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and Neotropical bats. However, when these data are analysed at the generic
level, families such as Lamiaceae, Rubiaceae, and Gesneriaceae are charac-
terized by high mean sucrose percentages of 65.5%, 84.9%, and 75.7%,
respectively, and coefficients of variation (CV) of less than 30%. In contrast,
the large, specialized and diverse family Asteraceae gave a mean value of
40.0% sucrose, with a CV of 71% indicating great variation among the 60
genera analysed. Torres and Galetto (2002) demonstrated an evolutionary
trend towards generalist pollination systems in Argentinian Asteraceae, with
shorter corollas, higher hexose proportions, and more diverse visitors. Also
in Argentina, Galetto and Bernardello (2003) looked for convergence of nec-
tar sugars in plants pollinated by bees, moths, and butterflies in two widely
separated regions, and found plant phylogeny to be a stronger determinant
of nectar sugars than visitor guilds. Hexose nectars were characteristic of
Asteraceae, Fabaceae, Solanaceae, and Verbenaceae, whereas sucrose nectars
prevailed in Bromeliaceae and Onagraceae. In general, nectars from Patago-
nia demonstrated very high hexose contents, regardless of floral syndrome or
systematic relatedness.
Wolff (2006) examined the nectar of 47 species of Gentianales in a mon-
tane forest in Ecuador in relation to observed floral visitors. Nectar con-
centrations did not differ significantly between pollination systems, and
sugar composition was different only for fly- and bat-pollinated flowers,
which had higher proportions of hexose. The main difference among pollina-
tion systems was in the volume of nectar offered. This rather conservative
sugar composition seems to be a common finding in recent studies of South
American floral nectars (see also Chalcoff et al., 2006).
The dichotomy between high-sucrose nectars in hummingbird-pollinated
plants and predominantly high hexose nectars in sunbird-pollinated plants
was formerly attributed to the preferences of hummingbirds for sucrose and
the fact that some passerine birds are unable to digest sucrose and therefore
obliged to utilize hexose nectars (MartÃnez del Rio et al., 1992). This is, how-
ever, not true of more specialized passerine nectar feeders (Lotz & Nicolson,
1996). More recent work shows that the dichotomy between sucrose and hex-
ose nectars is not related to bird preferences, because both hummingbirds
and sunbirds show a lack of sugar type preference when the solutions offered
are equicaloric (Fleming et al., 2004). A more important factor may be the
general lack of overlap in the major plant families visited by hummingbirds
and by passerine birds (the genus
Erythrina
is a notable exception). Geo-
graphical consideration of bird-flower associations shows that they vary
greatly on different continents (Stiles, 1981).