Big rib bones hold a clue about the
woolly mammoth's extinction
By Los Angeles Times, adapted by Newsela staff on 04.03.14
Word Count 615
A display of the woolly mammoth in the Royal BC Museum in Victoria, Canada, from 1979. The fur is musk
ox hair. Photo: Wikimedia Commons
Woolly mammoths are known for their thick fur coats and enormous tusks. But
their so-called cervical ribs are capturing scientists' attention these days. These
distinctive bones are giving scientists new clues about what made the woolly
mammoth die off roughly 10,000 years ago.
Cervical ribs are extra rib bones that stick out from the spine at the base of the
neck. Elephants living today grow them sometimes, too. But these ribs were
about 10 times more common in mammoths living in the Late Pleistocene age.
Dutch researchers published these findings Tuesday in the science magazine
PeerJ.
The cervical ribs themselves aren’t necessarily dangerous. But they tend to
appear in animals that didn't develop normally during the early stages of
pregnancy. Babies who don't develop normally may have short lifespans. In
humans, about 90 percent of babies born with a cervical rib die before they are
old enough to have their own babies.
The authors of the study became interested in cervical ribs in mammoths after
mammoth bones were discovered in the Netherlands. The bones were found
during a project to extend Rotterdam Harbor into the North Sea. Three of the
bones were from the lower portion of the neck, just above the part of the spine
that connects with rib bones. Two of the three specimens showed signs of
having been connected with ribs.
Checking For Cervical Ribs
That chance discovery made the researchers wonder just how common cervical
ribs were.
“We knew these were just about the last mammoths living there, so we
suspected something was happening,” study leader Jelle Reumer said. Reumer
is a paleontologist at Utrecht University. Paleontologists like Reumer study
ancient life, before the existence of modern humans. Reumer also runs the
Natural History Museum in Rotterdam.
Reumer and two co-workers looked in the collections of the Natural History
Museum and the Naturalis Biodiversity Center, both in the Netherlands. They
found 16 samples of mammoth vertebrae from the base of the neck. Seven of
the samples were missing the part that would have clued the researchers in on
whether a cervical rib had been attached. Of the remaining nine, six were
normal. But, three once had a cervical rib. Of the samples then, 33 percent had
the rib.
For the sake of comparison, the researchers also analyzed bones from 21 Asian
elephants and seven African elephants that were in museum collections in
Europe. Among the 28 elephants, only one had a cervical rib. The frequency of
one occurring, therefore, was 3.6 percent.
Unusually Big Bones
The mammoths were nearly 10 times more likely than the elephants to have a
cervical rib. This led researchers to conclude that the frequency of the extra ribs
was “extremely high” among the woolly mammoths.
But they weren't just unusually common. The shape of the vertebrae suggested
the bones were unusually large as well. In mice with cervical ribs, those with
bigger bones have more health problems than those with smaller bones.
“There was indeed a problem in this population,” Reumer said.
There are two explanations for the high frequency of cervical ribs in mammoths,
and both are probably part of the story, the researchers wrote. The first is that
the mammoths were interbreeding. Mating with close relatives would be
expected among a population that was dying off. The second explanation is that
the mammoths were struggling with widespread hunger, disease and other
environmental conditions. This would have made it difficult for female
mammoths to bear healthy offspring.
The Dutch researchers concluded that the high number of cervical ribs was
probably because of "a combination of inbreeding and harsh conditions.” They
went on to say that this vulnerability might have contributed to the mammoths'
eventual extinction.
Quiz
1
2
3
Why are scientists interested in studying cervical ribs of woolly mammoths?
(A)
Woolly mammoths are the only species with cervical ribs.
(B)
Cervical ribs were common among woolly mammoths in the Late
Pleistocene age.
(C)
Scientists believe that cervical ribs contributed to the extinction of
woolly mammoths.
(D)
Cervical ribs appear in animals that don't develop normally during
pregnancy.
Read the section "Checking For Cervical Ribs." Which of the following details
supports the main idea that cervical ribs were common among woolly mammoths?
(A)
It was a chance discovery.
(B)
All of the woolly mammoths found in the Netherlands had cervical
ribs.
(C)
Cervical ribs were more commonly found in Asian and African
elephants.
(D)
One in three mammoths found had a cervical rib.
Read the sentence from the article.
These distinctive bones are giving scientists new clues about
what made the woolly mammoth die off roughly 10,000 years
ago.
Based on how cervical ribs are described throughout the article, what does the
phrase "distinctive bones" MOST likely mean?
(A)
ancient bones
(B)
noteworthy bones
(C)
fragile bones
(D)
dangerous bones
4
Read the sentence from the section "Unusually Big Bones."
They went on to say that this vulnerability might have
contributed to the mammoths' eventual extinction.
As used here, "vulnerability" MOST nearly means:
(A)
weakness
(B)
strength
(C)
defense
(D)
understanding
Answer Key
1
2
3
Why are scientists interested in studying cervical ribs of woolly mammoths?
(A)
Woolly mammoths are the only species with cervical ribs.
(B)
Cervical ribs were common among woolly mammoths in the Late
Pleistocene age.
(C)
Scientists believe that cervical ribs contributed to the
extinction of woolly mammoths.
(D)
Cervical ribs appear in animals that don't develop normally during
pregnancy.
Read the section "Checking For Cervical Ribs." Which of the following details
supports the main idea that cervical ribs were common among woolly mammoths?
(A)
It was a chance discovery.
(B)
All of the woolly mammoths found in the Netherlands had cervical
ribs.
(C)
Cervical ribs were more commonly found in Asian and African
elephants.
(D)
One in three mammoths found had a cervical rib.
Read the sentence from the article.
These distinctive bones are giving scientists new clues about
what made the woolly mammoth die off roughly 10,000 years
ago.
Based on how cervical ribs are described throughout the article, what does the
phrase "distinctive bones" MOST likely mean?
(A)
ancient bones
(B)
noteworthy bones
(C)
fragile bones
(D)
dangerous bones
4
Read the sentence from the section "Unusually Big Bones."
They went on to say that this vulnerability might have
contributed to the mammoths' eventual extinction.
As used here, "vulnerability" MOST nearly means:
(A)
weakness
(B)
strength
(C)
defense
(D)
understanding