The reproductive energetics of gentoo (Pygoscelis papua) and macaroni (Eudyptes chrysolophus) penguins at South Georgia
(1) The doubly-labelled water method was used in the first comprehensive study of penguin energetics throughout the reproduction and moult season, comparing gentoo penguins (inshore feeding, sexually monomorphic, share breeding duties equally) with macaroni penguins (offshore feeding, dimorphic, duties unequally shared). (2) While fasting during courtship and incubation, macaronis (both sexes) and female gentoos had field metabolic rates (FMR) of 4.1 and 3.8 W kg-1, respectively. Male gentoos also spending time at sea had an FMR of 7.5 W kg-1. Male macaroni FMR for fasting while brooding was 3.8 W kg-1. During chick-rearing, gentoo FMR was 6.8 W kg-1 early on, rising to 7.4 W kg-1 later; for macaronis FMR was 13 W kg-1. Estimated energy cost of foraging at sea was 16 and 22 W kg-1 for gentoos and macaronis, respectively. Gentoo and macaroni FMR during moult was 5.3 and 6.7 W kg-1. (3) Over the breeding and moult season, male and female gentoos have total and average daily energy expenditure (DEE) only 1% different, agreeing with expectation for monomorphic species sharing duties equally. Female macaronis (10% smaller than males) have average DEE 9% (17% on a mass-specific basis) greater than males, mainly because they spend more time rearing chicks, involving intensive foraging at sea. However, males spent more days at the colony so over the whole season female energy expenditure is only 3.5% above males. (4) Scaled mass-specific average DEE is 18% greater for macaronis than gentoos, chiefly because of the energy costs of offshore foraging trips. As their breeding and moult season is 20% shorter than gentoos, average absolute DEE is 13% less for macaronis; however, macaroni chick production is only half that of gentoos.