Résumé
1. Diel migrations (DM; back and forth diel movements along an ecological
gradient) undertaken by prey to avoid predators during the day have been
demonstrated in many taxa in aquatic ecosystems. In terrestrial
ecosystems, prey often shift between various vegetation types whose cover
determine their vulnerability (i.e. likelihood of being killed when
attacked). 2. We conceptualized that in terrestrial ecosystems DM could
also occur, and that the contribution of DM and shifts in vegetation cover
use in reducing predation risk should depend upon the predator behaviour
and the correlation between encounter risk and vulnerability across
vegetation types. We further hypothesized that when the predator
distribution is predictable, terrestrial prey could have evolved DM
strategies taking them away from the predator when it is active or
efficient. 3. We investigated whether plains zebras Equus quagga perform
DM in Hwange National Park (Zimbabwe). There, zebras can forage in large
patches of open grasslands located near waterholes where they can also
easily detect predators. However, they are there at high risk of
encountering their main predator, lions Panthera leo, especially at night.
4. We found out that zebras employ a DM anti-predator strategy. Zebras
forage near waterholes during the day but move away from them at sunset,
when lions become active. We demonstrated that this DM, occurring over a
few kilometres, dramatically reduces their nighttime risk of encountering
lions, which generally remain close to waterholes. Zebra changes in
nighttime selection for vegetation cover types reduced their risk of
encountering lions much less. This may arise from a trade-off between
encounter risk and vulnerability across vegetation types, with zebras
favouring low vulnerability once DM has reduced encounter risk. 5. In
summary, here we (1) quantify, in a terrestrial system, the effect of a
predator-induced DM on the likelihood of encountering a predator, and (2)
distinguish the effects of the DM on encounter risk from those related to
day/night changes in selection for vegetation types. We discuss how prey
partition their risk between encounter risk and habitat-driven
vulnerability and why it is likely critical to understand the emergence of
anti-predator behavioural strategies.