NECTAR CHEMISTRY - Nectaries and Nectar - page 229

3.3 Is sugar composition determined by floral visitors

or common ancestry?

The sugar composition data used by Baker and Baker (1982a, 1983b) to cal-
culate sugar ratios remain largely unpublished today, so it is not possible to
compare the average percentages of sucrose in nectars consumed by the
various pollinator classes. Evidence is available from other publications,
however, and there has been considerable interest in the dichotomy between
nectars of hummingbird and passerine bird flowers. Stiles and Freeman
(1993) analysed the nectars of 112 species of bird-visited plants in Costa
Rica, and found high sucrose in all the putatively hummingbird-pollinated
species (mean 73% of total sugars). The sugar chemistry of both nectar and
fruit is correlated with bird and bat consumers (Baker et al., 1998): in this
large data set, species pollinated by hummingbirds had a mean nectar sucrose
concentration of 58%, while passerine-pollinated species in the Old and New
Worlds averaged only 8% and 3% sucrose respectively. Baker et al. (1998)
also examined their data on the basis of genera and families, because species
are not independent units and more intensive sampling of some genera than
others may distort the findings. Also with this in mind, Stiles and Freeman
(1993) verified that 23 species of the major hummingbird-pollinated genus
Heliconia had the same mean sucrose concentration as the rest of their sam-
ple. In a recent survey, 278 hummingbird-visited species were found to have
a mean 64.4% of their nectar sugar as sucrose, while 259 species of sunbird-
visited plants showed a bimodal pattern, some having high sucrose levels
but nearly half producing nectar with less than 10% sucrose (Nicolson &

Fleming, 2003). A similar dichotomy in sucrose proportions occurs in honey-
eater flowers, although fewer species have been examined (Nicolson &

Fleming, 2003).

The distinctive ornithophilous genus Erythrina (Fabaceae) has a pan-
tropical distribution and is unusual in including both hummingbird and
passerine pollinators. Hummingbirds are pollinators of Erythrina species
with concentrated nectar high in sucrose, and passerine birds pollinate spe-
cies with dilute nectar high in hexoses (Baker & Baker, 1982b). The sucrose
concentration averages 54.6 ± 1.9% (mean ± SE) in the nectars of 25 hum-
mingbird-pollinated species, in sharp contrast to 4.0 ± 0.4% in the nectars of
23 passerine-pollinated species from both the Old and New World (Baker
et al., 1998). Amino acid concentrations also differ, being much lower in the
hummingbird nectars than in the passerine nectars (Baker & Baker, 1982b);
they are exceptionally high in some southern African species such as
Erythrina lysistemon (S.W. Nicolson, unpublished data). Bruneau's (1996)
hypothesis of phylogenetic relationships within Erythrina is based on both