Reproductive behaviour, space-use and endocrine correlates of free-ranging impala (Aepyceros melampus) during the pre-rut, rut and post-rut phases

Abstract

Reproductive success is closely tied to an animal’s ability to adapt to social and environmental changes, particularly during physiologically stressful periods. Impala (Aepyceros melampus) are seasonally breeding, gregarious antelope and are considered a keystone species, given they contribute significantly to the game meat and tourism industry. Additionally, this species plays a crucial role in vegetation community composition, ecosystem functioning and facilitating the persistence of other species. Despite being an easily accessible study species, there is still limited information on how impala in the South African Highveld respond behaviourally and hormonally to the different phases of the rut. The aim of this study was to investigate reproductive behaviour, space-use and changes in faecal androgen metabolite (fAM) and faecal glucocorticoid metabolite (fGCM) concentrations in free-ranging male and female impala (Aepyceros melampus) over the pre-rut, rut and post-rut phases. As a pre-requisite, the phases of the rut had to be identified in the South African Highveld. Further, two enzyme immune-assays (EIA) had to be biologically validated to determine the most suitable assay for fAM quantification in this species. Lastly, changes in fAM and fGCM concentrations post-defaecation had to be investigated to assess how time and environmental variables affect the stability of hormone metabolites in faeces. Pre-copulatory (investigation, flehmen and following/chasing) and copulatory (mounting) behaviours were recorded as behavioural events and later converted to weekly frequencies. Additionally, latitude and longitude coordinates for breeding and bachelor herds were recorded during each phase of the rut and subsequently analysed. After the successful establishment of a non-invasive method for steroid hormone assessment, faecal samples were collected from male and female impala. Comparisons of the respective quantified hormone metabolite concentrations were made between males of different age classes and social ranks, the phases of the rut and across the two different sections of the Telperion Nature Reserve (TNR). The data indicate that the impala rut in the South African Highveld begins on 9 May and concludes on 15 June. Further, the observed differences in the frequency of reproductive behaviours within and across the phases of the rut provide insights into behavioural investment patterns. Breeding and bachelor herds adapted their space-use in response to the intra-specific interactions and resource availability. We found that the testosterone EIA was the best-suited assay for fAM analysis in male and female impala compared to the epiandrosterone EIA. The hormone stability study showed that immunoreactive fAM concentrations of male and female impala remained stable for up to 12h post-defaecation regardless of the exposed treatment. However, a sex-specific difference in fGCM post-defaecation stability was evident. The observed variation in fAM and fGCM concentrations among the compared variables is driven by a range of complex factors, including ethological, social and environmental aspects. This study increases our understanding of impala population dynamics, showing that they adapt their behavioural and hormonal responses to local social and environmental conditions associated with the rut to enhance reproductive success. As a result, reproduction cannot be viewed as an isolated event; instead, it must be understood in the context of the surrounding environment to fully capture its dynamics. The significance of understanding these aspects is emphasised in this study, as it may promote effective management strategies to ensure the long-term sustainability of impala and their ecosystem.