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The Origin and Function of 'Bizarre' Structures: Antler Size and Skull Size in the 'Irish Elk,' Megaloceros giganteus

Stephen Jay Gould
Evolution
Vol. 28, No. 2 (Jun., 1974), pp. 191-220
DOI: 10.2307/2407322
Stable URL: http://www.jstor.org/stable/2407322
Page Count: 30
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The Origin and Function of 'Bizarre' Structures: Antler Size and Skull Size in the 'Irish Elk,' Megaloceros giganteus
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Abstract

The misnamed 'Irish Elk' is a late Pleistocene giant deer that ranged southward to North Africa and eastward to China. Since its first scientific description in 1697, it has played a major role in debates about the history of life. Cuvier used it to prove the fact of extinction and set the basis for a geologic time scale. Later, Megaloceros became the rallying point for anti-Darwinians; they invoked orthogenesis to deny natural selection and attributed extinction to an inadaptive trend towards immense antlers. The antlers posed a severe difficulty for the modern synthesis: they were generally explained as allometric correlates of advantageous increases in body size that offset the problems of admittedly disproportionate antlers. Virtually every textbook in evolution cites Megaloceros as a case of allometry contra orthogenesis: nonetheless, no one has ever generated any quantitative data about it. I measured 79 skulls and antlers of Megaloceros to resolve two questions bearing upon the allometric hypothesis: 1) Does Megaloceros, at its maximal body size among cervine deer, lie on the extrapolated line for positive allometry of antlers among smaller cervines? 2) Does the intraspecific variation among adult stags of Megaloceros yield relatively large antlers in stags of large body size? The answer to both questions is undoubtedly yes (Figs. 1-2, 7-10). 1) the static, interspecific allometry of adult cervines is strongly positive; Megaloceros has the predicted antler size for its body size (moose have relatively small antlers for their body size). 2) The exponent of intraspecific allometry is about 2.5. If selection acted to preserve deer of large body size, relatively large antlers would follow as a consequence of correlation. Yet the fact of positive allometry need not provoke the usual interpretation: large bodies might be a consequence of advantageous antlers, or both antlers and bodies might be selected in concert. The assumption of disproportionate antlers is based on the a priori notion that antlers must function as weapons in battle: 90 pound antlers, mounted with tines pointing backward on a 5 pound skull cannot be regarded as well-designed for such a purpose. But deer often use their antlers to establish dominance and win access to females by display and ritualized combat. Moreover, display is especially important in large deer and deer with palmated antlers. The antler morphology of Megaloceros is ideally suited for display: smaller deer must rotate their heads to show the palm. The torque produced by rotation in Megaloceros would have posed severe mechanical problems. But, alone among deer with palmated antlers, Megaloceros displayed its full palm when simply looking straight ahead. The immense antlers of Megaloceros were advantageous in themselves. Its extinction may be traced to late glacial changes in climate.

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