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Wednesday, October 30, 2013

A real giraffe riddle

Judging from my Facebook wall, giraffes seem to be the rage right now. Here’s a riddle that’s a bit more meaningful than what to open first when your parents come over for early morning sandwiches: how do the different African giraffe species maintain their distinct lineages when their territories overlap?

There are three giraffe species in East Africa: Masai (Giraffa tippelskirchi), Rothschild’s (G. camelopardalis), and Reticulated (G. reticulata). While members of these different species will readily hybridize in zoos, this rarely happens in the wild. Mitochondrial DNA data suggests that the different species haven’t co-mingled for at least 200,000 years, and in some cases, over 1.5 million years.  What’s been keeping them apart? 

A detailed climactic analysis conducted by UCLA scientists shows that rainfall follows a predictable pattern in the region, resulting in a progression of ‘green-up’ areas. It’s this seasonal timing of rainfall that keeps the species separate. How? Each species synchronizes its reproductive cycle to a particular rainfall schedule so that its calves can take advantage of rain-lush vegetation.

When the scientists modeled where they expected to find the three giraffe species, based on monthly rainfall, it aligned extremely well with locations where giraffes were actually found. 


Predicted localities are indicated in red dots (Rothschild's), blue dots (Reticulated), and green dots (Masai). 
Observed localities are plotted in triangles (Rothschild's), asterisks (Reticulated), and pluses (Masai). 

It’s interesting that these subtle differences in habitat seem to be capable to separating otherwise compatible species.

Henri A. Thomassen, Adam H. Freedman, David M. Brown, Wolfgang Buermann, & David K. Jacobs (2013). Regional Differences in Seasonal Timing of Rainfall Discriminate between Genetically Distinct East African Giraffe Taxa PloS ONE : doi:10.1371/journal.pone.0077191.g003.