Genetic evidence of population subdivision among Masai giraffes separated by the Gregory Rift Valley in Tanzania

Abstract

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 number of Masai giraffe are geographically separated by the steep cliffs of the Gregory Rift escarpments (GRE) in Tanzania and Kenya dividing them into two populations, one west and one east of the GRE. The cliffs of the GRE are formidable barriers to east–west dispersal and gene flow and the few remaining natural corridors through the GRE are occupied by human settlements. To assess the impact of the GRE on Masai giraffe gene flow, we examined whole genome sequences of nuclear and mitochondrial DNA (mtDNA) variation in populations located east (Tarangire ecosystem) and west (Serengeti ecosystem) of the GRE in northern Tanzania. Evidence from mtDNA variation, which measures female‐mediated gene flow, suggests that females have not migrated across the GRE between populations in the Serengeti and Tarangire ecosystems in the past ~289,000 years. The analysis of nuclear DNA variation compared to mtDNA DNA variation suggests that male‐mediated gene flow across the GRE has occurred more recently but stopped a few thousand years ago. Our findings show that Masai giraffes are split into two populations and fulfill the criteria for designation as distinct evolutionary significant units (ESUs), which we denote as western Masai giraffe and eastern Masai giraffe. While establishing giraffe dispersal corridors across the GRE is impractical, conservation efforts should be focused on maintaining connectivity among populations within each of these two populations. The importance of these efforts is heightened by our finding that the inbreeding coefficients are high in some of these Masai giraffe populations, which could result in inbreeding depression in the small and fragmented populations.

Sexual dimorphisms in body proportions of Masai giraffes and the evolution of the giraffe’s neck

“…The founders of the captive Masai giraffe population in North American zoos and safari parks originated from southeastern Kenya and northeastern Tanzania (Cantwell 2018) from populations that are located east of the steep escarpments of the Gregory Rift Valley. The wild Masai giraffes used in this study (described below) are located in the same general region east of the Gregory Rift Valley escarpments and have recently been described as Eastern Masai giraffes (Lohay et al 2023). Genetic distances measured by whole genome sequence analysis of nuclear and mitochondrial DNA sequence of several captive and wild giraffes indicates that they are closely related as compared to wild Western Masai giraffes west of Gregory Rift Valley escarpments in the Serengeti Ecosystem (DRC, unpublished results).…”