ENERGETICS OF TOUCANS, A BARBET, AND A HORNBILL: IMPLICATIONS FOR AVIAN FRUGIVORY

Abstract Rate of oxygen consumption was measured in eight species of toucans, one barbet, and one hornbill to examine factors that influence their energy expenditures. Basal rate of metabolism in those species, supplemented with measurements from three woodpeckers and three mousebirds, is correlated with body mass and either food habits or climate: temperate acorn- and insect-eating species have basal rates that are ∼70% greater than tropical fruit-eating species. The temperate, insect- or acorn-eating species are woodpeckers, so level of basal rate is also correlated with familial affiliation. The toucans, barbet, and hornbill have basal rates similar to those of frugivorous pigeons and bats, which collectively are low by avian standards. The effects of climate, food habits, and family affiliation on basal rates in endotherms are difficult to separate, given the restricted data set available. Earlier conclusions that the basal rates of birds greatly exceed those of mammals are confounded by character interactions that influence dependence of basal rate on body mass. The largest toucan showed a remarkable ability to reduce energy expenditure at low ambient temperatures without reducing core body temperature, possibly as a result of peripheral vasoconstriction.