The diel niche of brown bears: constraints on adaptive capacity in human-modified landscapes

Diel activity rhythms, representing the behavioral pattern of the
sleep-wake cycle, may be adjusted by wildlife in response to changes in
environmental conditions. An increase in nocturnality is typically
recognized as an adaptive strategy to segregate from humans and mitigate
heat stress. Numerous studies have investigated spatial patterns and
habitat use of large carnivores in human-modified landscapes, but little
research has examined their activity rhythms. We compiled Global
Positioning System data (2004–2022) for 139 brown bears (Ursus arctos)
from 6 populations across Europe, representing a human-modified landscape,
and the Greater Yellowstone Ecosystem, U.S.A., representing a landscape
with limited human impact, which we used to calculate hourly movement
rates as an activity proxy. Using a Bayesian approach to model the
temporal autocorrelation of activity data, we tested if the extent of
nocturnality in brown bears is modulated by the intensity of human
encroachment, accounting for primary productivity and maximum ambient
temperature. All bear populations exhibited a predominantly bimodal,
crepuscular pattern of activity, although Yellowstone bears were
proportionally more crepuscular and diurnal. Whereas the effect of primary
productivity was variable, all European populations became more nocturnal
in response to higher human encroachment and reduced diurnal and
crepuscular activity at higher summer temperatures, decreasing overall
diel activity levels. Yellowstone bears displayed the greatest shift
towards nocturnality among all populations in response to increasing human
encroachment, and increased nocturnal activity to compensate for lower
diurnal and crepuscular activity at higher summer temperatures. Our
research indicates that European bears in human-modified landscapes may be
reaching a limit in the behavioral plasticity they can manifest in their
activity patterns, being already constrained into increased nocturnality.
Our findings enhance the understanding of brown bears adaptive capacity to
accommodate future changes, such as urbanization and increasing
temperatures, to the ecosystems they inhabit.