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 Keywords sexual selection; ancestral giraffids; competing browsers; necks-for-sex; female choice; allometry. 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 Received 4 December 2009; revised 23 March 2010; accepted 23 March 2010 doi:10.1111/j.1469-7998.2010.00711.x 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 Zimbab- wean 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. New data and tests of the origin of the elongated neck of the giraffe Giraffa camelopardalis have provided fresh insight into the origin of this trait. All attempt to test one of two main hypotheses – that of natural selection via advantages from feeding on trees above competitors (Darwin, 1871) or intra-sexual (male–male) competition via advantages found in clubbing rivals more effectively in competition for females (Simmons & Scheepers, 1996). Both mechanisms should result in longer necks and both are demonstrably advantageous to their bearers (Pratt & Anderson, 1982, 1985; Cameron & du Toit, 2007). Although both mechan- isms can explain long necks in giraffes, no tests have attempted to distinguish between them as the origin or maintenance of the long neck. The two hypotheses may not be mutually exclusive, and it may not be possible to differentiate between them if both have provided selective pressures to neck lengthening. The question, therefore, should be what is the relative importance of the two mechanisms in explaining the origin and maintenance of the giraffe’s long neck? Here, we review and clarify the main hypotheses, explore the tests that have been advanced and provide future tests for prising them apart. Evolution of the long necks: main hypotheses The ‘competing browsers’ idea, elucidated by later editions of Darwin (1871) proposed that the competition between browsing species for access to leaves from tall trees created an advantage for giraffe to out-reach their tallest rivals (Darwin, 1871). The individuals doing so survive food bottlenecks during dry-season dearths and live to pass on their long-necked genes to future generations. Observing giraffes browse from 5-m-tall Acacia suggests that the idea is secure with little need for further debate. However, Journal of Zoology Journal of Zoology 282 (2010) 6–12 c  2010 The Authors. Journal compilation c  2010 The Zoological Society of London 6 Journal of Zoology. Print ISSN 0952-8369