Biology Reference
In-Depth Information
4.3
Bats
Nectarivorous bats occur mainly in the tropics and subtropics in two phy-
logenetically distant families; the Neotropical Phyllostomidae, derived from
insectivorous Microchiroptera that radiated into flower-visiting niches, and
the larger and generally less specialized fruit bats or Pteropodidae (Megachi-
roptera), which are restricted to Palaeotropical regions (Winter & von
Helverson, 2001). They feed on similar nectars to birds, sometimes sharing
flowers. The most specialized nectarivorous bats are small, long-tongued
bats of the phyllostomid subfamily Glossophaginae: like hummingbirds, they
hover to feed and need space for wing movement directly in front of flowers
(Westerkamp, 1990). There are often no barriers between bird and bat polli-
nation: examples are hummingbirds and phyllostomid bats pollinating Bur-
meistera tenuiflora (Campanulaceae) in Costa Rica (Muchhala, 2003), and
sunbirds and fruit bats pollinating Musa itinerans (Musaceae) in southwest-
ern China (Liu et al., 2002).
Depending on pollinator size, nightly nectar production by bat flowers
varies from 100 µl to several millilitres; the concentration is typically around
15% w/w and the nectar, like that of many passerine bird flowers, is domi-
nated by hexose sugars (Baker et al., 1998; Winter & von Helverson, 2001;
Wolff, 2006 and references therein). It has been suggested that high noctur-
nal humidities probably help to keep bat nectars dilute (Búrquez & Corbet,
1998). Schondube et al. (2001) demonstrated increased intestinal sucrase
activity in nectar- and fruit-feeding phyllostomid bats compared to insecti-
vores, although this seems unnecessary for digestion of hexose nectars. The
characteristic sugar composition of bat nectars is not a result of bats prefer-
ring hexoses to sucrose or digesting hexoses more efficiently (Herrera,
1999). Correlated with the water loading due to dilute nectars, the shift from
insectivory to nectarivory in phyllostomid bats has also been accompanied
by a decrease in renal concentrating ability (Schondube et al., 2001). Energy
acquisition by phyllostomid bats has been studied under both laboratory and
field conditions (von Helversen & Reyer, 1984; Winter & von Helversen,
1998, 2001). Because the nectar on which they feed is so dilute, these bats
consume about 150% of their body mass in a night's foraging, visiting indi-
vidual flowers repeatedly during the night and covering long distances. In
captivity, this involves alternating between different feeders in indoor flight
enclosures or wind tunnels, behaviour which has been very helpful for stud-
ies of their flight costs and aerodynamics (Winter & von Helversen, 1998).
Hovering is apparently less expensive for both bats and hummingbirds than
has been commonly assumed (Winter & von Helversen, 2001).
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