AboutZoos, Since 2008


Weather pre­vents dif­fer­ent Giraffe species from interbreeding

pub­lished 24 Octo­ber 2013 | mod­i­fied 13 Sep­tem­ber 2014

We tend to think of giraffes as a sin­gle species, but in Kenya not one but three types of giraffe occupy the same scruffy grass­lands. These three species-​the Masai, Retic­u­lated and Rothschild’s giraffe-​often encounter one another in the wild and look sim­i­lar, but they each main­tain a unique genetic makeup and do not inter­breed. And yet, throw a male Masai and a female Rothschild’s giraffe, a male Roth­schild or a female Reticulated-​or any com­bi­na­tion thereof-​together in a zoo enclo­sure, and those dif­fer­ent species will hap­pily devote them­selves to mak­ing hybrid giraffe babies.

What is it, then, that keeps these species apart in the wild?
Giraffe familyResearchers from the Uni­ver­sity of Cal­i­for­nia, Los Ange­les, may be close to an answer. In nature, at least one of four poten­tial bar­ri­ers typ­i­cally keeps similar-​looking and similar-​acting but dis­tinct species from becom­ing inti­mate: dis­tance, phys­i­cal blocks, dis­parate habi­tats or sea­sonal dif­fer­ences, like rain­fall. In the case of the Kenyan
giraffes, the researchers could sim­ply look at the habi­tat and know that phys­i­cal bar­ri­ers could prob­a­bly be ruled out; no moun­tains, canyons or great bod­ies of water pre­vent the giraffes from find­ing one another. Like­wise, giraffes some­times have home ranges of up to 380 square miles, and those ranges may over­lap. Dis­tance alone, there­fore, was prob­a­bly not stop­ping the ani­mals from meeting.

Either habi­tat or sea­sonal dif­fer­ences, they sus­pected, was the likely fire­wall pre­vent­ing species from get­ting up close and per­sonal with one another. To tease out the roles of these poten­tial dri­vers, the authors built com­puter mod­els that took into account a range of fac­tors, includ­ing cli­mate, habi­tat, human pres­ence and geno­types from 429 giraffes that they sam­pled from 51 sites around Kenya. Just to make sure they weren’t unfairly exclud­ing dis­tance and phys­i­cal obsta­cles from the list of pos­si­ble dividers, they also included ele­va­tion val­ues — some giraffes were found in the steep Rift Val­ley — and the dis­tance between pop­u­la­tions of giraffes sampled.

Accord­ing to their sta­tis­ti­cal model, regional dif­fer­ences in rain — and the sub­se­quent green­ing of the plains that it trig­gers — best explain genetic diver­gence between giraffe species, the researchers write on 23 Octo­ber in the jour­nal PloS One. East Africa expe­ri­ences three dif­fer­ent regional peaks in rain per year — April and May, July and August and Decem­ber through March — and those dis­tinct weather envelopes tri­sect Kenya.

Giraffe division eastafrica

Genetic subdivision giraffe

So, although the trio of giraffe species some­times over­lap in range, the authors sam­ples as well as pre­vi­ous stud­ies revealed that they tend to each live and mate in one of those three geo­graphic rain pock­ets, both within Kenya and through­out the greater East Africa region.

Giraffe species sync their preg­nan­cies up with rain pat­terns to ensure enough veg­e­ta­tion is around to sup­port the ener­get­i­cally tax­ing processes of ges­ta­tion, birth and lac­ta­tion for mother giraffes, the authors think. Not much infor­ma­tion is avail­able on giraffe births, but the few obser­va­tions on this topic do con­firm that giraffe species tend to have their babies dur­ing the local wet sea­son, they report.

And while the mod­els indi­cate that rain is the pri­mary divider keep­ing giraffes apart, the authors point out that the ani­mals also may be recog­nis­ing dif­fer­ences in one another’s coat pat­terns, for exam­ple. But sci­en­tists do not know enough about how giraffes chose mates or whether they can dis­tin­guish poten­tial mates between species to give the species pos­si­ble due credit for recog­nis­ing one another.

Whether rain alone or some com­bi­na­tion of rain and (visual) recog­ni­tion trig­ger mat­ing, in the wild, at least, those mech­a­nisms seem to work well for keep­ing giraffe species apart. It will be inter­est­ing to see whether this sep­a­ra­tion is main­tained as cli­mate changes.

(Source: Smith­son­ian mag­a­zine sur­pris­ing sci­ence, 23.10.2013)

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