Behavioral plasticity, or the mechanism by which an organism can adjust its behavior in response to exogenous change, has been highlighted as a potential buffer against extinction risk. Giraffes (Giraffa spp.) are gregarious, long-lived, highly mobile megaherbivores with a large brain size, characteristics that have been associated with high levels of behavioral plasticity. However, while there has been a recent focus on genotypic variability and morphological differences among giraffe populations, there has been relatively little discussion centered on behavioral flexibility within giraffe populations. In large wild herbivores, one measure of behavioral plasticity is the ability to adjust herd size in line with local environmental conditions. Here, we examine whether a genetically isolated population of Angolan giraffes (G. g. angolensis) in a heterogeneous environment adjust their herd sizes in line with spatiotemporal variation in habitat. Our results suggest that ecological factors play a role in driving herd size, but that social factors also shape and stabilize herd-size dynamics. Specifically, we found that 1) mixed-sex herds were larger than single-sex herds, suggesting that sexual composition of herds played a role in driving herd size; 2) the presence of young did not influence herd size, suggesting that giraffes did not make use of the dilution effect to safeguard their young from predation; and 3) there was a strong relationship between herd size and spatial, but not seasonal, variation in food biomass availability, suggesting stability in herd sizes over time, but temporary variation in line with resource availability. These findings indicate that giraffes adjust herd size in line with local exogenous factors, signaling high behavioral plasticity, but also suggest that this mechanism operates within the constraints of the social determinants of giraffe herd size.