Alpine ungulates adjust diel activity to the natural return of wolves amid anthropogenic pressures

As wolves recolonise their historical range across Europe, ungulates face predation once more – but in landscapes profoundly altered by human activity. This shift raises crucial questions about their capacity to express adaptive antipredator behaviours. Using a quasi-experimental camera-trap design, we examined diel activity responses of ungulates along the ongoing wolf recolonisation in the south-eastern Alps. Red deer showed higher summer diurnality in sites with a longer history of wolf presence (7% increase over five years, on average) and progressively reduced nocturnality within sites as local wolf establishment advanced (5% decrease per year, on average), also heightening activity overlap with humans. This ‘diel shield effect' disappeared when human hunting occurred. Roe deer and Alpine chamois did not exhibit significant diel activity shifts in relation to wolves, though both species responded to human disturbance, with roe deer adjusting activity to hunting (18% less diurnal, on average) and chamois reducing diurnality in areas of intense outdoor use (up to 38% difference in diurnality between undisturbed and highly disturbed areas). Red deer, too, were less diurnal (up to 27% difference) and more nocturnal (up to 37% difference) in such highly disturbed areas, as well as near human settlements (up to 42% difference in diurnality between remote areas and villages). Our findings show that wolf recovery can induce detectable diel activity shifts in large herbivores over relatively short timescales, yet responses depend on species biology and behavioural plasticity. Importantly, human risk and disturbance can offset or override these behavioural adjustments, potentially altering the ecosystemic effects of returning large carnivores