How the Snake Lost Its Legs by Lewis I. Held, Jr.

Giraffes (second from left) evolved from short-necked ancestors that resembled okapis (far left), but they elongated their necks without adding more vertebrae. Giraffes have only sevencervical vertebrae like every other mammal except sloths (7-10) and sirenians (6-7). The only other animals with any obvious neck constraint were pterosaurs. In contrast, cervical vertebrae vary freely in birds and reptiles: there can be as many as 16 neck vertebrae in ducks (second from right), 17 in stegosaurs, 19 in sauropods, 25 in swans (far right), and 76 in plesiosaurs! Until 2011the favored explanation for the neck constraint was that mutations causing cervical ribs in mammals (but not other groups) have harmful side effects (cancer, etc.) that are selected against. A rebuttal of this paradigm was published in 2011, and in 2012a new idea was proposed. The new hypothesis stems from the fact that a critical organ depends on the neck somites that form neck vertebrae. Mammals differ from other vertebrates insofar as they breathe using a diaphragm. This novelty arose ~200MY ago. Oddly, the diaphragm muscle comes not from nearby thoracic somites but rather from the distant neck somites C3-C5, whose myoblasts migrate over the intervening distance! Because of this quirk, the mammal neck specialized into three modules whose morphological integration may be too difficult to disrupt without risking death: C1-C2 (skull articulation), C3-C5 (diaphragm musculature and innervation), and C6-C7 (forelimb musculature and innervation). Until recently, the most popular explanation for the long neck was that taller individuals were better equipped for reaching higher leaves during longer drought. However, the actual foraging behavior of giraffes during dry seasons refutes this notion. Instead, the driving force was more likely to have been sexual selection: (1) males use their necks in jousting contests to win mates, (2) they have longer necks than females, and (3) their necks exhibit the positive allometry expected for a sexually selected trait.

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