The giraffe's long neck may have evolved to help the male compete for mates, suggests new research
The giraffe's long neck may have evolved to help the male compete for mates, suggests new research.
Nearly 15 million years ago the giraffes were antelope-like animals roaming the dry grasslands of Africa. They had no distinguishing characteristics, except that some their necks were a bit long.
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But within 6 million years, they had evolved into animals that looked like modern giraffes, even as we know the modern species only emerged around 1 million years ago. Today's giraffe, the tallest living land animal, stands between 4.5 and 5 metres tall with its neck making up nearly half that height.
It is largely believed that giraffes' long necks evolved to help them eat leaves on tall trees that their rivals couldn't reach.
But the evidence supporting the high-feeding theory is surprisingly weak.
The latest theory is that the long necks are the result of sexual selection - they evolved in males as a way of competing for females.
Male giraffes fight for females by "necking". They stand side by side and swing the backs of their heads into each others'' ribs and legs. Helping them are their unusually thick skulls and horn-like growths called ossicones on the tops of their heads.
A long and powerful neck would be an advantage in these duels, and it has emerged males with long necks tend to win, and also that females prefer them.
The "necks for sex" idea also answers why giraffes have extended their necks so much more than their legs. If giraffes'' long necks evolved to reach higher branches, their legs should have been lengthened as fast as their necks, but they haven't.
The only problem for the sex idea is that it implies that female giraffes shouldn't have long necks, and they plainly do.
Research conducted last year by Graham Mitchell of the University of Pretoria in South Africa and colleagues apparently debunked the "necks for sex" theory. Mitchell's team demonstrated that, in Zimbabwe at least, males and females had necks that were almost exactly the same length, and that if anything the females' necks were longer.
However, Rob Simmons and Res Altwegg of the University of Cape Town, South Africa, have reviewed Mitchell's results and are not convinced. According to them, the figures do show that males have proportionally longer necks, and that "Mitchell et al. appear to have misinterpreted this result", reports The New Scientist.
They point to a study in Namibia which found that males consistently had heavier necks than females with the same body mass, and that only the males' necks kept growing throughout their lives. Males' heads were also heavier than females', which is what you would expect if they were being selected for their ability to fight.
Simmons and Altwegg believe giraffes' necks may have begun growing as a way of eating hard-to-reach food, but that they were then "hijacked" for mating purposes. Once the necks reached a certain length, males could use them for necking and clubbing - and at that point sexual selection took over, driving the necks to their current extreme lengths.
Simmons and Altwegg's research appears in Journal of Zoology.
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