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.
