Higher and bigger: How riparian bats react to climate change

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The altitudinal distribution of animals and changes in their body size are effective indicators of climate change. Bats are sensitive to climate change due to their dependence on temperature during critical life stages. However, long-term studies documenting responses over extended periods are rare. We present a 24-year investigation of Myotis daubentonii, a riparian bat known for altitudinal sexual segregation, along a river course in Central Italy. While males occupy the entire river course, females are confined to downstream warmer areas supporting successful reproduction due to improved foraging site productivity. In 2000, females were absent above 900 m a.s.l in our study area. We hypothesise that a) this altitude threshold is now higher, due to thermal gradient changes along the river course; and b) thermoregulatory costs for reproductive females have declined, leading to increased energy investment in offspring and subsequent generational growth in bat body size. Confirming our hypotheses, females exhibited a 175-m upward shift in altitude limit. Furthermore, we found a concurrent increase in body size (but not condition). Temperatures increased in the 24 years, likely allowing females to extend their range to higher elevations and favouring an increase in newborn body mass. Riparian vegetation remained unchanged, excluding habitat quality changes as the cause for the observed responses. The rapid female elevation rise might imply future disruption of established social structures, altering intra- and intersexual competition for roosts and food. Given the global decline in insect populations, larger bats might face future difficulties in finding food to sustain their body size, increasing mortality. However, the full impact of such changes on bat fitness remains unexplored and warrants further investigation, including other bat populations. This knowledge is crucial for informing conservation in the face of ongoing climate change and preserving the ecosystem services bats deliver in riparian ecosystems.