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Wednesday, May 30, 2007

Why do giraffes have horns?


My family recently returned from a trip to Africa with a bunch of photos of wildlife. We were halfway into a marathon of lions, wildebeests, and secretary birds when we came upon a close-up shot of a giraffe (not pictured above). And suddenly, it occurred to me that I didn't know why giraffes have horns. I raised the question, promised to research, and report back.

And the result of my exhaustive search of the internets: we don't really know. Based on what I've found, it's likely that the horns of the giraffe are an example of what Steven J. Gould called "spandrels" -- structures or adaptations that served as a support for some other function. In the case of giraffes, biologists know that the ancestors of giraffes had antlers, much like deer. Antlers are made of protrusions of bone which are shed and regrown each year. The giraffe's "horns" however, are not antlers -- they are permanent outcroppings of bone from the skull, called "ossicones." Giraffes are born with them, and they are covered with hair (except for adult males, who wear away the fur at the end). A best guess is that the giraffe's "horns" were originally support structures for their antlers -- sockets that supported the large racks which deer find so handy during mating season in their tests of strength and dominance. To speculate a bit, as giraffes grew taller, and their necks thinner, the violent frontal assaults of the mating ritual would have become dangerous. Instead, giraffes joust by wrapping their necks around each other (pictured at right) and banging the back of their skull, and sometimes their forehead, into the skull of their opponent. For this purpose, giraffes additionally have bony protuberances above their eyes and at the back of their heads -- just visible in the illustration above. But this method of fighting renders antlers at the top of the head useless -- as you can see, the horns remain pointing more or less into the air. There's a strong chance that it was partly because of this that giraffes lost their horns.
But it is possible that the bone structures which supported those horns were not lost so easily -- resulting in ossicones.
Thus, the "horns" of the giraffe may present a strong example of a structure which no longer serves a purpose, because the antlers these horns were meant to anchor no longer exist. Gould loved to talk about such "spandrels" because they provided evidence of evolutionary by-products, evidence that creatures are not designed from the ground up, but adjusted and shifted over time. Sometimes, spandrels find a new, secondary function. And it may be that the horns of the giraffe do have some new purpose which biologists have been unable to suss out as yet. But it may be that they are evolutionary flotsam -- illustrations of the odd side-effects produced as evolution fiddles with a few thousand genes in order to produce the wild variety of physical forms we call life.

20 comments:

Anonymous said...

OMG! Thank You So Much! You just squashed my curiosity. I was watching a program and wondered the same thing!

Anonymous said...

we just went to the zoo and I was asked why giraffes have horns!! Thank you soo much for the answers, next time I'll be able to shed some light on the situation!!!!

Anonymous said...

Thanks for the answer. We just had the exact question you researched and your speculations make a whole lot of sense.

Anonymous said...

Thanks for the explnantion :) But it still drives me up the wall seeing them there but not having a proper use.

Anonymous said...

"Why do giraffes have horns?" is the wrong question. The real question is "Seeing that giraffes are the only african animal born with horns, why is this?"

Anonymous said...

thank you i am gald that you were able to answer this question for me my 2 yr old daughter had asked me this question at the zoo yesterday and i couldn't answer it for her so thank you

Anonymous said...

How can you say that giraffes gradually evolved from some extinct animal since acquired characteristics (like stretched necks) cannot be passed on to the next generation? Steven Gould once stated the fossil evidence did not support graduated evolution. Aren't evolutionary ideas being debunked as science brings more light? I really think so....:)

Sarah said...

Thanks for this! Very nice explanation!! I think their little horn nubs are pretty cute, even if they have no use now.

Scott said...

Anyone want to explain to Anonymous how evolution by natural selection works? I think he or she must have missed a few classes.

Trevor said...

The Lamarckian theory of evolution (that characteristics acquired during an individual's life could be inherited by future generations) was replaced by the theory of natural selection (Darwinian theory). According to Darwinian theory, the predecessors of giraffes had various genes for shorter or longer necks. During famines, the animals with shorter necks more often starved, so through a gradual sorting process, genes for short necks disappeared and genes for long necks survived. Hence the giraffe.
If Anon is a creationist trying to debunk Lamarck, he is about 150 years late.

Anonymous said...

ummm thats actually wrong because giraffes use it for mating purposes. the males swing it into the other males side and whoever can withstand the most hits, or can hit harder, gets the female.

Steven said...

I think evolution is one of the most absurd theories in the universe. Just look at the complexity of the human body ... can you honestly believe that it came about by chance. Even Darwin doubted his own theory because of the complexity of the human eye with auto-color correction.

Anonymous said...

Someone needs to explain to Scott that evolution, especially Darwinian Evolution, is NOT the only way to explain our natural world and living things in it. There is a LOT we simply don't know. Much of evolution is assumption upon assumption upon assumption, that is assumed to be true since it has been repeated so many times. BAD science.

Anonymous said...

yeah, jesus invented them and put them there at the same time as the dinosaurs. Then he rode a dinosaur into the sunset.

Anonymous said...

No, animals with longer necks tended to survive. Animals that could stretch their necks tended to get cirque de soliel jobs and eventually evolved into flubber.

Anonymous said...

There is a perpose. They use them as a defence meganisa. They are very territorial animals and they use those to break the others girraffes neck if they come on there territory

Anonymous said...

It seems to me that these ossicones might still be useful for prodding during necking. At least it seems so from some necking videos.

Anonymous said...

interressting could these be successer of those megaceroses

Anonymous said...

thanks for the info but i thought they had horns from hurting their eyes. is that true?

Anonymous said...

I always called their 'horns', antennas. Haha.