Masai giraffe rifting apart: Loss of genetic connectivity across the Gregory Rift Valley in Tanzania

The Masai giraffe has experienced a population decline from 70,000 to 35,000 in the past three decades and was declared an endangered subspecies by the IUCN in 2019. The remaining population is divided into smaller subpopulations dispersed west and east of the Gregory Rift Valley (GRV) in Tanzania, Kenya, and Zambia. The steep escarpments of the GRV are formidable barriers to migration and gene flow and the few remaining natural corridors are now occupied by human settlements. To assess the impact of the GRV on Masai giraffe gene flow, we examined nuclear and mitochondrial DNA variation in subpopulations east and west of the Manyara and Eyasi escarpments of the Rift in northern Tanzania. Strikingly, evidence from mtDNA variation, which measures female-mediated gene flow, suggests that females have not migrated across the GRV between major subpopulations in the Serengeti and Tarangire ecosystems since the approximate time that Masai giraffes emerged as a (sub)species 250kya. In contrast the analysis of nuclear DNA variation shows that male-mediated gene flow across the GVR has occurred over most of the history of Masai giraffes but may have ceased in the recent past as natural migratory routes across the escarpment have been blocked by human activities. These findings suggest that the Masai giraffe is now split into two separate metapopulations and in the absence of gene flow will eventually become separate species with populations of less than 20,000.