Evaluation of anthropogenic disturbance on African clawless otter (Aonyx capensis) physiological stress, behaviour, and population density using non-invasive methods

Abstract

Land transformation for anthropogenic use is the leading cause of species declines globally, but some species are able to succeed in these environments. African clawless otters (Aonyx capensis) are the most widely distributed otter species in Africa and occur in a wide variety of habitats, including transformed landscapes. Thus, they are a good model species to investigate animal adaptions to anthropogenic environments. The aim of the study was to examine and compare the stress-related endocrine responses, population density, and behaviour of African clawless otters (ACOs) from a transformed area and natural areas using non-invasive techniques. An enzyme-immunoassay for measuring faecal glucocorticoid metabolite (fGCM) concentrations as a measure of stress in ACOs was established by comparing fGCM concentrations from a captive otter before and after a stress event caused by translocation to a different site. Thereafter, fresh faeces from ACO latrines were collected, and camera traps were set up at a transformed area (Millstream Farm) and two natural areas (Verloren Vallei Nature Reserve and Cobham Nature Reserve). Camera trap arrays, consisting of between 18 and 24 cameras, were placed on all three sites, recording otter presence for a total of 2439 camera days. From the five different enzyme-immunoassays (EIAs) tested, the cortisol and oxoaetiocholanolone (measuring 11,17 dioxoandrostanes) EIAs showed the highest response (74% and 48% increase, respectively) 30-, 24- hours after the stress event. For both EIAs, a desiccation experiment illustrated that alterations in fGCM concentrations after defecation is acceptable (≤ 16.2 %) for samples collected up to 3 hours postdefecation. Using the cortisol EIA for subsequent analyses, fGCM concentrations of animals from the transformed area (n = 20; mean (± SD): 0.468 (± 0.539) μg/g dry weight (DW)) were significantly higher (p = 0.019) than those from otters in the natural areas (n = 17; 0.242 (± 0.226) μg/g DW), with an overall difference of 220%. Using a random encounter model approach, the transformed area was estimated to have the highest density of ACOs (8.2 ± 2.3 km-2), whereas Verloren Vallei and Cobham Nature Reserve (natural areas) had estimated densities of 0.7 ± 0.2 km-2 and 2.1 ± 0.6 km-2, respectively. There was a significant difference (p = 0.007) between group sizes in the transformed area (detections = 112; group size range = 1 – 5) and natural areas (detections = 29; group size range = 1 – 3) and in otter activity time (p = 0.033, activity overlap = 66.5 ± 8.33 %) between Verloren Vallei and Millstream farm. With the newly established non-invasive method, this study demonstrates that ACOs show increased adrenocortical activity in a transformed environment. Conversely, the otters exhibit substantial behavioural plasticity to exploit the anthropogenic landscape. Transformed areas such as Millstream farm provide ACOs with suitable habitat and abundant food resources, evidently supporting higher than average otter population densities. Such densities likely exacerbate conflict with trout farm managers due to their perceived depredation impact on trout stocks. Further studies evaluating the potential causes of elevated fGCM concentrations and investigate approaches to reduce human-otter conflict will contribute to African clawless otter conservation.