342 Diane W. Davidson and Steven C. Cook
these food types are dispersed and unpredictable
in space and time, favoring their location and
collection by widely and independently search-
ing workers; for example, EFN production often
varies with leaf age (McKey 1984). Interspecific
variation in search intensity suggests additional
functional group structure among leaf-foragers
(Cook and Davidson in preparation). Predatory
taxa like New WorldCamponotus sericeiventrisand
Paraponera clavata(isotopic evidence in Davidson
et al.2003 and Tillberg and Breed 2004) visit
leaves in a coarse-grained way, either out to the
tip and back, or a trip around the leaf edge.
Fine-grained searchers, which directly contact a
large fraction of each visited leaf, include other
New WorldCamponotus(e.g., members of sub-
generaMyrmobrachysandMyrmaphaenus), most
CephalotesandPseudomyrmexspecies, and many
Old World Polyrhachis, Echinopla, Camponotus
(Colobopsis), andTetraponera.
Kaspari and Weiser (2000) have noted
a bimodal distribution of body sizes in a
Panamanian community of arboreal and terres-
trial ants, and this finding is consistent with
our experience in western Amazonia (authors’
unpublished data). Smaller-bodied taxa compris-
ing the lower mode are abundant and important
elements of the tropical arboreal fauna but do
not fall neatly into either of the above two major
functional groups. Most small-bodied exudate-
foragers, including manyCrematogaster,Wasman-
nia,Azteca, andTechnomyrmexspecies, tend to
forage with nestmates, in accord with their size-
limited forage capacities. However, liquid-feeding
performances vary, and consistent with their ple-
siomorphic proventriculi (DeMoss 1973), tested
Crematogasterspecies exhibit relatively poor feed-
ing performances (both load sizes and uptake
rates; Davidsonet al.2004). Nevertheless, they
can dominate resources to the exclusion of much
larger ants by virtue of powerful contact tox-
ins (Daloze et al.1986). The same may be
true ofWasmannia, defended by a potent sting
(Howardet al.1982), though its proventriculus
has apparently not been studied. In contrast, two
dolichoderines with moderately derived proventri-
culi (Eisner 1957) exhibit relatively rapid liquid
uptake (Technomyrmex) or large load capacities
(Azteca) for their body sizes (Davidsonet al.2004),
as well as volatile alarm/defense secretions that
can quickly attract nestmates (Do Nascimento
et al. 1998 and Brophy et al. 1993, respec-
tively). Future research may resolve the question
of whether these genera differ on average in their
capacities for resource defense versus exploita-
tion, as well as whether small-bodied taxa are
constrained to forage nearer to nest sites or
pavilions.Foraging functional groups and ant
community structureBased on emerging knowledge of forag-
ing functional groups in tropical arboreal
exudate-foragers, can we predict how member-
ship in particular groups should affect inter-
specific interactions and community structure?
First, some have argued that dominant status
and true spatial territoriality (see Hölldobler and
Wilson1990)aremainlyafeatureof trophobiont-
tenders (Blüthgenet al.2004b). However, our
own studies reveal that many leaf-foragers defend
spatial territories centered around live nest
trees (Joneset al.2004, for AsianCamponotus
(Colobopsis) in the species-richcylindricusgroup,
andDavidsonetal.2007,forneotropicalCampono-
tus sericeiventris). Therefore, territoriality appears
to correlate more strongly with expectation of
long-term gain from spatially defined resources
than with foraging functional groupper se.
In contrast, and correlated with differences in
proventricular anatomy, large-bodied formicines
and dolichoderines may have diverged early
on in ways that differentiate both their con-
temporary roles as exploitative versus interfer-
ence competitors and their potential roles as
predators and scavengers in tropical forests.
With greater numbers of independently search-
ing workers and disproportionately large crop
capacities and rapid uptake rates, leaf-foragers
may be superior exploitative competitors for both
non-honeydew exudates and prey. In contrast,
densely populous foraging groups of trophobiont-
tenders (Figure 20.3) are likely better adapted
than are leaf-foragers for interference competition
over locally concentrated resources (see especially
Blüthgenet al.2004b).Thus, variousDolichoderus