Ecology, Behavior, and Conservation 15mixed conifer stands, C. C. Smith has noted that Tamia s c i-
urus systematically remove and larder-hoard the cones from
the most profi table species of trees, then concentrate on the
next most useful species in the forest, and so on, until cones
are harvested from all species.
Clearly the effi ciency with which tree squirrels select
and harvest cones places strong selection pressure on the
trees, as the tree characteristics do on the squirrels. C. C.
Smith fi rst suggested that attributes such as cone hardness,
most likely the result of serotiny, place strong selection on
squirrels for a larger body size and more robust jaw muscu-
lature to handle the cones. However, such adaptations in
squirrels also probably infl uence tree and cone characteris-
tics, such as the orientation and strength of the point of at-
tachment to the branch, as well as the number of seeds per
cone, all of which help to reduce squirrel feeding effi ciency
and deter cone predation. Recent studies by C. W. Benkman
and associates show how these evolutionary interactions
between squirrels and conifers may have resulted in mosa-
ics of selection across large geographic regions.
Tree squirrels that reside in hardwood forests and in
many parts of the subtropical and tropical regions of the
world also are seed predators, but they often double as both
seed predators and seed dispersers. In these environments
squirrels certainly exert selective pressures as seed preda-
tors, which is evidenced by the heavy outer seed coats of
many nuts (e.g., Carya and Juglans). Indeed, for some tree
species squirrels are their exclusive seed predators, suggest-
ing a long evolutionary history between squirrels and these
trees. For many genera of trees, however, tree squirrels also
function as a primary agent of seed dispersal. This compli-
cates the relationship; in a number of cases squirrels are
agents of dispersal and forest regeneration, even though
they depend on these seeds as a food resource (see below).
Squirrels are equally effi cient and well adapted for her-
bivory. Although granivorous tree squirrels seasonally shift
to herbivory only during periods of low food availability,
ground squirrels and prairie dogs have mastered the art of
specializing on plant material. However, at least one species
of Sciurus, Abert’s squirrel (S. aberti), has perfected the art of
herbivory. Found throughout the range of ponderosa pine
(Pinus ponderosa) in the southwestern USA, S. aberti is almost
entirely dependent on this conifer species for its survival.
The squirrel nests in the ponderosa and feeds on the imma-
ture female cones. But, after the cones mature and the seeds
disperse, the squirrel shifts its diet to the nutrient-poor
cambium and phloem of the terminal twigs of the tree. As
shown by M. A. Snyder, Abert’s squirrels are highly selec-
tive with respect to the trees in which they feed, eating the
cambium of the same trees each year. This selectivity, based
on the chemical characteristics of the xylem and phloem
and the concentration of limiting nutrients (e.g., sodium in
the phloem), results in repeated defoliation and strong selec-
tive pressure on the tree.a b(a) A Siberian flying squirrel (Pteromys volans) foraging on European aspen (Populus tremula) leaves. Photo courtesy Marko
Schrader. (b) An Indian palm squirrel (Funambulus palmarum) feeding on nectar. Photo courtesy Natasha Mhatre.