Tropical Forest Community Ecology

Tropical Arboreal Ants 341

Defining foraging functional groups

Correlations between proventricular anatomy
and diet suggest potential utility of these struc-
tures in defining taxonomically based niche differ-
ences among the ants. Foraging performances of
relatively large-bodied formicines and dolichoder-
ines correlate with both proventricular anatomy
and (more loosely) membership in one of two
broad foraging functional groups defined by
predominant worker activities (Davidsonet al.
2004). “Trophobiont-tenders” include all studied
dolichoderines and a small subset of formicines
with atypically slow uptake rates. In contrast,
most formicines are “leaf-foragers,” defined oper-
ationally by regular searching of leaf laminae by
solitary workers. Although ants in the two cate-
gories certainly overlap in their activities, foraging
modes differ on average (Davidsonet al.2004,
supplementary online dataset). With the most
rapid, body size-adjusted uptake rates and the
largest load sizes, sepalous formicines are better
equipped to forage solitarily than are compara-
bly sized species ofDolichoderuswith plesiomor-
phic proventriculi and much slower drinking
rates. In contrast,Dolichoderusspecies specialize
in aggregate trophobiont-tending (Figure 20.3),
with nestmates present to assist one another
in handling honeydew production that, where
measured, is gradual (Tjallingii 1995, Yao and
Akimoto 2002). Workers may not then have been
selected for rapid liquid uptake, and a require-
ment for microbial assistance in food processing

in the hindgut may also slow digestion (Cook

and Davidson 2006). In bothDolichoderusand

highly trophobiont-tending formicines (e.g.,Oeco-

phylla spp.), workers do not stray far from

nestmates (Dillet al.2002), and unlike most

formicines, are not systematic leaf searchers

(Davidsonet al.2004). Guarding trophobionts

both day and night, workers regularly com-

mute over chemical trails (Hölldobler and Wilson

1990). Compared with the monotonous lifestyles

ofDolichoderusspecies,foragingmodesof sepalous

formicines are many and diverse, perhaps due to

their capacity for independent foraging (Davidson

et al.2004).

Additional foraging functional groups almost

certainly remain to be defined within both

leaf-foraging and trophobiont-tending exudate-

foragers. Trophobiont-tenders might specialize

on certain categories of sap-feeders based on

ease of controlling these associates, and/or on

resource quality (including CHO:N ratios) and

the plant parts from which they feed. For exam-

ple, Coccidae are mobile only as tiny crawlers

and cannot later be relocated to new, relatively

N-rich growth. Nevertheless, they might be more

easily controlled by small-bodied ants than are

Membracidae, mobile at all developmental stages

and sizes. Leaf-foragers exhibit diverse habits,

searching leaf laminae for EFNs, cast-off honey-

dews and wound secretions, pollen and fungal

spores (Wheeler and Bailey 1920, Andrade and

Baroni-Urbani 1999), prey, and even bird drop-

pings (potentially recyclable urate N). Several of

Figure 20.3 Crematogaster ants tending
hemipteran egg mass from which nymphs
will emerge and be tended individually.