Sexual dimorphisms in body proportions of Masai giraffes and the evolution of the giraffe’s neck

Female giraffes have proportionally longer necks than males. Male Masai giraffes are larger overall, but a study separating size from shape found that females devote more of their potential height to neck and less to foreleg. That distinction matters to the idea that long necks evolved primarily as weapons for competing males.

Douglas Cavener, Monica Bond and colleagues measured body proportions in photographs of captive and wild Masai giraffes. A set of 357 images of 179 known-age captive animals let them examine how proportions changed during growth. They then compared mature zoo giraffes with wild adults from Tanzania to check whether the same differences appeared outside captivity.

Females had relatively longer necks and trunks, while males had relatively longer forelegs and wider neck bases. The photographic area of males’ necks was also proportionally larger, although area is not a direct measurement of mass. These patterns do not establish how necks originally evolved. They challenge the prediction that male competition should produce proportionally longer male necks, while remaining compatible with selection for a heavier fighting neck. A giraffe’s neck length and its bulk tell different parts of the evolutionary story.