Abstract:
Metabolic rate is a fundamental characteristic of all organisms.
It covaries most significantly with activity, body mass, seasonality,
and temperature. Nonetheless, substantial additional variation
in metabolic rate, especially either resting rate or basal rate,
is associated with a range of factors including phylogenetic position,
ecological distinctiveness, range position, and diet. Understanding
this variation is a key goal of physiological ecology.
The black-faced sheathbill is a phylogenetically distinct, highlatitude,
island-endemic bird occurring exclusively on several
archipelagos in the southern Indian Ocean. Here we examined
the idea that the unique phylogenetic position and ecology of
the black-faced sheathbill may lead to a basal metabolic rate
(BMR) different from that predicted by its body mass. When
compared with BMR data available for all birds and a subset
of island species, it was clear that the BMR of the black-faced
sheathbill on subantarctic Marion Island, estimated at 157C using
indirect calorimetry (2.3705 0.464 W, mean5SD; n 22),
for a group of birds with a mean mass of 459 + 64 g, is no
different from that expected based on body mass. However,
variation in BMR, associated with habitat use and diet, even
when correcting for variation in mass, was found. Sheathbills
foraging year-round in comparatively resource-rich king penguin
colonies have a higher BMR (2.758 5 0.291 W, n 12)
than sheathbills that split their foraging between rockhopper
penguin colonies and the intertidal zone (2.04750.303 W, n 10), which are poorer in resources. Because these populations
coexist at relatively small spatial extents (the entire island is
290 km2), other factors seem unlikely as causes of this variation.
