Testing the microclimatic effects of sprinklers aimed at mitigating heat stress in flying-fox roosts

Context: Extreme heat events are a serious concern for the conservation management of wildlife. In flying-foxes (Pteropus spp.), exposure to air temperatures (T_a) > 42°C can result in mass mortality, sometimes at catastrophic scales. To mitigate the worst of the impacts on flying-foxes, sprinklers are increasingly being deployed in roosts to cool flying-fox roosting habitat and/or the flying-foxes directly. However, while anecdotal reports suggest positive outcomes from these interventions, the effects of sprinklers on microclimatic conditions and flying-fox thermoregulatory responses have not been studied empirically. Aims: We aimed to test experimentally the impacts of sprinklers on microclimatic conditions of a flying-fox roost and so provide a much-needed evidence base for flying-fox heat stress mitigation. Methods: We used an automated split-system sprinkler setup in the understory of a permanently occupied grey-headed flying-fox (P. poliocephalus) roost site in Campbelltown, New South Wales. High-resolution weather data were systematically collected at sprinkler and control areas throughout the roost across a range of daily meteorological conditions between the austral summers of 2020–2021 and 2023–2024, including during an extreme heat event that resulted in mortality of flying-foxes across the region. Key results: Our results showed that on days ≥ 35°C, sprinklers cooled T_a by 1.5°C and increased dewpoint (T_dew) by 1.2°C, on average; and during an extreme heat event, the sprinklers kept the local microclimatic conditions within known thermoregulatory tolerances of flying-foxes, in contrast to the lethal conditions that were observed elsewhere in the roost. The effects of the sprinklers were highly localised to the treatment area both horizontally and vertically; and the timing and duration of effect differed for T_a and T_dew. Conclusions: This study made important progress in identifying the impacts of understory-based sprinklers on roost microclimatic conditions, and the results are promising for the utility of sprinklers for flying-fox heat stress mitigation. Implications: To understand where, when, and how sprinklers can be used to achieve net-positive outcomes for heat stressed flying-foxes, it remains necessary to determine directly how this management intervention influences flying-fox thermoregulatory responses.