Regional Differences in Seasonal Timing of Rainfall Discriminate between Genetically Distinct East African Giraffe Taxa

Giraffes’ genetic boundaries follow when the rains arrive. In East Africa, Masai, reticulated and Rothschild’s giraffes remained genetically distinct even where their ranges approached one another. Their ability to travel long distances made that separation puzzling. A possible dividing line was not a mountain or a river, but the seasonal timing of fresh leaves.

Henri Thomassen, Adam Freedman and colleagues combined previous genetic data from 429 giraffes at 51 locations with climate records and satellite measurements. They compared four explanations: distance, physical or human-created barriers, general habitat differences, and the timing of rainfall and vegetation growth. They also revisited published birth records from two genetically distinct Masai populations, in Nairobi and the Serengeti.

Rainfall timing distinguished the three groups more effectively than the other tested explanations. Models using monthly rainfall alone nearly matched those with the full set of predictors. The two Masai populations also had birth peaks separated by about two to three months, paralleling differences in local rainfall timing. These associations do not prove that seasonal breeding prevents interbreeding; seasonal habitat use could also contribute. The study identifies an unexpected possibility: giraffes can occupy adjoining landscapes yet encounter food and reproductive opportunities on different annual schedules.