Abstract 169 – Main drivers of brown bears’ circadian activity: a global assessment.

Aurora Donatelli, Dusko Cirovic, Mark Haroldson, Djuro Huber, Jonas Kindberg, Ilpo
Kojola, Josip Kusak, Gianluca Mastrantonio, Andres Ordiz, Slaven Reljic, Luca Santini, Frank T.
van Manen, Paolo Ciucci

The scope of our study was to evaluate effects of anthropogenic pressure, land productivity,
and ambient temperature on circadian activity rhythms of brown bears (Ursus arctos) at a
transcontinental scale. We quantified activity through hourly movement rates and used this as
the currency to compare activity of bears from the Central Apennines, Dinaric-Pindus, Greater
Yellowstone Ecosystem, Karelian, and Scandinavian populations. Based on a Bayesian modeling
approach, activity rhythms of bears in all populations were best represented by a bimodal curve
with crepuscular peaks. Bears at northern latitudes (i.e., Sweden and Finland) generally display
higher activity levels compared to the other populations, while bears in Yellowstone exhibit
slightly lower nocturnal activity, and higher diurnal and crepuscular activity compared to
various study areas in Europe. The overall effect of human disturbance is to increase
crepuscular/nocturnal activity and/or decrease diurnal activity in all study areas, with slight
differences across seasons. However, the magnitude of this effect is stronger for bears residing
in less disturbed landscapes, as they may be less habituated to human activity. In addition, all
populations tend to increase nocturnal activity and decrease diurnal activity, as a response to
higher daily maximum temperatures. Lastly, the effect of the Normalized Difference Vegetation
Index on circadian activity is much more variable across study areas and seasons, depending on
the key foods that characterize each population’s diet. Future research should focus on possible
effects of modifications in activity rhythms on survival and reproduction. Whereas flexibility in
large carnivores’ circadian activity is a key component of their adaptive strategies for co-
habiting with humans, drastic changes in activity may occur at the expense of the individual's
fitness, with potentially cascading effects across ecological communities.