A significant change in the geography of livestock raising over the past 30 years is the southerly movement of FulBe herds into the humid Sudanian and Guinean savannas of West Africa. The literature suggests that the severe droughts of the early 1970s and mid-1980s were the driving force behind this southern expansion of mobile livestock raising. The conventional view is that drought forced herders to seek greener pastures to the south, an area that zebu cattle have previously avoided because of the presence of tsetse flies, the vector of animal sleeping sickness (trypanosomiasis). This "sudden push" hypothesis places Sahelian herds in savanna pastures in a matter of a 1-3 years. This stimulus-response model runs counter to our observations and understanding of the social and ecological processes influencing FulBe herd movements. We challenge the "sudden shift" thesis at the regional scale by arguing that the southerly expansion of FulBe herds has proceeded according to a more complex temporal frame that includes generational, biological, and social historical timeframes and periodicities. We distinguish between short-term shifts ("test movements") and more permanent shifts ("migration movements"). These mobility patterns are linked to contingent factors such as cattle diseases, drought, and political instability, as well as to more structural and adaptive features such as the establishment of social networks, herding contracts, and cattle cross-breeding. Shifts in livestock ownership and the social differentiation among herders are important variables for understanding changes in herd movements. We conclude that the permanent shift of herds to the humid savannas of West Africa has been preceded by a series of social and agroecological adjustments that operate on decadal and generational time scales.