The neck of a giraffe isn’t all that different from any other mammal’s. There are seven neck vertebrae, like those of humans, but they are much bigger. (This is a different strategy than other long-necked creatures in history. Marine reptiles called plesiosaurs, for instance, had 38 to 42 vertebrae to lengthen their necks.)
How giraffes evolved their long necks has long been the subject of debate, dating back to the early days of evolutionary theory. French naturalist Jean Baptiste Lamarck, for instance, suggested that the giraffe neck lengthened as the animals stretched to reach leaves high in trees, with a “nervous fluid” flowing into the neck to make it longer. The giraffe’s offspring would inherit the longer neck, then stretch to reach even higher leaves, and that even longer neck would get passed on.
That’s not how evolution works, though. Animals that had longer necks had some sort of advantage, and they were able to pass on their genes, eventually resulting in super-long necked animals. Just what that advantage might have been is still unknown. Perhaps the feature let giraffes access more food resources high in the treetops. But since female giraffes prefer males with longer necks, sexual selection might also be involved.
But what did that evolution look like? And when did it happen? Insight into those questions comes from a study published October 6 in Royal Society Open Science. Melinda Danowitz and colleagues at the New York Institute of Technology College of Osteopathic Medicine in Old Westbury analyzed neck vertebrae from 11 species — nine that are extinct, modern giraffes (of course) and okapi. (Okapi look like a cross between a zebra and a deer, but they belong to the same family as giraffes.)
Giraffes, it turns out, are not the first species in their lineage to have a long neck — they just have the longest one. The species started off with a shorter neck, 7.5 million years ago, when it first appeared on the scene, after which the neck became even longer. But the lengthening began even earlier in the giraffe’s lineage, the fossil analysis revealed. The extinct Giraffa sivalensis shares some of the elongation features found in the modern giraffe. But even it wasn’t the beginning of the journey. Two other extinct creatures from the giraffe family Giraffidae, Samotherian and Palaeotragus, also show some of these features, as does the primitive giraffid Canthumeryx. But what was surprising was that the neck length appears to predate the giraffe family — an even earlier species, Prodremotherium, which disappeared some 23 million years ago, also showed neck elongation.
“Neck length, the most distinguishing and popular attribute of Giraffa, is apparently not a defining feature of the family,” the researchers note.
All living giraffes are considered to belong to one species, G. camelopardis. But a 2007 genetic study suggested that the animal’s many subspecies may actually be separate species. If that’s true, I’d be curious how the animal’s long neck — and unique bones — have continued to evolve within each of those groups.
Source: Science News