indicators of stress, the extent to which large mammals, particularly ungulates, exhibit hormonal responses to predation risk remains unclear. This review examines whether consistent patterns of physiological stress responses to predation risk occur among vertebrates, with a particular focus on ungulates, and identifies ecological or methodological factors underlying observed variation. A systematic literature search in Web of Science yielded 107 empirical studies measuring GC levels in vertebrates under predation risk. Studies were classified by stress type (acute vs. chronic), conditions (natural vs. experimental), and sampling method (invasive vs. non-invasive). Experimental studies included laboratory rodents. Across vertebrates, 54 % of studies reported elevated GCs under predation risk, while 46 % did not. Acute stress responses were more consistently detected, especially in experimental studies (66 %) and with invasive methods (69 %). Chronic stress effects were less evident, with similar numbers of studies showing increases or no change in GC levels. Among ungulates, 10 of 13 studies showed no clear hormonal changes, likely due to ecological and evolutionary adaptations or the influence of confounding environmental factors. Chronic stress from predation risk is inconsistently detected in wild vertebrates, particularly ungulates. Long-term exposure to predators may not always trigger measurable GC elevation, or prey may mitigate chronic stress via behavioural strategies. Future research should employ multi-method approaches to better capture the complexity of stress responses in large mammals.