Biology Reference
In-Depth Information
sucrose nectars, and found that total energy is relatively stable because nec-
tar volume and concentration vary inversely. As pointed out by Mitchell and
Paton (1990), who used “equal volume” and “equal sugar” presentations to
measure the nectar intake rates of honeyeaters, this is biologically more real-
istic than investing different amounts of sugar in the same volume of nectar.
Increasing the water component of the nectar could explain the widespread
shifts from insect to bird pollination in the flora of western North America
(Grant, 1993), as discussed by Nicolson and Thornburg (2007, Chapter 5 in
this volume).
The large genus
Penstemon
(Scrophulariaceae), with 270 species, is an
excellent example because hummingbird pollination has evolved repeatedly
from bee pollination (Wilson et al., 2006). Thomson et al. (2000) listed sys-
tematic rules that contrast ornithophily to melittophily in
Penstemon
, and
compared pairs of closely related bird and bee species: nectar volumes were
invariably higher in the bird-visited species, and nectar concentrations were
usually lower. Colour and morphological characters were also clearly dis-
tinct in the two categories. Using another pair of closely related species of
Penstemon
, Castellanos et al. (2002) showed that nectar refilling rates were
much faster in hummingbird-pollinated
P. barbatus
than in bee-pollinated
P. strictus
. Since the latter species produces smaller volumes of more concen-
trated nectar (Wilson et al., 2006), the differences in refilling rates are likely
to be due to differences in water transport in the nectary.
Changes in nectar
volume and concentration are predicted by Wilson et al. (2006) to come first
during evolutionary shifts from bee to hummingbird pollination in
Penste-
mon
, preceding changes in nectar sugar composition or in other floral
characters such as colour or size. They suggest that birds care more about
nectar, but bees are choosier about colour, given that all species of
Penste-
mon
produce enough nectar to interest bees.
However, for a shift to passerine or bat pollination, which is associated
with the most copious and dilute floral nectars, hydrolysis of the nectar su-
crose to glucose and fructose seems to be required. This maintains the
gradient for sucrose transport, and the increased osmolality draws additional
water from the nectary and dilutes the nectar (Nicolson, 1998, 2002). The
nectar of two bat-pollinated species of Sinningieae (Gesneriaceae), which
have evolved independently, is much more copious, dilute, and rich in hex-
oses than that of related species pollinated by moths, bees, or hummingbirds
(Perret et al., 2001). Examples are more numerous for passerine pollination.
The classic case of
Erythrina
, in which nectar concentration and nectar
chemistry are highly labile and associated with repeated shifts from passerine
to hummingbird pollination, is discussed by Nicolson and Thornburg (2007,
