BULLETIN FLORIDA STATE MUSEUM


 What mechanisms permit so many similar species to occupy tropical
 forest communities?
 Orians (1969) concluded that tropical forests support more bird
 species than temperate forests because of two attributes unique to the
 tropics. First, many kinds of food items that are ephemeral in
 temperate latitudes are available year-round in tropical latitudes (e.g.
 fruits and insects). Second, vegetation structure in tropical forests is
 much more heterogenous than in temperate forests, thus providing
 more microhabits for foraging specialization (e.g. more vertical layers,
 bromeliads and lianas).
 The great variety of year-round food items and habitat heterogenei-
 ty explain to some degree the abundant numbers and kinds of bat
 species in the tropics. All Nearctic bats north of 320 N latitude are ful-
 ly insectivorous. In Panama 42 species are probably fully insec-
 tivorous, but the other 63 species feed on fruit, pollen and nectar, fish,
 vertebrates, blood, or some combination of the above. At least 10 in-
 sectivore species in Panama feed to some extent by gleaning foliage,
 whereas only one bat species in South Carolina does so. A number-of
 emballonurids appear to feed much of the time on aerial insects that
 are flying around foliage, flowers, or fruits. Several species feed chiefly
 on insects found over water or on its surface (Hooper and Brown 1968,
 Gardner 1977).
 A third factor related to the high species diversity of the tropics is
 the great range in food particle sizes. Flowers, fruits, and insects
 available to bats all range widely in size. I earlier demonstrated that
 large canopy frugivores select fruits in proportion to their body size. It
 is worth considering whether such a mechanism is a common means of
 partitioning food resources both at intra- and interspecific levels of the
 bat community.
 Hutchinson (1959), McNab (1971a and b), and May (1973) all
theorized that similar species may avoid competition for food by differ-
ing in body weights by a factor of at least 1.3 (McNab and Hutchinson
use the figure 2.0). Each species thus specializes in food particles pro-
portional to its body weight, an easily measurable indicator of the
linear dimensions of food handling apparatus such as tooth row, gape,
tongue length, etc.
 Food size plays an important role in the partitioning of food
resources among similar species in many types of animals (e.g. Dia-
mond 1973, Brown and Liberman 1973) and may be particularly im-
portant for fruit bats because of the behavior of carrying fruits in
flight to feeding roosts. In accordance with Schoener's (1969) theory of
optimal foraging, each bat should attempt to maximize the amount of
food it harvests per unit of time and thus select the largest food par-


Vol. 24, No. 4