Ecological drivers of group size in female Alpine chamois, Rupicapra rupicapra

In large mammalian herbivores, an increase in
herd size not only reduces predation risk but also energy
intake. As a consequence, the size of the groups made
up by herbivores is often assumed to be the outcome of a
trade-off depending on local predation risk and food availability.
We studied Alpine chamois ( Rupicapra rupicapra )
group size in a 3660 ha wide area (central-Eastern Alps,
Italy) where mouflon ( Ovis gmelinii ) were also present. We
walked seven hiking trails (spanning a total of 33.9 km) in
a high-elevation environment every 15 days from May to
October in 2007 – 2009, and located all the groups of freeranging
ungulates within our study site. We analyzed the
size variation in female groups with kids of Alpine chamois
in relation to (i) forage availability (as estimated using
the normalized difference vegetation index); (ii) distance
from safe areas; (iii) presence of potential competitor
species (i.e., mouflon); and (iv) environmental variables
(elevation, slope exposure). We found that forage availability
was one of the most important drivers of group
size patterns. Larger groups were found in meadows with
higher productivity, whereas smaller groups were located
in lower quality meadows in sheltered areas, indicating
how the female group size may be the outcome of both
resource availability and predation risk avoidance. Interestingly,
proximity of a mouflon group was able to modify
the size of Alpine chamois female groups, especially if the
nearest mouflon group was concurrently close and inclusive
of a higher percentage of rams.