Field metabolic rate of lactating female Galapagos fur seals (Arctocephalus galapagoenis): the influence of offspring age and environment

Galapagos fur seals, Arctocephalus galapagoensis, live in a seasonal enviro nment which varies strongly in productivity from year to year. We measured how the field metabolic rates (FMR) of lactating females varied with season , pup age and year. Energy expenditure was measured using doubly labeled wa ter (DLW) during the cold seasons of 1984 and 1985 in 9 mothers of 1-3-mont h-old pups and 5 mothers of yearlings, and during the 1986 warm season in 8 mothers of B-month-old pups. Young pups gained 0.84% mass/day during the c old season, but larger pups during the warm season lost 1.25% mass/day. Dur ing the warm season, females had lower relative total body water than durin g the cold season suggesting higher fat content during the warm, less produ ctive season, but the effect was even more marked when comparing different years of the study: fat content was high in 1984 and 1986 and low in 1985. The FMR of mothers varied from 134 to 167 W but did not show significant di fferences between any of the pup age-groups. Among the years of the study, FMR showed only a trend towards law energy expenditure of mothers of young pups in 1984. The mean FMR was lower than for other otariids. Mothers may l imit energy expenditure independent of pup age and season to minimize their own risk of starvation in an environment of comparatively low productivity , varying unpredictably due to frequent El Nino events. (C) 2001 Elsevier S cience Inc. All rights reserved.