Biology Reference
In-Depth Information
reduce the water load on their kidneys, subsequent studies have shown that
this is not the case (McWhorter & Martínez del Rio, 1999; Hartman Bakken
& Sabat, 2006). Water shunting directly through the intestine has, however,
been demonstrated in sunbirds (McWhorter et al., 2003). Variation in frac-
tional water reabsorption by the kidney has been shown to be important for
managing water excess in all three main lineages of nectarivorous birds
(Nicolson, 2006). Salts must be recovered from the large volumes of dilute
urine, and measurements on the excreted fluid of both hummingbirds and
sunbirds show this process to be highly efficient (Calder & Hiebert, 1983;
Fleming & Nicolson, 2003). Another consequence of feeding on dilute nec-
tar, especially at low ambient temperatures, is the energetic cost of warming
large volumes of nectar to body temperature (Lotz et al., 2003).
The sugar and concentration preferences of nectar-feeding birds have at-
tracted considerable interest, prompted initially by studies on mainly
frugivorous passerine birds (reviewed by Martínez del Rio et al., 1992). For
example, starlings (
Sturnus vulgaris
) strongly prefer hexose solutions when
given a choice between isocaloric diets containing hexoses or sucrose, while
in less frugivorous bird species the preference for hexoses is weaker; the
physiological basis of these sugar preferences lies in the relative activity of
the intestinal enzyme sucrase, which is completely absent in starlings
(Martínez del Rio et al., 1988). Sucrose aversion, resulting from the loss of
intestinal sucrase, appears to be restricted mainly to the frugivorous families
of the large sturnid-muscicapid lineage of birds: the starlings, thrushes, and
mockingbirds (Nicolson & Fleming, 2003b; Lotz & Schondube, 2006).
Among other avian nectarivores, intestinal sucrase activity is ten times higher
in hummingbirds than in a variety of passerine birds, including flower-
piercers (
Diglossa
), which are the most specialized nectar-feeding passerines
in the Neotropics (Schondube & Martínez del Rio, 2004). Sucrase activity of
a sunbird species,
Nectarinia osea
, has been found to be as high as that of
hummingbirds (T.J. McWhorter & J.E. Schondube, unpublished data). Hex-
ose absorption is another possible digestive constraint for specialized nectar
feeders, but McWhorter et al. (2006) have recently shown that humming-
birds, like many passerine frugivores, rely substantially on paracellular
uptake of hexose sugars, which suggests that sucrose hydrolysis is more
likely to be limiting. As in honeybees (Gmeinbauer & Crailsheim, 1993) and
the hawkmoths studied by O'Brien (1999), hummingbirds use newly in-
gested carbohydrate to fuel their hovering flight; another example of
convergence between nectar-feeding animals (Welch et al., 2006).
It is now apparent that the sugar preferences of nectar-feeding birds are
concentration-dependent (Lotz & Schondube, 2006). The sugar preferences
