Singing in the Rain: Seasonal rainfall patterns isolate three species of giraffes in East Africa

Why are there so many species? Since Hutchinson addressed the American Society of Naturalist in 1958 (“Homage to Santa Rosalia or why are there so many kinds of animals”), ecologists have long sought to answer this question. Recently, scientists from UCLA examined this basic question (report in PLOS ONE) and asked why are there three species of giraffe - Masai Giraffe (Giraffa tippelskirchi), Reticulated Giraffe (G. reticulata), and Rothschild’s Giraffe (G. camelopardalis) - instead of one in East Africa. Below are the three species of giraffes. 

Giraffe’s are large, mobile, herbivores that rely on plants that grow during seasonal rains. For example, giraffe’s in Niger have an average home range of more than 90 square km. In East Africa, there are 3 distinct populations of giraffes even though they have the ability to come into contact and breed. Yet the giraffes living in East Africa are not one species, but probably three distinct species. Recent research suggests that there is a distinct genetic structure in their mitochondrial DNA and nuclear microsatelllies and minimal gene flow among the three adjacent populations. Furthermore, these giraffes interbred in zoos. In the absence of any appreciable geographic barriers that reproductively isolates each population, how do these highly mobile giraffes, that regularly interbred in zoos, remain distinct as three species as opposed to a single species? Below is a map that shows the geographic range of the three giraffe species. 

Thomassen et al. (2013) examined 4 factors that are hypothesized to keep the Rothschild’s, Reticulated, and the Masai giraffe isolated from one another and prevent interbreeding. Ultimately, their results found that only differences in seasonal rainfall and growing season, influenced the time of year that giraffes breed. (In East Africa, there are three geographically distinct seasonal patterns of rain). Thus, these giraffes are only reproductively active during the growing season, the growing season is different in these three parts of East Africa, and natural selection acts on the timing of reproduction so it corresponds with the seasonal rains. Simply, giraffes that breed and reproduce when food is plentiful will grow better and faster and pass more of their genes into the next generations - voila, there are three species of giraffes instead of one. This is a great example of the simple power of divergent natural selection.

This figures shows the population range of Giraffe's in Africa. Notice how the population range of the Rothschild's, Masai, and Reticulated giraffe abut. Thomassen et al. (2013) showed that seasonal patterns of rainfall are the likely reason there are three distinct species of giraffe in East Africa as opposed to one. From Wikipedia Commons

In case you wanted to know: 
Here is the citation to the study:Thomassen HA, Freedman AH, Brown DM, Buermann W, Jacobs DK (2013) Regional Differences in Seasonal Timing of Rainfall Discriminate between Genetically Distinct East African Giraffe Taxa. PLoS ONE 8(10): e77191. doi:10.1371/journal.pone.0077191

The 4 hypotheses Thomassen et al. 2013 examined were: 1) Isolation by distance, 2) Barriers to Dispersal (limit or prevent gene flow among populations), 3) Spatial habitat differences that are not due to phenology (seasonal differences), and 4) Seasonal differences related to the “greening up” of the habit - basically differences in rainfall that influence the growing season of the plants that the giraffes eat. Using multivariate, spatially non-explicit, and spatially explicit analyses, Thomassen et al. 2013 only found that only seasonal patterns in rainfall and corresponding “green up” of plants were responsible for the reproductive isolation in the three species of giraffes.