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G. Mitchell 1,2 , S. J. van Sittert 1 & J. D. Skinner 1
1 Centre for Veterinary Wildlife Studies, University of Pretoria, Pretoria, South Africa
2 Department of Zoology and Physiology, University of Wyoming, Laramie, WY, USA
Correspondence
G. Mitchell, Centre for Veterinary Wildlife Studies, University of Pretoria, Pretoria, South Africa.
Email: mitchg@uwyo.edu
Editor: Virginia Hayssen
KEYWORDS
giraffe • evolution • morphology • sexual selection
ABSTRACT
The evolutionary origin of the long neck of giraffes is enigmatic. One theory (the 'sexual selection' theory) is that their shape evolved because males use their necks and heads to achieve sexual dominance. Support for this theory would be that males invest more in neck and head growth than do females. We have investigated this hypothesis in 17 male and 21 female giraffes with body masses ranging from juvenile to mature animals, by measuring head mass, neck mass, neck and leg length and the neck length to leg length ratio. We found no significant differences in any of these dimensions between males and females of the same mass, although mature males, whose body mass is significantly (50%) greater than that of mature females, do have significantly heavier (but not longer) necks and heavier heads than mature females. We conclude that morphological differences between males and females are minimal, that differences that do exist can be accounted for by the larger final mass of males and that sexual selection is not the origin of a long neck in giraffes.
Received 5 February 2009; revised 26 February 2009; accepted 26 February 2009
DIGITAL OBJECT IDENTIFIER (DOI)10.1111/j.1469-7998.2009.00573.x About DOI
2. A seleção sexual explica a origem do pescoço longo da girafa.
Early View (Articles online in advance of print)
Published Online: 2 Jul 2010
Necks-for-sex or competing browsers? A critique of ideas on the evolution of giraffe
R. E. Simmons 1 & R. Altwegg 2,3
1 DST/NRF Centre of Excellence at the Percy FitzPatrick Institute, University of Cape Town, Rondebosch, South Africa
2 South African National Biodiversity Institute, Kirstenbosch Research Centre, Claremont, South Africa
3 Animal Demography Unit, University of Cape Town, Department of Zoology, Rondebosch, South Africa
Correspondence
DST/NRF Centre of Excellence at the Percy FitzPatrick Institute, University of Cape Town, Rondebosch 7701, South Africa.
Email: rob.simmons@uct.ac.za
Editor: Nigel Bennett
KEYWORDS
sexual selection • ancestral giraffids • competing browsers • necks-for-sex • female choice • allometry
ABSTRACT
Recent years have witnessed a resurgence in tests of the evolution and origin of the great height and long neck of the giraffe Giraffa camelopardalis. The two main hypotheses are (1) long necks evolved through competition with other browsers allowing giraffe to feed above them ('competing browsers' hypothesis); or (2) the necks evolved for direct use in intra-sexual combat to gain access to oestrous females ('necks-for-sex' hypothesis). Here, we review recent developments and their relative contribution in explaining giraffe evolution. Trends from Zimbabwean giraffes show positive allometry for male necks and isometry for female necks relative to body mass, while comparative analyses of the cervical versus the total vertebral column of the giraffe, okapi and fossil giraffe suggest selection specifically on neck length rather than on overall height. Both support the necks-for-sex idea. Neither study, however, allows us to refute one of the two ideas. We suggest new approaches for quantifying the relative importance of the two hypotheses. A direct analysis of selection pressure on neck length via survival and reproduction should clarify the mechanism maintaining the trait, while we predict that short robust ossicones should have arisen concurrently with incipient neck elongation if sexual selection was the main selective driver. The main challenge for the competing browser hypothesis is to explain why giraffe have remained about 2 m taller than their tallest competitors for over 1 Myr, whereas the sexual selection hypothesis cannot provide an adaptive explanation for the long neck of female giraffe. We conclude that probably both mechanisms have contributed to the evolution and maintenance of the long neck, and their relative importance can be clarified further.
Received 4 December 2009; revised 23 March 2010; accepted 23 March 2010
DIGITAL OBJECT IDENTIFIER (DOI)10.1111/j.1469-7998.2010.00711.x About DOI
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