NECTAR CHEMISTRY - Nectaries and Nectar - page 221

Exposed nectars may sometimes be diluted by high humidity, and both
nectar and pollen may need protection from rain (Corbet, 1990). When bee-
pollinated Pulsatilla cernua (Ranunculaceae) flowers become pendulous
during anthesis, rain damage to pollen is avoided (Huang et al., 2002), but
the nectar of the relatively open flowers is also protected. Downward flower
orientation is common in bird-pollinated flowers, and it is suggested that the
corolla serves as an umbrella (Aizen, 2003). However, this has seldom been
tested. Tadey and Aizen (2001) found that the narrow tubular shape and sur-
face properties of the petals were more important than orientation in
preventing flooding in the mistletoe Tristerix corymbosus (Loranthaceae).
Unwettable floral surfaces, constrictions, and hairs all serve to prevent con-
tact between nectar and rainwater, but nectar may still gain water in the
vapour phase from rain droplets inside flowers (Corbet & Delfosse, 1984).
Pubescent hairs protect nectar from dilution by rain as well as from evapora-
tion, and a good example is seen in Protea species (Proteaceae), which
flower during winter in the southwestern Cape of South Africa and are a ma-
jor nectar source for the endemic Cape sugarbird, Promerops cafer . In those
Protea species which have heavily furred involucral bracts surrounding the
tightly packed florets, the abundant nectar which pools at the base of the in-
florescence is little affected even by heavy rain (Table 1).

Variation in nectar concentration, and the low concentration of many
nectars, means that nectar feeders must often ingest and process excess water
in order to meet their energy requirements (Nicolson, 1998; Martínez del Rio
et al., 2001). The consequences for animal physiology, such as chronic
diuresis, food warming costs for endotherms consuming large volumes of
cold dilute nectar, additional metabolic costs for bees carrying larger nectar
loads, are discussed by Nicolson (2007, Chapter 7 in this volume). The most
immediate effect of varying concentration is on ingestion rates, and here vis-
cosity is an important property of sugar solutions.

2.3 Viscosity and feeding rates

Both the temperature and solute concentration of nectar have substantial ef-
fects on its viscosity (Fig. 1). Viscosity is inversely proportional to tem-
perature, which suggests advantages to feeding on warm nectar (Heyneman,
1983). In the case of insect nectarivores, both the decreased viscosity and the
increased body temperature at higher ambient temperatures contribute to the
volume ingested (Pivnick & McNeil, 1985). While energy content increases
linearly with sugar concentration, viscosity increases exponentially (Fig. 1),
with the important consequence for nectar feeders that the most efficient en-
ergy intake occurs at intermediate sugar concentrations. It was Baker (1975)