Friday, July 2, 2010
Saber-toothed cats may be best known for their supersized canines,
but they also had exceptionally strong forelimbs for pinning prey
before delivering the fatal bite, says a new study in the
journalPLoS
ONE.
Commonly called the "saber-toothed tiger," the extinct
cat Smilodon
fatalisroamed North and South America until 10,000 years ago,
preying on large mammals such as bison, camels, mastodons and
mammoths. Telltale clues from bones and teeth suggest they relied
on their forelimbs as well as their fangs to catch and kill their
prey.
The size and shape of sabertooth canines made them more vulnerable
to fracture than cats living today, said author Julie
Meachen-Samuels, a paleontologist at the National Evolutionary
Synthesis Center in Durham, NC.
"Cats living today have canines that are round in cross-section, so
they can withstand forces in all directions. If the prey is
struggling it doesn't matter which way it's pulling — their teeth
are unlikely to break," she explained.
In contrast, the elongated canines of saber-toothed cats were oval
in cross-section, which made them more vulnerable to breaking than
their conical-toothed cousins. "Many scientists infer that
saber-toothed cats killed prey differently from other cats because
their teeth were thinner side-to-side," said Meachen-Samuels.
Despite their vulnerable canines, prominent muscle attachment scars
on sabertooth limb bones suggest the cat was powerfully built.
Saber-toothed cats may have used their muscular arms to immobilize
prey and protect their teeth from fracture, she explained.
To estimate how strong sabertooth forelimbs were relative to other
cats, the researchers used x-rays to measure the cross-sectional
dimensions of the upper arm and leg bones of fossils recovered from
the La Brea Tar Pits in Los Angeles. They also measured the limb
bones of 28 cat species living today — ranging in size from the
6-pound margay to the 600-pound tiger — as well as the extinct
American lion, the largest conical-toothed cat that ever
lived.
The researchers used their cross-sectional measurements to estimate
bone strength and rigidity for each species. When they plotted
rigidity against length for the 30 species in their study, species
with longer limbs generally had stronger bones. But the data for
the saber-toothed cat fell well outside the normal range —while
their leg bones scaled to size, their arm bones were exceptionally
thick for their length.
"When I looked at the arm bones, Smilodon
fatalis was way out in left field," said
Meachen-Samuels.
Sabertooth arm bones were not only larger in diameter than other
cats, they also had thicker cortical bone, the dense outer layer
that makes bones strong and stiff. Thicker cortical bone is
consistent with the idea that sabertooth forelimbs were under
greater stress than would be expected for cats their size,
Meachen-Samuels explained. Just like weight-bearing exercise
remodels our bones and improves bone density over time, the
repeated strain of grappling with prey may have resulted in thicker
and stronger arm bones in saber-toothed cats.
"As muscles pull on bones, bones respond by getting stronger," said
Meachen-Samuels. "Because saber-toothed cats had thicker arm bones
we think they must have used their forelimbs more than other cats
did."
"The findings give us new information about how strong their
forelimbs were and how they were built," she added. "This is the
first study to look inside sabertooth arm bones to see exactly how
much stress and strain they could handle."
The findings will be published online in the June 30 issue
of PLoS
ONE.
###
National Evolutionary Synthesis Center (NESCent)
Thanks
to National Evolutionary Synthesis Center (NESCent) for this
article.