If sexual selection were responsible for the giraffe's long neck, male giraffes would have evolved longer necks than their female counterparts, said the researchers, but they didn't find that to be the case.
Credit: iStockphoto
BOSTON: Theories to explain the giraffe's long neck – an age-old riddle of evolution – are both plentiful and controversial, says a new study, which refutes the role of sexual selection.
One popular theory has been that longer necks make male giraffes more sexually attractive to females, causing necks to grow through evolution as males with genes for longer necks are more likely to have offspring.
Another common example of a sexually selected trait is the peacock's tail, said study author Graham Mitchell, a zoologist at the University of Wyoming in Laramie, USA. "It has no function other than to attract peahens."
Battle of the necks
A different way sexual selection can work to make a male animal a more successful mate is by helping it to compete for females. A second hypothesis proposes this as the mechanism for the giraffe's neck, following observations of the way males spar with one another.
They battle for dominance through "necking" and "head-clubbing," says the study, published in a recent edition of the Journal of Zoology. The theory goes that giraffes with longer, thicker necks win more fights and therefore more mates. As a result, they are more likely to pass on their genes conferring longer necks.
However, the new study has found evidence that sexual selection can't be the key. If sexual selection were responsible for the giraffe's long neck, male giraffes would have evolved longer necks than their female counterparts, said Mitchell.
To test the theory, he led a team of researchers who studied 17 male and 21 female giraffes in Zimbabwe. They measured height, weight, and the length of necks and legs – but found little variation between the sexes. "This is not what you would expect if necks were a male feature," Mitchell said.
Poaching problem
"Our data showed that neck length… is not unique to one sex and so does not meet the requirements of sexual selection," says the study. "The neck must have other functions."
Rob Simmons, an ecologist and giraffe expert from the Percy FitzPatrick Institute of African Ornithology in Cape Town, South Africa, said he had found conflicting evidence, however.
His own study, which had a larger sample size, did find a difference in neck length between the sexes, he said. "To try to turn over a unique and well-supported theory with a sample of 38 giraffe is not very wise."
Simmons argued that studying giraffes in Zimbabwe could have skewed the results because poachers have killed most large males there.
Johan Du Toit, a large mammal ecologist at Utah State University in Logan, disagreed with Simmons and told Cosmos Online that the new study offers "solid evidence" against the sexual selection hypothesis.
Instead, he favours the feeding advantage theory, which states that long necks evolved to give the animals access to food on higher branches.
"Those individuals with higher reach will have a higher probability of surviving when food is limited," he said, "and will thus be more likely to pass on their genes to future generations."
Temperature control
Mitchell said other factors could also have played a role. For example, giraffes are much better at spotting prey at a distance than species with their eyes closer to the ground. Furthermore, the long neck may be helpful for keeping the animals cool, by increasing relative surface area for heat loss.
"We think that temperature control, vigilance and feeding are all major benefits of having a long neck," he said.
Research into the origin of the giraffe's long neck could help scientists answer some more general questions too. For example, studying the development of vertebrae in the neck could provide important insight into what controls the length of bones in other animals, including humans.
"In general, the principle is that species that have extraordinary adaptations are the ones that will give the most insight into how things work [in all animals]," Mitchell said.
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