Diel variations in plasma glucose concentration in some South African avian frugivores

Abstract

We examined diel variations in the plasma glucose concentrations (PG1u) of avian blood, by measuring PG1u in four South African fruit-eating birds: the Cape White-eye(Zosterops virens), Knysna Turaco (Tauraco corythaix), Red-winged Starling (Onychognathus morio) and Speckled Mousebird (Colius striatus). Species were chosen as representatives of a range of body sizes and from the passerines and non-passerines. Birds were maintained in controlled environment rooms and PG1u were measured at four times of the day, at sunrise (0600 hours), midday (1200 hours), sunset (1800 hours) and midnight (2400 hours). Interspecific variation in mass and both the interspecific and intraspecific variation in PG1u were found to be significant for all species. PG1u of Red-winged Starlings and Knysna Turacos were highest after overnight fasting, whereas peak PG1u in Speckled Mousebirds and Cape White-eyes were recorded at midday. The lowest PG1u was recorded after overnight fasting in Speckled Mousebirds whereas the lowest PG1u were recorded at midnight in the three other species. Whereas the Red-winged Starlings, Knysna Turacos and Cape White-eyes clearly increase their PG1u via the processes of gluconeogenesis, Speckled Mousebirds apparently do not. Previous studies involving the comparison of avian PG1u have not specified sampling times and this study clearly demonstrates that PG1u of birds vary over time, thereby highlighting the need for the specification of sampling times in future sampling of avian PG1u.