Body size influences metabolic rate, which impacts feeding ecology. Body mass differs by sex in size-dimorphic species, such as giraffes, and also by age. Giraffes reside in a fission–fusion social system, which influences feeding ecology due to frequent changes in group size and composition. We analysed 40 years of feeding records collected from a population of Thornicroft’s giraffes (Giraffa camelopardalis thornicrofti) living in the Luangwa Valley, Zambia. We examined the influence of herd composition, age and sex on diet. Solitary males and herd males did not differ in diet. Dietary diversity was comparable for females and males, with sex differences in plant species eaten present during the dry season. Age differences in feeding ecology were pronounced, with juveniles often feeding on bushes and smaller trees, while adults tended to feed upon taller trees. Both sexes have evolved foraging strategies that maximize nutrient and energy intake commensurate with their reproductive strategies, with male metabolic requirements sometimes greater, and sometimes less, than that of females. We propose that females are not exchanging foraging efficiency with reproductive tactics by feeding on smaller trees in the open, but are increasing prospects for their calves to survive when confronted with interspecific competition by browsers.